go/types, types2: better error message if type is not in type set

Fixes #40350.

Change-Id: Ia654d6b854971700ca618692a864265557122b23
Reviewed-on: https://go-review.googlesource.com/c/go/+/410876
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -277,7 +277,7 @@ func (check *Checker) implements(V, T Type) error {
 		if alt != nil {
 			return errorf("%s does not implement %s (possibly missing ~ for %s in constraint %s)", V, T, alt, T)
 		} else {
-			return errorf("%s does not implement %s", V, T)
+			return errorf("%s does not implement %s (%s missing in %s)", V, T, V, Ti.typeSet().terms)
 		}
 	}
 

src/cmd/compile/internal/types2/testdata/fixedbugs/issue40350.go

@@ -0,0 +1,16 @@
+// 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
+
+type number interface {
+	~float64 | ~int | ~int32
+	float64 | ~int32
+}
+
+func f[T number]() {}
+
+func _() {
+	_ = f[int /* ERROR int does not implement number \(int missing in float64 | ~int32\)*/]
+}

src/go/types/instantiate.go

@@ -277,7 +277,7 @@ func (check *Checker) implements(V, T Type) error {
 		if alt != nil {
 			return errorf("%s does not implement %s (possibly missing ~ for %s in constraint %s)", V, T, alt, T)
 		} else {
-			return errorf("%s does not implement %s", V, T)
+			return errorf("%s does not implement %s (%s missing in %s)", V, T, V, Ti.typeSet().terms)
 		}
 	}
 

src/go/types/testdata/fixedbugs/issue40350.go

@@ -0,0 +1,16 @@
+// 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
+
+type number interface {
+	~float64 | ~int | ~int32
+	float64 | ~int32
+}
+
+func f[T number]() {}
+
+func _() {
+	_ = f[int /* ERROR int does not implement number \(int missing in float64 | ~int32\)*/]
+}