test: add more switch error handling tests

Some of these errors are reported in the wrong places.
That’s issue #15911 and #15912.

Change-Id: Ia09d7f89be4d15f05217a542a61b6ac08090dd87
Reviewed-on: https://go-review.googlesource.com/23588
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/switch5.go

+// errorcheck
+
+// Copyright 2016 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.
+
+// Verify that switch statements with duplicate cases are detected by the compiler.
+// Does not compile.
+
+package main
+
+import "fmt"
+
+func f0(x int) {
+	switch x {
+	case 0:
+	case 0: // ERROR "duplicate case 0 in switch"
+	}
+
+	switch x {
+	case 0:
+	case int(0): // ERROR "duplicate case 0 in switch"
+	}
+}
+
+func f1(x float32) {
+	switch x {
+	case 5:
+	case 5: // ERROR "duplicate case 5 in switch"
+	case 5.0: // ERROR "duplicate case 5 in switch"
+	}
+}
+
+func f2(s string) {
+	switch s {
+	case "":
+	case "": // ERROR "duplicate case .. in switch"
+	case "abc":
+	case "abc": // ERROR "duplicate case .abc. in switch"
+	}
+}
+
+func f3(e interface{}) {
+	switch e {
+	case 0:
+	case 0: // ERROR "duplicate case 0 in switch"
+	case int64(0):
+	case float32(10):
+	case float32(10): // ERROR "duplicate case float32\(10\) in switch"
+	case float64(10):
+	case float64(10): // ERROR "duplicate case float64\(10\) in switch"
+	}
+}
+
+func f4(e interface{}) {
+	switch e.(type) {
+	case int:
+	case int: // ERROR "duplicate case int in type switch"
+	case int64:
+	case error: // ERROR "duplicate case error in type switch"
+	case error:
+	case fmt.Stringer:
+	case fmt.Stringer: // ERROR "duplicate case fmt.Stringer in type switch"
+	case struct {
+		i int "tag1"
+	}:
+	case struct {
+		i int "tag2"
+	}:
+	case struct {
+		i int "tag1"
+	}: // ERROR "duplicate case struct { i int .tag1. } in type switch"
+	}
+}
+
+func f5(a [1]int) {
+	switch a {
+	case [1]int{0}:
+	case [1]int{0}: // OK -- see issue 15896
+	}
+}

test/switch6.go

+// errorcheck
+
+// Copyright 2016 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.
+
+// Check the compiler's switch handling that happens
+// at typechecking time.
+// This must be separate from other checks,
+// because errors during typechecking
+// prevent other errors from being discovered.
+
+package main
+
+// Verify that type switch statements with impossible cases are detected by the compiler.
+func f0(e error) {
+	switch e.(type) {
+	case int: // ERROR "impossible type switch case: e \(type error\) cannot have dynamic type int \(missing Error method\)"
+	}
+}
+
+// Verify that the compiler rejects multiple default cases.
+func f1(e interface{}) {
+	switch e { // ERROR "multiple defaults in switch"
+	default:
+	default:
+	}
+	switch e.(type) { // ERROR "multiple defaults in switch"
+	default:
+	default:
+	}
+}