Source file src/go/types/call.go

Documentation: go/types

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file implements typechecking of call and selector expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"go/ast"
    11  	"go/internal/typeparams"
    12  	"go/token"
    13  	. "internal/types/errors"
    14  	"strings"
    15  	"unicode"
    16  )
    17  
    18  // funcInst type-checks a function instantiation.
    19  // The incoming x must be a generic function.
    20  // If ix != nil, it provides some or all of the type arguments (ix.Indices).
    21  // If target != nil, it may be used to infer missing type arguments of x, if any.
    22  // At least one of T or ix must be provided.
    23  //
    24  // There are two modes of operation:
    25  //
    26  //  1. If infer == true, funcInst infers missing type arguments as needed and
    27  //     instantiates the function x. The returned results are nil.
    28  //
    29  //  2. If infer == false and inst provides all type arguments, funcInst
    30  //     instantiates the function x. The returned results are nil.
    31  //     If inst doesn't provide enough type arguments, funcInst returns the
    32  //     available arguments and the corresponding expression list; x remains
    33  //     unchanged.
    34  //
    35  // If an error (other than a version error) occurs in any case, it is reported
    36  // and x.mode is set to invalid.
    37  func (check *Checker) funcInst(T *target, pos token.Pos, x *operand, ix *typeparams.IndexExpr, infer bool) ([]Type, []ast.Expr) {
    38  	assert(T != nil || ix != nil)
    39  
    40  	var instErrPos positioner
    41  	if ix != nil {
    42  		instErrPos = inNode(ix.Orig, ix.Lbrack)
    43  	} else {
    44  		instErrPos = atPos(pos)
    45  	}
    46  	versionErr := !check.verifyVersionf(instErrPos, go1_18, "function instantiation")
    47  
    48  	// targs and xlist are the type arguments and corresponding type expressions, or nil.
    49  	var targs []Type
    50  	var xlist []ast.Expr
    51  	if ix != nil {
    52  		xlist = ix.Indices
    53  		targs = check.typeList(xlist)
    54  		if targs == nil {
    55  			x.mode = invalid
    56  			x.expr = ix
    57  			return nil, nil
    58  		}
    59  		assert(len(targs) == len(xlist))
    60  	}
    61  
    62  	// Check the number of type arguments (got) vs number of type parameters (want).
    63  	// Note that x is a function value, not a type expression, so we don't need to
    64  	// call under below.
    65  	sig := x.typ.(*Signature)
    66  	got, want := len(targs), sig.TypeParams().Len()
    67  	if got > want {
    68  		// Providing too many type arguments is always an error.
    69  		check.errorf(ix.Indices[got-1], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
    70  		x.mode = invalid
    71  		x.expr = ix.Orig
    72  		return nil, nil
    73  	}
    74  
    75  	if got < want {
    76  		if !infer {
    77  			return targs, xlist
    78  		}
    79  
    80  		// If the uninstantiated or partially instantiated function x is used in
    81  		// an assignment (tsig != nil), infer missing type arguments by treating
    82  		// the assignment
    83  		//
    84  		//    var tvar tsig = x
    85  		//
    86  		// like a call g(tvar) of the synthetic generic function g
    87  		//
    88  		//    func g[type_parameters_of_x](func_type_of_x)
    89  		//
    90  		var args []*operand
    91  		var params []*Var
    92  		var reverse bool
    93  		if T != nil && sig.tparams != nil {
    94  			if !versionErr && !check.allowVersion(check.pkg, instErrPos, go1_21) {
    95  				if ix != nil {
    96  					check.versionErrorf(instErrPos, go1_21, "partially instantiated function in assignment")
    97  				} else {
    98  					check.versionErrorf(instErrPos, go1_21, "implicitly instantiated function in assignment")
    99  				}
   100  			}
   101  			gsig := NewSignatureType(nil, nil, nil, sig.params, sig.results, sig.variadic)
   102  			params = []*Var{NewVar(x.Pos(), check.pkg, "", gsig)}
   103  			// The type of the argument operand is tsig, which is the type of the LHS in an assignment
   104  			// or the result type in a return statement. Create a pseudo-expression for that operand
   105  			// that makes sense when reported in error messages from infer, below.
   106  			expr := ast.NewIdent(T.desc)
   107  			expr.NamePos = x.Pos() // correct position
   108  			args = []*operand{{mode: value, expr: expr, typ: T.sig}}
   109  			reverse = true
   110  		}
   111  
   112  		// Rename type parameters to avoid problems with recursive instantiations.
   113  		// Note that NewTuple(params...) below is (*Tuple)(nil) if len(params) == 0, as desired.
   114  		tparams, params2 := check.renameTParams(pos, sig.TypeParams().list(), NewTuple(params...))
   115  
   116  		targs = check.infer(atPos(pos), tparams, targs, params2.(*Tuple), args, reverse)
   117  		if targs == nil {
   118  			// error was already reported
   119  			x.mode = invalid
   120  			x.expr = ix // TODO(gri) is this correct?
