cmd/compile: clearer error when non-bool used as "||" and "&&" operand

Fixes #41500

Change-Id: I658d8921b7769b6e4288ca781cbdca5ff14a84ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/255899
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

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

@@ -630,6 +630,22 @@ func typecheck1(n *Node, top int) (res *Node) {
 			break
 		}
 
+		// For "x == x && len(s)", it's better to report that "len(s)" (type int)
+		// can't be used with "&&" than to report that "x == x" (type untyped bool)
+		// can't be converted to int (see issue #41500).
+		if n.Op == OANDAND || n.Op == OOROR {
+			if !n.Left.Type.IsBoolean() {
+				yyerror("invalid operation: %v (operator %v not defined on %s)", n, n.Op, typekind(n.Left.Type))
+				n.Type = nil
+				return n
+			}
+			if !n.Right.Type.IsBoolean() {
+				yyerror("invalid operation: %v (operator %v not defined on %s)", n, n.Op, typekind(n.Right.Type))
+				n.Type = nil
+				return n
+			}
+		}
+
 		// ideal mixed with non-ideal
 		l, r = defaultlit2(l, r, false)
 

test/fixedbugs/issue41500.go

@@ -0,0 +1,20 @@
+// errorcheck
+
+// 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 s struct {
+	slice []int
+}
+
+func f() {
+	var x *s
+
+	_ = x == nil || len(x.slice) // ERROR "invalid operation: .+ \(operator \|\| not defined on int\)"
+	_ = len(x.slice) || x == nil // ERROR "invalid operation: .+ \(operator \|\| not defined on int\)"
+	_ = x == nil && len(x.slice) // ERROR "invalid operation: .+ \(operator && not defined on int\)"
+	_ = len(x.slice) && x == nil // ERROR "invalid operation: .+ \(operator && not defined on int\)"
+}