Source file src/cmd/compile/internal/types2/call.go

Documentation: cmd/compile/internal/types2

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

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