   121  			return nil, nil
   122  		}
   123  		got = len(targs)
   124  	}
   125  	assert(got == want)
   126  
   127  	// instantiate function signature
   128  	expr := x.expr // if we don't have an index expression, keep the existing expression of x
   129  	if ix != nil {
   130  		expr = ix.Orig
   131  	}
   132  	sig = check.instantiateSignature(x.Pos(), expr, sig, targs, xlist)
   133  
   134  	x.typ = sig
   135  	x.mode = value
   136  	x.expr = expr
   137  	return nil, nil
   138  }
   139  
   140  func (check *Checker) instantiateSignature(pos token.Pos, expr ast.Expr, typ *Signature, targs []Type, xlist []ast.Expr) (res *Signature) {
   141  	assert(check != nil)
   142  	assert(len(targs) == typ.TypeParams().Len())
   143  
   144  	if check.conf._Trace {
   145  		check.trace(pos, "-- instantiating signature %s with %s", typ, targs)
   146  		check.indent++
   147  		defer func() {
   148  			check.indent--
   149  			check.trace(pos, "=> %s (under = %s)", res, res.Underlying())
   150  		}()
   151  	}
   152  
   153  	inst := check.instance(pos, typ, targs, nil, check.context()).(*Signature)
   154  	assert(inst.TypeParams().Len() == 0) // signature is not generic anymore
   155  	check.recordInstance(expr, targs, inst)
   156  	assert(len(xlist) <= len(targs))
   157  
   158  	// verify instantiation lazily (was go.dev/issue/50450)
   159  	check.later(func() {
   160  		tparams := typ.TypeParams().list()
   161  		if i, err := check.verify(pos, tparams, targs, check.context()); err != nil {
   162  			// best position for error reporting
   163  			pos := pos
   164  			if i < len(xlist) {
   165  				pos = xlist[i].Pos()
   166  			}
   167  			check.softErrorf(atPos(pos), InvalidTypeArg, "%s", err)
   168  		} else {
   169  			check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
   170  		}
   171  	}).describef(atPos(pos), "verify instantiation")
   172  
   173  	return inst
   174  }
   175  
   176  func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
   177  	ix := typeparams.UnpackIndexExpr(call.Fun)
   178  	if ix != nil {
   179  		if check.indexExpr(x, ix) {
   180  			// Delay function instantiation to argument checking,
   181  			// where we combine type and value arguments for type
   182  			// inference.
   183  			assert(x.mode == value)
   184  		} else {
   185  			ix = nil
   186  		}
   187  		x.expr = call.Fun
   188  		check.record(x)
   189  	} else {
   190  		check.exprOrType(x, call.Fun, true)
   191  	}
   192  	// x.typ may be generic
   193  
   194  	switch x.mode {
   195  	case invalid:
   196  		check.use(call.Args...)
   197  		x.expr = call
   198  		return statement
   199  
   200  	case typexpr:
   201  		// conversion
   202  		check.nonGeneric(nil, x)
   203  		if x.mode == invalid {
   204  			return conversion
   205  		}
   206  		T := x.typ
   207  		x.mode = invalid
   208  		switch n := len(call.Args); n {
   209  		case 0:
   210  			check.errorf(inNode(call, call.Rparen), WrongArgCount, "missing argument in conversion to %s", T)
   211  		case 1:
   212  			check.expr(nil, x, call.Args[0])
   213  			if x.mode != invalid {
   214  				if call.Ellipsis.IsValid() {
   215  					check.errorf(call.Args[0], BadDotDotDotSyntax, "invalid use of ... in conversion to %s", T)
   216  					break
   217  				}
   218  				if t, _ := under(T).(*Interface); t != nil && !isTypeParam(T) {
   219  					if !t.IsMethodSet() {
   220  						check.errorf(call, MisplacedConstraintIface, "cannot use interface %s in conversion (contains specific type constraints or is comparable)", T)
   221  						break
   222  					}
   223  				}
   224  				check.conversion(x, T)
   225  			}
   226  		default:
   227  			check.use(call.Args...)
   228  			check.errorf(call.Args[n-1], WrongArgCount, "too many arguments in conversion to %s", T)
   229  		}
   230  		x.expr = call
   231  		return conversion
   232  
   233  	case builtin:
   234  		// no need to check for non-genericity here
   235  		id := x.id
   236  		if !check.builtin(x, call, id) {
   237  			x.mode = invalid
   238  		}
   239  		x.expr = call
   240  		// a non-constant result implies a function call
   241  		if x.mode != invalid && x.mode != constant_ {
   242  			check.hasCallOrRecv = true
   243  		}
   244  		return predeclaredFuncs[id].kind
   245  	}
   246  
   247  	// ordinary function/method call
   248  	// signature may be generic
   249  	cgocall := x.mode == cgofunc
   250  
   251  	// a type parameter may be "called" if all types have the same signature
   252  	sig, _ := coreType(x.typ).(*Signature)
   253  	if sig == nil {
   254  		check.errorf(x, InvalidCall, invalidOp+"cannot call non-function %s", x)
   255  		x.mode = invalid
   256  		x.expr = call
   257  		return statement
   258  	}
   259  
   260  	// Capture wasGeneric before sig is potentially instantiated below.
   261  	wasGeneric := sig.TypeParams().Len() > 0
   262  
   263  	// evaluate type arguments, if any
   264  	var xlist []ast.Expr
   265  	var targs []Type
   266  	if ix != nil {
   267  		xlist = ix.Indices
   268  		targs = check.typeList(xlist)
   269  		if targs == nil {
   270  			check.use(call.Args...)
