[dev.typeparams] cmd/compile/internal/types2: report error for invalid (but empty) expr switch

Enable one more errorcheck test.

Updates #43110.
Updates #43200.

Change-Id: Ib7b971d5e9989c65320579f75d65266bbbbeec53
Reviewed-on: https://go-review.googlesource.com/c/go/+/278132
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/fixedbugs/issue43110.src

@@ -0,0 +1,43 @@
+// Copyright 2020 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 P *struct{}
+
+func _() {
+	// want an error even if the switch is empty
+	var a struct{ _ func() }
+	switch a /* ERROR cannot switch on a */ {
+	}
+
+	switch a /* ERROR cannot switch on a */ {
+	case a: // no follow-on error here
+	}
+
+	// this is ok because f can be compared to nil
+	var f func()
+	switch f {
+	}
+
+	switch f {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case f /* ERROR cannot compare */ :
+	}
+
+	switch nil /* ERROR use of untyped nil in switch expression */ {
+	}
+
+	// this is ok
+	switch P(nil) {
+	case P(nil):
+	}
+}

src/cmd/compile/internal/types2/stmt.go

@@ -605,6 +605,10 @@ func (check *Checker) switchStmt(inner stmtContext, s *syntax.SwitchStmt) {
 		// By checking assignment of x to an invisible temporary
 		// (as a compiler would), we get all the relevant checks.
 		check.assignment(&x, nil, "switch expression")
+		if x.mode != invalid && !Comparable(x.typ) && !hasNil(x.typ) {
+			check.errorf(&x, "cannot switch on %s (%s is not comparable)", &x, x.typ)
+			x.mode = invalid
+		}
 	} else {
 		// spec: "A missing switch expression is
 		// equivalent to the boolean value true."

test/run.go

@@ -1937,7 +1937,6 @@ var excluded = map[string]bool{
 	"initializerr.go": true, // types2 reports extra errors
 	"linkname2.go":    true, // error reported by noder (not running for types2 errorcheck test)
 	"shift1.go":       true, // issue #42989
-	"switch3.go":      true, // issue #43110
 	"switch4.go":      true, // error reported by noder (not running for types2 errorcheck test)
 	"typecheck.go":    true, // invalid function is not causing errors when called
 

test/switch3.go

@@ -28,21 +28,21 @@ func bad() {
 	var m, m1 map[int]int
 	switch m {
 	case nil:
-	case m1: // ERROR "can only compare map m to nil|map can only be compared to nil"
+	case m1: // ERROR "can only compare map m to nil|map can only be compared to nil|cannot compare"
 	default:
 	}
 
 	var a, a1 []int
 	switch a {
 	case nil:
-	case a1: // ERROR "can only compare slice a to nil|slice can only be compared to nil"
+	case a1: // ERROR "can only compare slice a to nil|slice can only be compared to nil|cannot compare"
 	default:
 	}
 
 	var f, f1 func()
 	switch f {
 	case nil:
-	case f1: // ERROR "can only compare func f to nil|func can only be compared to nil"
+	case f1: // ERROR "can only compare func f to nil|func can only be compared to nil|cannot compare"
 	default:
 	}