go/types, types2: better error message for invalid use of constraint type

Fixes #42881.

Change-Id: If800c5f90c0034d192bf8b6649e5cfda96df48cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/410954
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

4 files changed, 36 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue42881.go b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue42881.go
new file mode 100644
index 0000000000..7122d1c787
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue42881.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 (
+	T1 interface{ comparable }
+	T2 interface{ int }
+)
+
+var (
+	_ comparable // ERROR cannot use type comparable outside a type constraint: interface is \(or embeds\) comparable
+	_ T1         // ERROR cannot use type T1 outside a type constraint: interface is \(or embeds\) comparable
+	_ T2         // ERROR cannot use type T2 outside a type constraint: interface contains type constraints
+)
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index f0cd236050..692feb9751 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -167,9 +167,9 @@ func (check *Checker) validVarType(e syntax.Expr, typ Type) {
 			tset := computeInterfaceTypeSet(check, pos, t) // TODO(gri) is this the correct position?
 			if !tset.IsMethodSet() {
 				if tset.comparable {
-					check.softErrorf(pos, "interface is (or embeds) comparable")
+					check.softErrorf(pos, "cannot use type %s outside a type constraint: interface is (or embeds) comparable", typ)
 				} else {
-					check.softErrorf(pos, "interface contains type constraints")
+					check.softErrorf(pos, "cannot use type %s outside a type constraint: interface contains type constraints", typ)
 				}
 			}
 		}
diff --git a/src/go/types/testdata/fixedbugs/issue42881.go b/src/go/types/testdata/fixedbugs/issue42881.go
new file mode 100644
index 0000000000..7122d1c787
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue42881.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 (
+	T1 interface{ comparable }
+	T2 interface{ int }
+)
+
+var (
+	_ comparable // ERROR cannot use type comparable outside a type constraint: interface is \(or embeds\) comparable
+	_ T1         // ERROR cannot use type T1 outside a type constraint: interface is \(or embeds\) comparable
+	_ T2         // ERROR cannot use type T2 outside a type constraint: interface contains type constraints
+)
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index a881d33654..b02929df22 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -163,9 +163,9 @@ func (check *Checker) validVarType(e ast.Expr, typ Type) {
 			tset := computeInterfaceTypeSet(check, e.Pos(), t) // TODO(gri) is this the correct position?
 			if !tset.IsMethodSet() {
 				if tset.comparable {
-					check.softErrorf(e, _MisplacedConstraintIface, "interface is (or embeds) comparable")
+					check.softErrorf(e, _MisplacedConstraintIface, "cannot use type %s outside a type constraint: interface is (or embeds) comparable", typ)
 				} else {
-					check.softErrorf(e, _MisplacedConstraintIface, "interface contains type constraints")
+					check.softErrorf(e, _MisplacedConstraintIface, "cannot use type %s outside a type constraint: interface contains type constraints", typ)
 				}
 			}
 		}