   271  			x.mode = invalid
   272  			x.expr = call
   273  			return statement
   274  		}
   275  		assert(len(targs) == len(xlist))
   276  
   277  		// check number of type arguments (got) vs number of type parameters (want)
   278  		got, want := len(targs), sig.TypeParams().Len()
   279  		if got > want {
   280  			check.errorf(xlist[want], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
   281  			check.use(call.Args...)
   282  			x.mode = invalid
   283  			x.expr = call
   284  			return statement
   285  		}
   286  
   287  		// If sig is generic and all type arguments are provided, preempt function
   288  		// argument type inference by explicitly instantiating the signature. This
   289  		// ensures that we record accurate type information for sig, even if there
   290  		// is an error checking its arguments (for example, if an incorrect number
   291  		// of arguments is supplied).
   292  		if got == want && want > 0 {
   293  			check.verifyVersionf(atPos(ix.Lbrack), go1_18, "function instantiation")
   294  			sig = check.instantiateSignature(ix.Pos(), ix.Orig, sig, targs, xlist)
   295  			// targs have been consumed; proceed with checking arguments of the
   296  			// non-generic signature.
   297  			targs = nil
   298  			xlist = nil
   299  		}
   300  	}
   301  
   302  	// evaluate arguments
   303  	args, atargs, atxlist := check.genericExprList(call.Args)
   304  	sig = check.arguments(call, sig, targs, xlist, args, atargs, atxlist)
   305  
   306  	if wasGeneric && sig.TypeParams().Len() == 0 {
   307  		// Update the recorded type of call.Fun to its instantiated type.
   308  		check.recordTypeAndValue(call.Fun, value, sig, nil)
   309  	}
   310  
   311  	// determine result
   312  	switch sig.results.Len() {
   313  	case 0:
   314  		x.mode = novalue
   315  	case 1:
   316  		if cgocall {
   317  			x.mode = commaerr
   318  		} else {
   319  			x.mode = value
   320  		}
   321  		x.typ = sig.results.vars[0].typ // unpack tuple
   322  	default:
   323  		x.mode = value
   324  		x.typ = sig.results
   325  	}
   326  	x.expr = call
   327  	check.hasCallOrRecv = true
   328  
   329  	// if type inference failed, a parameterized result must be invalidated
   330  	// (operands cannot have a parameterized type)
   331  	if x.mode == value && sig.TypeParams().Len() > 0 && isParameterized(sig.TypeParams().list(), x.typ) {
   332  		x.mode = invalid
   333  	}
   334  
   335  	return statement
   336  }
   337  
   338  // exprList evaluates a list of expressions and returns the corresponding operands.
   339  // A single-element expression list may evaluate to multiple operands.
   340  func (check *Checker) exprList(elist []ast.Expr) (xlist []*operand) {
   341  	if n := len(elist); n == 1 {
   342  		xlist, _ = check.multiExpr(elist[0], false)
   343  	} else if n > 1 {
   344  		// multiple (possibly invalid) values
   345  		xlist = make([]*operand, n)
   346  		for i, e := range elist {
   347  			var x operand
   348  			check.expr(nil, &x, e)
   349  			xlist[i] = &x
   350  		}
   351  	}
   352  	return
   353  }
   354  
   355  // genericExprList is like exprList but result operands may be uninstantiated or partially
   356  // instantiated generic functions (where constraint information is insufficient to infer
   357  // the missing type arguments) for Go 1.21 and later.
   358  // For each non-generic or uninstantiated generic operand, the corresponding targsList and
   359  // xlistList elements do not exist (targsList and xlistList are nil) or the elements are nil.
   360  // For each partially instantiated generic function operand, the corresponding targsList and
   361  // xlistList elements are the operand's partial type arguments and type expression lists.
   362  func (check *Checker) genericExprList(elist []ast.Expr) (resList []*operand, targsList [][]Type, xlistList [][]ast.Expr) {
   363  	if debug {
   364  		defer func() {
   365  			// targsList and xlistList must have matching lengths
   366  			assert(len(targsList) == len(xlistList))
   367  			// type arguments must only exist for partially instantiated functions
   368  			for i, x := range resList {
   369  				if i < len(targsList) {
   370  					if n := len(targsList[i]); n > 0 {
   371  						// x must be a partially instantiated function
   372  						assert(n < x.typ.(*Signature).TypeParams().Len())
   373  					}
   374  				}
   375  			}
   376  		}()
   377  	}
   378  
   379  	// Before Go 1.21, uninstantiated or partially instantiated argument functions are
   380  	// nor permitted. Checker.funcInst must infer missing type arguments in that case.
   381  	infer := true // for -lang < go1.21
   382  	n := len(elist)
   383  	if n > 0 && check.allowVersion(check.pkg, elist[0], go1_21) {
   384  		infer = false
   385  	}
   386  
   387  	if n == 1 {
   388  		// single value (possibly a partially instantiated function), or a multi-valued expression
   389  		e := elist[0]
   390  		var x operand
   391  		if ix := typeparams.UnpackIndexExpr(e); ix != nil && check.indexExpr(&x, ix) {
   392  			// x is a generic function.
   393  			targs, xlist := check.funcInst(nil, x.Pos(), &x, ix, infer)
   394  			if targs != nil {
   395  				// x was not instantiated: collect the (partial) type arguments.
