go/types: better error message when using comparable in union

This is a port of CL 372674 from types2 to go/types with
minor adjustments for error handling.

For #49602.

Change-Id: I726081325a2ff2d5690d11ddc8a830bbcbd8ab33
Reviewed-on: https://go-review.googlesource.com/c/go/+/372954
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 29 insertions, 1 deletions

diff --git a/src/go/types/testdata/fixedbugs/issue49602.go2 b/src/go/types/testdata/fixedbugs/issue49602.go2
new file mode 100644
index 0000000000..208501fafd
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue49602.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 M interface {
+	m()
+}
+
+type C interface {
+	comparable
+}
+
+type _ interface {
+	int | M          // ERROR cannot use p\.M in union \(p\.M contains methods\)
+	int | comparable // ERROR cannot use comparable in union
+	int | C          // ERROR cannot use p\.C in union \(p\.C embeds comparable\)
+}
diff --git a/src/go/types/union.go b/src/go/types/union.go
index 7cd5b2a88b..9c59279447 100644
--- a/src/go/types/union.go
+++ b/src/go/types/union.go
@@ -111,7 +111,16 @@ func parseUnion(check *Checker, uexpr ast.Expr) Type {
 			// in the beginning. Embedded interfaces with tilde are excluded above. If we reach
 			// here, we must have at least two terms in the union.
 			if f != nil && !f.typeSet().IsTypeSet() {
-				check.errorf(tlist[i], _InvalidUnion, "cannot use %s in union (interface contains methods)", t)
+				switch {
+				case f.typeSet().NumMethods() != 0:
+					check.errorf(tlist[i], _InvalidUnion, "cannot use %s in union (%s contains methods)", t, t)
+				case t.typ == universeComparable.Type():
+					check.error(tlist[i], _InvalidUnion, "cannot use comparable in union")
+				case f.typeSet().comparable:
+					check.errorf(tlist[i], _InvalidUnion, "cannot use %s in union (%s embeds comparable)", t, t)
+				default:
+					panic("not a type set but no methods and not comparable")
+				}
 				continue // don't report another error for t
 			}