go/types, types2: ensure we have an interface before checking constraints

Due to delayed computation of constraint type sets it is possible that
a type argument is checked against a constraint before that constraint
has been wrapped into an implicit interface (if needed).

When the type checker is about to check whether a type argument
implements a constraint, it's always safe to force wrapping of
a constraint in an implicit interface (if necessary) before doing
the implements check.

Also, use a better position for tracing output related to constraint
type set computation.

Fixes #51048.

Change-Id: I52fecbf57814f09b62762452d7e17c2a230fdd59
Reviewed-on: https://go-review.googlesource.com/c/go/+/383834
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -135,6 +135,8 @@ func (check *Checker) validateTArgLen(pos syntax.Pos, ntparams, ntargs int) bool
 func (check *Checker) verify(pos syntax.Pos, tparams []*TypeParam, targs []Type) (int, error) {
 	smap := makeSubstMap(tparams, targs)
 	for i, tpar := range tparams {
+		// Ensure that we have a (possibly implicit) interface as type bound (issue #51048).
+		tpar.iface()
 		// The type parameter bound is parameterized with the same type parameters
 		// as the instantiated type; before we can use it for bounds checking we
 		// need to instantiate it with the type arguments with which we instantiated

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51048.go2

@@ -0,0 +1,11 @@
+// Copyright 2022 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
+
+func _[P int]() {
+	_ = f[P]
+}
+
+func f[T int]() {}

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

@@ -124,8 +124,9 @@ func (t *TypeParam) iface() *Interface {
 
 	// compute type set if necessary
 	if ityp.tset == nil {
-		// use the (original) type bound position if we have one
-		pos := nopos
+		// pos is used for tracing output; start with the type parameter position.
+		pos := t.obj.pos
+		// use the (original or possibly instantiated) type bound position if we have one
 		if n, _ := bound.(*Named); n != nil {
 			pos = n.obj.pos
 		}

src/go/types/instantiate.go

@@ -135,6 +135,8 @@ func (check *Checker) validateTArgLen(pos token.Pos, ntparams, ntargs int) bool
 func (check *Checker) verify(pos token.Pos, tparams []*TypeParam, targs []Type) (int, error) {
 	smap := makeSubstMap(tparams, targs)
 	for i, tpar := range tparams {
+		// Ensure that we have a (possibly implicit) interface as type bound (issue #51048).
+		tpar.iface()
 		// The type parameter bound is parameterized with the same type parameters
 		// as the instantiated type; before we can use it for bounds checking we
 		// need to instantiate it with the type arguments with which we instantiated

src/go/types/testdata/fixedbugs/issue51048.go2

@@ -0,0 +1,11 @@
+// Copyright 2022 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
+
+func _[P int]() {
+	_ = f[P]
+}
+
+func f[T int]() {}

src/go/types/typeparam.go

@@ -5,7 +5,6 @@
 package types
 
 import (
-	"go/token"
 	"sync/atomic"
 )
 
@@ -127,8 +126,9 @@ func (t *TypeParam) iface() *Interface {
 
 	// compute type set if necessary
 	if ityp.tset == nil {
-		// use the (original) type bound position if we have one
-		pos := token.NoPos
+		// pos is used for tracing output; start with the type parameter position.
+		pos := t.obj.pos
+		// use the (original or possibly instantiated) type bound position if we have one
 		if n, _ := bound.(*Named); n != nil {
 			pos = n.obj.pos
 		}