   396  				targsList = [][]Type{targs}
   397  				xlistList = [][]ast.Expr{xlist}
   398  				// Update x.expr so that we can record the partially instantiated function.
   399  				x.expr = ix.Orig
   400  			} else {
   401  				// x was instantiated: we must record it here because we didn't
   402  				// use the usual expression evaluators.
   403  				check.record(&x)
   404  			}
   405  			resList = []*operand{&x}
   406  		} else {
   407  			// x is not a function instantiation (it may still be a generic function).
   408  			check.rawExpr(nil, &x, e, nil, true)
   409  			check.exclude(&x, 1<<novalue|1<<builtin|1<<typexpr)
   410  			if t, ok := x.typ.(*Tuple); ok && x.mode != invalid {
   411  				// x is a function call returning multiple values; it cannot be generic.
   412  				resList = make([]*operand, t.Len())
   413  				for i, v := range t.vars {
   414  					resList[i] = &operand{mode: value, expr: e, typ: v.typ}
   415  				}
   416  			} else {
   417  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   418  				resList = []*operand{&x}
   419  			}
   420  		}
   421  	} else if n > 1 {
   422  		// multiple values
   423  		resList = make([]*operand, n)
   424  		targsList = make([][]Type, n)
   425  		xlistList = make([][]ast.Expr, n)
   426  		for i, e := range elist {
   427  			var x operand
   428  			if ix := typeparams.UnpackIndexExpr(e); ix != nil && check.indexExpr(&x, ix) {
   429  				// x is a generic function.
   430  				targs, xlist := check.funcInst(nil, x.Pos(), &x, ix, infer)
   431  				if targs != nil {
   432  					// x was not instantiated: collect the (partial) type arguments.
   433  					targsList[i] = targs
   434  					xlistList[i] = xlist
   435  					// Update x.expr so that we can record the partially instantiated function.
   436  					x.expr = ix.Orig
   437  				} else {
   438  					// x was instantiated: we must record it here because we didn't
   439  					// use the usual expression evaluators.
   440  					check.record(&x)
   441  				}
   442  			} else {
   443  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   444  				check.genericExpr(&x, e)
   445  			}
   446  			resList[i] = &x
   447  		}
   448  	}
   449  
   450  	return
   451  }
   452  
   453  // arguments type-checks arguments passed to a function call with the given signature.
   454  // The function and its arguments may be generic, and possibly partially instantiated.
   455  // targs and xlist are the function's type arguments (and corresponding expressions).
   456  // args are the function arguments. If an argument args[i] is a partially instantiated
   457  // generic function, atargs[i] and atxlist[i] are the corresponding type arguments
   458  // (and corresponding expressions).
   459  // If the callee is variadic, arguments adjusts its signature to match the provided
   460  // arguments. The type parameters and arguments of the callee and all its arguments
   461  // are used together to infer any missing type arguments, and the callee and argument
   462  // functions are instantiated as necessary.
   463  // The result signature is the (possibly adjusted and instantiated) function signature.
   464  // If an error occurred, the result signature is the incoming sig.
   465  func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type, xlist []ast.Expr, args []*operand, atargs [][]Type, atxlist [][]ast.Expr) (rsig *Signature) {
   466  	rsig = sig
   467  
   468  	// Function call argument/parameter count requirements
   469  	//
   470  	//               | standard call    | dotdotdot call |
   471  	// --------------+------------------+----------------+
   472  	// standard func | nargs == npars   | invalid        |
   473  	// --------------+------------------+----------------+
   474  	// variadic func | nargs >= npars-1 | nargs == npars |
   475  	// --------------+------------------+----------------+
   476  
   477  	nargs := len(args)
   478  	npars := sig.params.Len()
   479  	ddd := call.Ellipsis.IsValid()
   480  
   481  	// set up parameters
   482  	sigParams := sig.params // adjusted for variadic functions (may be nil for empty parameter lists!)
   483  	adjusted := false       // indicates if sigParams is different from sig.params
   484  	if sig.variadic {
   485  		if ddd {
   486  			// variadic_func(a, b, c...)
   487  			if len(call.Args) == 1 && nargs > 1 {
   488  				// f()... is not permitted if f() is multi-valued
   489  				check.errorf(inNode(call, call.Ellipsis), InvalidDotDotDot, "cannot use ... with %d-valued %s", nargs, call.Args[0])
   490  				return
   491  			}
   492  		} else {
   493  			// variadic_func(a, b, c)
   494  			if nargs >= npars-1 {
   495  				// Create custom parameters for arguments: keep
   496  				// the first npars-1 parameters and add one for
   497  				// each argument mapping to the ... parameter.
   498  				vars := make([]*Var, npars-1) // npars > 0 for variadic functions
   499  				copy(vars, sig.params.vars)
   500  				last := sig.params.vars[npars-1]
   501  				typ := last.typ.(*Slice).elem
   502  				for len(vars) < nargs {
   503  					vars = append(vars, NewParam(last.pos, last.pkg, last.name, typ))
   504  				}
   505  				sigParams = NewTuple(vars...) // possibly nil!
