cmd/compile/internal/types2: better error position for instantiation failure

- Thread type argument expressions (rather than posLists) through various
type-checker functions so we can provide a better error position.

- Adjust signatures that expect a syntax.Pos to accept a poser instead
to avoid gratuituous conversions from expressions to positions.

- Rename targsx to xlist so we use xlist consistently for expression
lists.

First step in providing a better error message for the issue below.

For #49179.

Change-Id: I8fc685a2ee4f5640f4abd35568ba32bcb34e9e84
Reviewed-on: https://go-review.googlesource.com/c/go/+/366757
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/builtins.go

@@ -129,7 +129,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			arg(&x, i)
 			xlist = append(xlist, &x)
 		}
-		check.arguments(call, sig, nil, xlist) // discard result (we know the result type)
+		check.arguments(call, sig, nil, xlist, nil) // discard result (we know the result type)
 		// ok to continue even if check.arguments reported errors
 
 		x.mode = value

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

@@ -50,14 +50,8 @@ func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
 	}
 	assert(got == want)
 
-	// determine argument positions (for error reporting)
-	poslist := make([]syntax.Pos, len(xlist))
-	for i, x := range xlist {
-		poslist[i] = syntax.StartPos(x)
-	}
-
 	// instantiate function signature
-	res := check.instantiateSignature(x.Pos(), sig, targs, poslist)
+	res := check.instantiateSignature(x.Pos(), sig, targs, xlist)
 	assert(res.TypeParams().Len() == 0) // signature is not generic anymore
 	check.recordInstance(inst.X, targs, res)
 	x.typ = res
@@ -65,7 +59,7 @@ func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
 	x.expr = inst
 }
 
-func (check *Checker) instantiateSignature(pos syntax.Pos, typ *Signature, targs []Type, posList []syntax.Pos) (res *Signature) {
+func (check *Checker) instantiateSignature(pos syntax.Pos, typ *Signature, targs []Type, xlist []syntax.Expr) (res *Signature) {
 	assert(check != nil)
 	assert(len(targs) == typ.TypeParams().Len())
 
@@ -79,17 +73,17 @@ func (check *Checker) instantiateSignature(pos syntax.Pos, typ *Signature, targs
 	}
 
 	inst := check.instance(pos, typ, targs, check.bestContext(nil)).(*Signature)
-	assert(len(posList) <= len(targs))
+	assert(len(xlist) <= len(targs))
 	tparams := typ.TypeParams().list()
 	if i, err := check.verify(pos, tparams, targs); err != nil {
 		// best position for error reporting
 		pos := pos
-		if i < len(posList) {
-			pos = posList[i]
+		if i < len(xlist) {
+			pos = syntax.StartPos(xlist[i])
 		}
-		check.softErrorf(pos, err.Error())
+		check.softErrorf(pos, "%s", err)
 	} else {
-		check.mono.recordInstance(check.pkg, pos, tparams, targs, posList)
+		check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
 	}
 
 	return inst
@@ -179,9 +173,10 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 	}
 
 	// evaluate type arguments, if any
+	var xlist []syntax.Expr
 	var targs []Type
 	if inst != nil {
-		xlist := unpackExpr(inst.Index)
+		xlist = unpackExpr(inst.Index)
 		targs = check.typeList(xlist)
 		if targs == nil {
 			check.use(call.ArgList...)
@@ -205,7 +200,7 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 	// evaluate arguments
 	args, _ := check.exprList(call.ArgList, false)
 	isGeneric := sig.TypeParams().Len() > 0
-	sig = check.arguments(call, sig, targs, args)
+	sig = check.arguments(call, sig, targs, args, xlist)
 
 	if isGeneric && sig.TypeParams().Len() == 0 {
 		// update the recorded type of call.Fun to its instantiated type
@@ -279,7 +274,8 @@ func (check *Checker) exprList(elist []syntax.Expr, allowCommaOk bool) (xlist []
 	return
 }
 
-func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []Type, args []*operand) (rsig *Signature) {
+// xlist is the list of type argument expressions supplied in the source code.
+func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []Type, args []*operand, xlist []syntax.Expr) (rsig *Signature) {
 	rsig = sig
 
