cmd/compile: improve error for wrong type in switch

Fixes #10561.

Provides a better diagnostic message for failed type switch
satisfaction in the case that a value receiver is being used
in place of the pointer receiver that implements and satisfies
the interface.

Change-Id: If8c13ba13f2a8d81bf44bac7c3a66c12921ba921
Reviewed-on: https://go-review.googlesource.com/35235
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/test/switch6.go b/test/switch6.go
index 32392d8f73..9d102fef51 100644
--- a/test/switch6.go
+++ b/test/switch6.go
@@ -30,3 +30,17 @@ func f1(e interface{}) {
 	default: // ERROR "multiple defaults in switch"
 	}
 }
+
+type I interface {
+	Foo()
+}
+
+type X int
+
+func (*X) Foo() {}
+func f2() {
+	var i I
+	switch i.(type) {
+	case X: // ERROR "impossible type switch case: i \(type I\) cannot have dynamic type X \(Foo method has pointer receiver\)"
+	}
+}