   506  				adjusted = true
   507  				npars = nargs
   508  			} else {
   509  				// nargs < npars-1
   510  				npars-- // for correct error message below
   511  			}
   512  		}
   513  	} else {
   514  		if ddd {
   515  			// standard_func(a, b, c...)
   516  			check.errorf(inNode(call, call.Ellipsis), NonVariadicDotDotDot, "cannot use ... in call to non-variadic %s", call.Fun)
   517  			return
   518  		}
   519  		// standard_func(a, b, c)
   520  	}
   521  
   522  	// check argument count
   523  	if nargs != npars {
   524  		var at positioner = call
   525  		qualifier := "not enough"
   526  		if nargs > npars {
   527  			at = args[npars].expr // report at first extra argument
   528  			qualifier = "too many"
   529  		} else {
   530  			at = atPos(call.Rparen) // report at closing )
   531  		}
   532  		// take care of empty parameter lists represented by nil tuples
   533  		var params []*Var
   534  		if sig.params != nil {
   535  			params = sig.params.vars
   536  		}
   537  		err := newErrorf(at, WrongArgCount, "%s arguments in call to %s", qualifier, call.Fun)
   538  		err.errorf(nopos, "have %s", check.typesSummary(operandTypes(args), false))
   539  		err.errorf(nopos, "want %s", check.typesSummary(varTypes(params), sig.variadic))
   540  		check.report(err)
   541  		return
   542  	}
   543  
   544  	// collect type parameters of callee and generic function arguments
   545  	var tparams []*TypeParam
   546  
   547  	// collect type parameters of callee
   548  	n := sig.TypeParams().Len()
   549  	if n > 0 {
   550  		if !check.allowVersion(check.pkg, call, go1_18) {
   551  			switch call.Fun.(type) {
   552  			case *ast.IndexExpr, *ast.IndexListExpr:
   553  				ix := typeparams.UnpackIndexExpr(call.Fun)
   554  				check.versionErrorf(inNode(call.Fun, ix.Lbrack), go1_18, "function instantiation")
   555  			default:
   556  				check.versionErrorf(inNode(call, call.Lparen), go1_18, "implicit function instantiation")
   557  			}
   558  		}
   559  		// rename type parameters to avoid problems with recursive calls
   560  		var tmp Type
   561  		tparams, tmp = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
   562  		sigParams = tmp.(*Tuple)
   563  		// make sure targs and tparams have the same length
   564  		for len(targs) < len(tparams) {
   565  			targs = append(targs, nil)
   566  		}
   567  	}
   568  	assert(len(tparams) == len(targs))
   569  
   570  	// collect type parameters from generic function arguments
   571  	var genericArgs []int // indices of generic function arguments
   572  	if enableReverseTypeInference {
   573  		for i, arg := range args {
   574  			// generic arguments cannot have a defined (*Named) type - no need for underlying type below
   575  			if asig, _ := arg.typ.(*Signature); asig != nil && asig.TypeParams().Len() > 0 {
   576  				// The argument type is a generic function signature. This type is
   577  				// pointer-identical with (it's copied from) the type of the generic
   578  				// function argument and thus the function object.
   579  				// Before we change the type (type parameter renaming, below), make
   580  				// a clone of it as otherwise we implicitly modify the object's type
   581  				// (go.dev/issues/63260).
   582  				asig = clone(asig)
   583  				// Rename type parameters for cases like f(g, g); this gives each
   584  				// generic function argument a unique type identity (go.dev/issues/59956).
   585  				// TODO(gri) Consider only doing this if a function argument appears
   586  				//           multiple times, which is rare (possible optimization).
   587  				atparams, tmp := check.renameTParams(call.Pos(), asig.TypeParams().list(), asig)
   588  				asig = tmp.(*Signature)
   589  				asig.tparams = &TypeParamList{atparams} // renameTParams doesn't touch associated type parameters
   590  				arg.typ = asig                          // new type identity for the function argument
   591  				tparams = append(tparams, atparams...)
   592  				// add partial list of type arguments, if any
   593  				if i < len(atargs) {
   594  					targs = append(targs, atargs[i]...)
   595  				}
   596  				// make sure targs and tparams have the same length
   597  				for len(targs) < len(tparams) {
   598  					targs = append(targs, nil)
   599  				}
   600  				genericArgs = append(genericArgs, i)
   601  			}
   602  		}
   603  	}
   604  	assert(len(tparams) == len(targs))
   605  
   606  	// at the moment we only support implicit instantiations of argument functions
   607  	_ = len(genericArgs) > 0 && check.verifyVersionf(args[genericArgs[0]], go1_21, "implicitly instantiated function as argument")
   608  
   609  	// tparams holds the type parameters of the callee and generic function arguments, if any:
   610  	// the first n type parameters belong to the callee, followed by mi type parameters for each
   611  	// of the generic function arguments, where mi = args[i].typ.(*Signature).TypeParams().Len().
   612  
   613  	// infer missing type arguments of callee and function arguments
   614  	if len(tparams) > 0 {
   615  		targs = check.infer(call, tparams, targs, sigParams, args, false)
   616  		if targs == nil {
   617  			// TODO(gri) If infer inferred the first targs[:n], consider instantiating
   618  			//           the call signature for better error messages/gopls behavior.
   619  			//           Perhaps instantiate as much as we can, also for arguments.
   620  			//           This will require changes to how infer returns its results.
