cmd/compile: diagnose invalid switch interface{} case earlier

Fixes #11737.

Change-Id: Id231b502ac5a44035dc3a02515b43bf665cb1e87
Reviewed-on: https://go-review.googlesource.com/17816Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -139,6 +139,8 @@ func typecheckswitch(n *Node) {
 						}
 					case nilonly != "" && !isnil(ll.N):
 						Yyerror("invalid case %v in switch (can only compare %s %v to nil)", ll.N, nilonly, n.Left)
+					case Isinter(t) && !Isinter(ll.N.Type) && algtype1(ll.N.Type, nil) == ANOEQ:
+						Yyerror("invalid case %v in switch (incomparable type)", Nconv(ll.N, obj.FmtLong))
 					}
 
 				// type switch

test/fixedbugs/issue11737.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Issue 11737 - invalid == not being caught until generated switch code was compiled
+
+package p
+
+func f()
+
+func s(x interface{}) {
+	switch x {
+	case f: // ERROR "invalid case f \(type func\(\)\) in switch \(incomparable type\)"
+	}
+}