go/types: report error for invalid (but empty) expr switch

This is a port of CL 278132 from the dev.typeparams branch. A notable
addition is a new error code, since no existing codes made sense and we
have an analogous code for type switches.

Fixes #43110

Change-Id: I22b3f9d8777063223f82785504e8b7d299bc5216
Reviewed-on: https://go-review.googlesource.com/c/go/+/278813
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>

src/go/types/errorcodes.go

@@ -1207,6 +1207,16 @@ const (
 	//  }
 	_InvalidTypeSwitch
 
+	// _InvalidExprSwitch occurs when a switch expression is not comparable.
+	//
+	// Example:
+	//  func _() {
+	//  	var a struct{ _ func() }
+	//  	switch a /* ERROR cannot switch on a */ {
+	//  	}
+	//  }
+	_InvalidExprSwitch
+
 	/* control flow > select */
 
 	// _InvalidSelectCase occurs when a select case is not a channel send or

src/go/types/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/go/types/stmt.go

@@ -528,6 +528,10 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			// 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, _InvalidExprSwitch, "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."