   621  			return // error already reported
   622  		}
   623  
   624  		// update result signature: instantiate if needed
   625  		if n > 0 {
   626  			rsig = check.instantiateSignature(call.Pos(), call.Fun, sig, targs[:n], xlist)
   627  			// If the callee's parameter list was adjusted we need to update (instantiate)
   628  			// it separately. Otherwise we can simply use the result signature's parameter
   629  			// list.
   630  			if adjusted {
   631  				sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(tparams[:n], targs[:n]), nil, check.context()).(*Tuple)
   632  			} else {
   633  				sigParams = rsig.params
   634  			}
   635  		}
   636  
   637  		// compute argument signatures: instantiate if needed
   638  		j := n
   639  		for _, i := range genericArgs {
   640  			arg := args[i]
   641  			asig := arg.typ.(*Signature)
   642  			k := j + asig.TypeParams().Len()
   643  			// targs[j:k] are the inferred type arguments for asig
   644  			arg.typ = check.instantiateSignature(call.Pos(), arg.expr, asig, targs[j:k], nil) // TODO(gri) provide xlist if possible (partial instantiations)
   645  			check.record(arg)                                                                 // record here because we didn't use the usual expr evaluators
   646  			j = k
   647  		}
   648  	}
   649  
   650  	// check arguments
   651  	if len(args) > 0 {
   652  		context := check.sprintf("argument to %s", call.Fun)
   653  		for i, a := range args {
   654  			check.assignment(a, sigParams.vars[i].typ, context)
   655  		}
   656  	}
   657  
   658  	return
   659  }
   660  
   661  var cgoPrefixes = [...]string{
   662  	"_Ciconst_",
   663  	"_Cfconst_",
   664  	"_Csconst_",
   665  	"_Ctype_",
   666  	"_Cvar_", // actually a pointer to the var
   667  	"_Cfpvar_fp_",
   668  	"_Cfunc_",
   669  	"_Cmacro_", // function to evaluate the expanded expression
   670  }
   671  
   672  func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *TypeName, wantType bool) {
   673  	// these must be declared before the "goto Error" statements
   674  	var (
   675  		obj      Object
   676  		index    []int
   677  		indirect bool
   678  	)
   679  
   680  	sel := e.Sel.Name
   681  	// If the identifier refers to a package, handle everything here
   682  	// so we don't need a "package" mode for operands: package names
   683  	// can only appear in qualified identifiers which are mapped to
   684  	// selector expressions.
   685  	if ident, ok := e.X.(*ast.Ident); ok {
   686  		obj := check.lookup(ident.Name)
   687  		if pname, _ := obj.(*PkgName); pname != nil {
   688  			assert(pname.pkg == check.pkg)
   689  			check.recordUse(ident, pname)
   690  			pname.used = true
   691  			pkg := pname.imported
   692  
   693  			var exp Object
   694  			funcMode := value
   695  			if pkg.cgo {
   696  				// cgo special cases C.malloc: it's
   697  				// rewritten to _CMalloc and does not
   698  				// support two-result calls.
   699  				if sel == "malloc" {
   700  					sel = "_CMalloc"
   701  				} else {
   702  					funcMode = cgofunc
   703  				}
   704  				for _, prefix := range cgoPrefixes {
   705  					// cgo objects are part of the current package (in file
   706  					// _cgo_gotypes.go). Use regular lookup.
   707  					_, exp = check.scope.LookupParent(prefix+sel, check.pos)
   708  					if exp != nil {
   709  						break
   710  					}
   711  				}
   712  				if exp == nil {
   713  					check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
   714  					goto Error
   715  				}
   716  				check.objDecl(exp, nil)
   717  			} else {
   718  				exp = pkg.scope.Lookup(sel)
   719  				if exp == nil {
   720  					if !pkg.fake {
   721  						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
   722  					}
   723  					goto Error
   724  				}
   725  				if !exp.Exported() {
   726  					check.errorf(e.Sel, UnexportedName, "%s not exported by package %s", sel, pkg.name)
   727  					// ok to continue
   728  				}
   729  			}
   730  			check.recordUse(e.Sel, exp)
   731  
   732  			// Simplified version of the code for *ast.Idents:
   733  			// - imported objects are always fully initialized
   734  			switch exp := exp.(type) {
   735  			case *Const:
   736  				assert(exp.Val() != nil)
   737  				x.mode = constant_
   738  				x.typ = exp.typ
   739  				x.val = exp.val
   740  			case *TypeName:
   741  				x.mode = typexpr
   742  				x.typ = exp.typ
   743  			case *Var:
   744  				x.mode = variable
   745  				x.typ = exp.typ
   746  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
   747  					x.typ = x.typ.(*Pointer).base
   748  				}
   749  			case *Func:
   750  				x.mode = funcMode
   751  				x.typ = exp.typ
   752  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
   753  					x.mode = value
   754  					x.typ = x.typ.(*Signature).results.vars[0].typ
   755  				}
   756  			case *Builtin:
   757  				x.mode = builtin
   758  				x.typ = exp.typ
   759  				x.id = exp.id
   760  			default:
   761  				check.dump("%v: unexpected object %v", e.Sel.Pos(), exp)
   762  				unreachable()
   763  			}
   764  			x.expr = e
   765  			return
   766  		}
   767  	}
   768  
   769  	check.exprOrType(x, e.X, false)
   770  	switch x.mode {
   771  	case typexpr:
   772  		// don't crash for "type T T.x" (was go.dev/issue/51509)
   773  		if def != nil && def.typ == x.typ {
   774  			check.cycleError([]Object{def})
   775  			goto Error
   776  		}
   777  	case builtin:
   778  		// types2 uses the position of '.' for the error
   779  		check.errorf(e.Sel, UncalledBuiltin, "cannot select on %s", x)
   780  		goto Error
   781  	case invalid:
   782  		goto Error
   783  	}
   784  
   785  	// Avoid crashing when checking an invalid selector in a method declaration
   786  	// (i.e., where def is not set):
   787  	//
   788  	//   type S[T any] struct{}
   789  	//   type V = S[any]
   790  	//   func (fs *S[T]) M(x V.M) {}
   791  	//
   792  	// All codepaths below return a non-type expression. If we get here while
   793  	// expecting a type expression, it is an error.
