cmd/compile/internal/types2: better error message when using comparable in union

Fixes #49602.

Change-Id: I3499f8a485a2c8ec8afc74c5ef7b20d42c943a05
Reviewed-on: https://go-review.googlesource.com/c/go/+/372674
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 29 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue49602.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue49602.go2
new file mode 100644
index 0000000000..9edbf14a55
--- /dev/null
+++ b/src/cmd/compile/internal/types2/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/cmd/compile/internal/types2/union.go b/src/cmd/compile/internal/types2/union.go
index 6f66260af4..3c0df04ccd 100644
--- a/src/cmd/compile/internal/types2/union.go
+++ b/src/cmd/compile/internal/types2/union.go
@@ -108,7 +108,16 @@ func parseUnion(check *Checker, uexpr syntax.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], "cannot use %s in union (interface contains methods)", t)
+				switch {
+				case f.typeSet().NumMethods() != 0:
+					check.errorf(tlist[i], "cannot use %s in union (%s contains methods)", t, t)
+				case t.typ == universeComparable.Type():
+					check.error(tlist[i], "cannot use comparable in union")
+				case f.typeSet().comparable:
+					check.errorf(tlist[i], "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
 			}