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/35235Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/swt.go

@@ -162,8 +162,13 @@ func typecheckswitch(n *Node) {
 							yyerror("impossible type switch case: %L cannot have dynamic type %v"+
 								" (wrong type for %v method)\n\thave %v%S\n\twant %v%S", n.Left.Right, n1.Type, missing.Sym, have.Sym, have.Type, missing.Sym, missing.Type)
 						} else if !missing.Broke {
-							yyerror("impossible type switch case: %L cannot have dynamic type %v"+
-								" (missing %v method)", n.Left.Right, n1.Type, missing.Sym)
+							if ptr != 0 {
+								yyerror("impossible type switch case: %L cannot have dynamic type %v"+
+									" (%v method has pointer receiver)", n.Left.Right, n1.Type, missing.Sym)
+							} else {
+								yyerror("impossible type switch case: %L cannot have dynamic type %v"+
+									" (missing %v method)", n.Left.Right, n1.Type, missing.Sym)
+							}
 						}
 					}
 				}

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 \(Foo method has pointer receiver\)"
+	}
+}