   794  	//
   795  	// See go.dev/issue/57522 for more details.
   796  	//
   797  	// TODO(rfindley): We should do better by refusing to check selectors in all cases where
   798  	// x.typ is incomplete.
   799  	if wantType {
   800  		check.errorf(e.Sel, NotAType, "%s is not a type", ast.Expr(e))
   801  		goto Error
   802  	}
   803  
   804  	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
   805  	if obj == nil {
   806  		// Don't report another error if the underlying type was invalid (go.dev/issue/49541).
   807  		if !isValid(under(x.typ)) {
   808  			goto Error
   809  		}
   810  
   811  		if index != nil {
   812  			// TODO(gri) should provide actual type where the conflict happens
   813  			check.errorf(e.Sel, AmbiguousSelector, "ambiguous selector %s.%s", x.expr, sel)
   814  			goto Error
   815  		}
   816  
   817  		if indirect {
   818  			if x.mode == typexpr {
   819  				check.errorf(e.Sel, InvalidMethodExpr, "invalid method expression %s.%s (needs pointer receiver (*%s).%s)", x.typ, sel, x.typ, sel)
   820  			} else {
   821  				check.errorf(e.Sel, InvalidMethodExpr, "cannot call pointer method %s on %s", sel, x.typ)
   822  			}
   823  			goto Error
   824  		}
   825  
   826  		var why string
   827  		if isInterfacePtr(x.typ) {
   828  			why = check.interfacePtrError(x.typ)
   829  		} else {
   830  			why = check.sprintf("type %s has no field or method %s", x.typ, sel)
   831  			// Check if capitalization of sel matters and provide better error message in that case.
   832  			// TODO(gri) This code only looks at the first character but LookupFieldOrMethod should
   833  			//           have an (internal) mechanism for case-insensitive lookup that we should use
   834  			//           instead (see types2).
   835  			if len(sel) > 0 {
   836  				var changeCase string
   837  				if r := rune(sel[0]); unicode.IsUpper(r) {
   838  					changeCase = string(unicode.ToLower(r)) + sel[1:]
   839  				} else {
   840  					changeCase = string(unicode.ToUpper(r)) + sel[1:]
   841  				}
   842  				if obj, _, _ = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
   843  					why += ", but does have " + changeCase
   844  				}
   845  			}
   846  		}
   847  		check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (%s)", x.expr, sel, why)
   848  		goto Error
   849  	}
   850  
   851  	// methods may not have a fully set up signature yet
   852  	if m, _ := obj.(*Func); m != nil {
   853  		check.objDecl(m, nil)
   854  	}
   855  
   856  	if x.mode == typexpr {
   857  		// method expression
   858  		m, _ := obj.(*Func)
   859  		if m == nil {
   860  			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
   861  			check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
   862  			goto Error
   863  		}
   864  
   865  		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
   866  
   867  		sig := m.typ.(*Signature)
   868  		if sig.recv == nil {
   869  			check.error(e, InvalidDeclCycle, "illegal cycle in method declaration")
   870  			goto Error
   871  		}
   872  
   873  		// the receiver type becomes the type of the first function
   874  		// argument of the method expression's function type
   875  		var params []*Var
   876  		if sig.params != nil {
   877  			params = sig.params.vars
   878  		}
   879  		// Be consistent about named/unnamed parameters. This is not needed
   880  		// for type-checking, but the newly constructed signature may appear
   881  		// in an error message and then have mixed named/unnamed parameters.
   882  		// (An alternative would be to not print parameter names in errors,
   883  		// but it's useful to see them; this is cheap and method expressions
   884  		// are rare.)
   885  		name := ""
   886  		if len(params) > 0 && params[0].name != "" {
   887  			// name needed
   888  			name = sig.recv.name
   889  			if name == "" {
   890  				name = "_"
   891  			}
   892  		}
   893  		params = append([]*Var{NewVar(sig.recv.pos, sig.recv.pkg, name, x.typ)}, params...)
