From b0f01e17f8f8165b1ae273282eec00d78105e2fe Mon Sep 17 00:00:00 2001
From: Rob Findley <rfindley@google.com>
Date: Wed, 16 Dec 2020 17:19:44 -0500
Subject: [PATCH] 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            | 10 +++++++
 src/go/types/fixedbugs/issue43110.src | 43 +++++++++++++++++++++++++++
 src/go/types/stmt.go                  |  4 +++
 3 files changed, 57 insertions(+)
 create mode 100644 src/go/types/fixedbugs/issue43110.src

diff --git a/src/go/types/errorcodes.go b/src/go/types/errorcodes.go
index e4c8311d62..c01a12c346 100644
--- a/src/go/types/errorcodes.go
+++ b/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
diff --git a/src/go/types/fixedbugs/issue43110.src b/src/go/types/fixedbugs/issue43110.src
new file mode 100644
index 0000000000..4a46945239
--- /dev/null
+++ b/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):
+	}
+}
diff --git a/src/go/types/stmt.go b/src/go/types/stmt.go
index 7b3f322ced..d88e47170c 100644
--- a/src/go/types/stmt.go
+++ b/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."
-- 
2.48.1