 	// TODO(gri) try to eliminate this extra verification loop
@@ -381,15 +377,13 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 				check.versionErrorf(call.Pos(), "go1.18", "implicit function instantiation")
 			}
 		}
-		// TODO(gri) provide position information for targs so we can feed
-		//           it to the instantiate call for better error reporting
 		targs := check.infer(call.Pos(), sig.TypeParams().list(), targs, sigParams, args)
 		if targs == nil {
 			return // error already reported
 		}
 
 		// compute result signature
-		rsig = check.instantiateSignature(call.Pos(), sig, targs, nil)
+		rsig = check.instantiateSignature(call.Pos(), sig, targs, xlist)
 		assert(rsig.TypeParams().Len() == 0) // signature is not generic anymore
 		check.recordInstance(call.Fun, targs, rsig)
 

src/cmd/compile/internal/types2/mono.go

@@ -168,11 +168,11 @@ func (w *monoGraph) recordCanon(mpar, tpar *TypeParam) {
 
 // recordInstance records that the given type parameters were
 // instantiated with the corresponding type arguments.
-func (w *monoGraph) recordInstance(pkg *Package, pos syntax.Pos, tparams []*TypeParam, targs []Type, posList []syntax.Pos) {
+func (w *monoGraph) recordInstance(pkg *Package, pos syntax.Pos, tparams []*TypeParam, targs []Type, xlist []syntax.Expr) {
 	for i, tpar := range tparams {
 		pos := pos
-		if i < len(posList) {
-			pos = posList[i]
+		if i < len(xlist) {
+			pos = syntax.StartPos(xlist[i])
 		}
 		w.assign(pkg, pos, tpar, targs[i])
 	}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue39754.go2

@@ -17,7 +17,5 @@ func f[V interface{}, A, B Box[V]]() {}
 func _() {
 	f[int, Optional[int], Optional[int]]()
 	_ = f[int, Optional[int], Optional /* ERROR does not implement Box */ [string]]
-	// TODO(gri) Provide better position information here.
-	//           See TODO in call.go, Checker.arguments.
-	f[int, Optional[int], Optional[string]]( /* ERROR does not implement Box */ )
+	_ = f[int, Optional[int], Optional /* ERROR Optional.* does not implement Box.* */ [string]]
 }

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49179.go2

@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type SliceConstraint[T any] interface {
+	[]T
+}
+
+func Map[S SliceConstraint[E], E any](s S, f func(E) E) S {
+	return s
+}
+
+type MySlice []int
+
+func f(s MySlice) {
+	Map[MySlice /* ERROR MySlice does not implement SliceConstraint\[int\] */, int](s, nil)
+}

src/cmd/compile/internal/types2/typexpr.go

@@ -402,9 +402,9 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 	return typ
 }
 
-func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def *Named) (res Type) {
+func (check *Checker) instantiatedType(x syntax.Expr, xlist []syntax.Expr, def *Named) (res Type) {
 	if check.conf.Trace {
-		check.trace(x.Pos(), "-- instantiating %s with %s", x, targsx)
+		check.trace(x.Pos(), "-- instantiating %s with %s", x, xlist)
 		check.indent++
 		defer func() {
 			check.indent--
@@ -424,18 +424,12 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 	}
 
 	// evaluate arguments
-	targs := check.typeList(targsx)
+	targs := check.typeList(xlist)
 	if targs == nil {
 		def.setUnderlying(Typ[Invalid]) // avoid later errors due to lazy instantiation
 		return Typ[Invalid]
 	}
 
-	// determine argument positions
-	posList := make([]syntax.Pos, len(targs))
-	for i, arg := range targsx {
-		posList[i] = arg.Pos()
-	}
-
 	// create the instance
 	ctxt := check.bestContext(nil)
 	h := ctxt.instanceHash(orig, targs)
@@ -484,12 +478,12 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 			if i, err := check.verify(x.Pos(), inst.tparams.list(), inst.targs.list()); err != nil {
 				// best position for error reporting
 				pos := x.Pos()
-				if i < len(posList) {
-					pos = posList[i]
+				if i < len(xlist) {
+					pos = syntax.StartPos(xlist[i])
 				}
-				check.softErrorf(pos, err.Error())
+				check.softErrorf(pos, "%s", err)
 			} else {
-				check.mono.recordInstance(check.pkg, x.Pos(), inst.tparams.list(), inst.targs.list(), posList)
+				check.mono.recordInstance(check.pkg, x.Pos(), inst.tparams.list(), inst.targs.list(), xlist)
 			}
 		}