   894  		x.mode = value
   895  		x.typ = &Signature{
   896  			tparams:  sig.tparams,
   897  			params:   NewTuple(params...),
   898  			results:  sig.results,
   899  			variadic: sig.variadic,
   900  		}
   901  
   902  		check.addDeclDep(m)
   903  
   904  	} else {
   905  		// regular selector
   906  		switch obj := obj.(type) {
   907  		case *Var:
   908  			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
   909  			if x.mode == variable || indirect {
   910  				x.mode = variable
   911  			} else {
   912  				x.mode = value
   913  			}
   914  			x.typ = obj.typ
   915  
   916  		case *Func:
   917  			// TODO(gri) If we needed to take into account the receiver's
   918  			// addressability, should we report the type &(x.typ) instead?
   919  			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
   920  
   921  			// TODO(gri) The verification pass below is disabled for now because
   922  			//           method sets don't match method lookup in some cases.
   923  			//           For instance, if we made a copy above when creating a
   924  			//           custom method for a parameterized received type, the
   925  			//           method set method doesn't match (no copy there). There
   926  			///          may be other situations.
   927  			disabled := true
   928  			if !disabled && debug {
   929  				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
   930  				// TODO(gri) This only works because we call LookupFieldOrMethod
   931  				// _before_ calling NewMethodSet: LookupFieldOrMethod completes
   932  				// any incomplete interfaces so they are available to NewMethodSet
   933  				// (which assumes that interfaces have been completed already).
   934  				typ := x.typ
   935  				if x.mode == variable {
   936  					// If typ is not an (unnamed) pointer or an interface,
   937  					// use *typ instead, because the method set of *typ
   938  					// includes the methods of typ.
   939  					// Variables are addressable, so we can always take their
   940  					// address.
   941  					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
   942  						typ = &Pointer{base: typ}
   943  					}
   944  				}
   945  				// If we created a synthetic pointer type above, we will throw
   946  				// away the method set computed here after use.
   947  				// TODO(gri) Method set computation should probably always compute
   948  				// both, the value and the pointer receiver method set and represent
   949  				// them in a single structure.
   950  				// TODO(gri) Consider also using a method set cache for the lifetime
   951  				// of checker once we rely on MethodSet lookup instead of individual
   952  				// lookup.
   953  				mset := NewMethodSet(typ)
   954  				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
   955  					check.dump("%v: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
   956  					check.dump("%s\n", mset)
   957  					// Caution: MethodSets are supposed to be used externally
   958  					// only (after all interface types were completed). It's
   959  					// now possible that we get here incorrectly. Not urgent
   960  					// to fix since we only run this code in debug mode.
   961  					// TODO(gri) fix this eventually.
   962  					panic("method sets and lookup don't agree")
   963  				}
   964  			}
   965  
   966  			x.mode = value
   967  
   968  			// remove receiver
   969  			sig := *obj.typ.(*Signature)
   970  			sig.recv = nil
   971  			x.typ = &sig
   972  
   973  			check.addDeclDep(obj)
   974  
   975  		default:
   976  			unreachable()
   977  		}
   978  	}
   979  
   980  	// everything went well
   981  	x.expr = e
   982  	return
   983  
   984  Error:
   985  	x.mode = invalid
   986  	x.expr = e
   987  }
   988  
   989  // use type-checks each argument.
   990  // Useful to make sure expressions are evaluated
   991  // (and variables are "used") in the presence of
   992  // other errors. Arguments may be nil.
   993  // Reports if all arguments evaluated without error.
   994  func (check *Checker) use(args ...ast.Expr) bool { return check.useN(args, false) }
   995  
   996  // useLHS is like use, but doesn't "use" top-level identifiers.
   997  // It should be called instead of use if the arguments are
   998  // expressions on the lhs of an assignment.
   999  func (check *Checker) useLHS(args ...ast.Expr) bool { return check.useN(args, true) }
  1000  
  1001  func (check *Checker) useN(args []ast.Expr, lhs bool) bool {
  1002  	ok := true
  1003  	for _, e := range args {
  1004  		if !check.use1(e, lhs) {
  1005  			ok = false
  1006  		}
  1007  	}
  1008  	return ok
  1009  }
  1010  
  1011  func (check *Checker) use1(e ast.Expr, lhs bool) bool {
  1012  	var x operand
  1013  	x.mode = value // anything but invalid
  1014  	switch n := unparen(e).(type) {
  1015  	case nil:
  1016  		// nothing to do
  1017  	case *ast.Ident:
  1018  		// don't report an error evaluating blank
  1019  		if n.Name == "_" {
  1020  			break
  1021  		}
  1022  		// If the lhs is an identifier denoting a variable v, this assignment
  1023  		// is not a 'use' of v. Remember current value of v.used and restore
  1024  		// after evaluating the lhs via check.rawExpr.
  1025  		var v *Var
  1026  		var v_used bool
  1027  		if lhs {
  1028  			if _, obj := check.scope.LookupParent(n.Name, nopos); obj != nil {
  1029  				// It's ok to mark non-local variables, but ignore variables
  1030  				// from other packages to avoid potential race conditions with
  1031  				// dot-imported variables.
  1032  				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
  1033  					v = w
  1034  					v_used = v.used
  1035  				}
  1036  			}
  1037  		}
  1038  		check.exprOrType(&x, n, true)
  1039  		if v != nil {
  1040  			v.used = v_used // restore v.used
  1041  		}
  1042  	default:
  1043  		check.rawExpr(nil, &x, e, nil, true)
  1044  	}
  1045  	return x.mode != invalid
  1046  }
  1047  

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