cmd/compile: recognize bool(true) as a constant expression

Fixes #13821.

Change-Id: I4a28a92d137edac3061537af25ac9d7aba411a66
Reviewed-on: https://go-review.googlesource.com/18262Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -664,7 +664,8 @@ func evconst(n *Node) {
 		case OCONV_ | CTINT_,
 			OCONV_ | CTRUNE_,
 			OCONV_ | CTFLT_,
-			OCONV_ | CTSTR_:
+			OCONV_ | CTSTR_,
+			OCONV_ | CTBOOL_:
 			convlit1(&nl, n.Type, true)
 
 			v = nl.Val()

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

@@ -688,17 +688,7 @@ OpSwitch:
 				n.Right = r
 			}
 		} else if n.Op == OANDAND || n.Op == OOROR {
-			if l.Type == r.Type {
-				t = l.Type
-			} else if l.Type == idealbool {
-				t = r.Type
-			} else if r.Type == idealbool {
-				t = l.Type
-			}
-		} else
-		// non-comparison operators on ideal bools should make them lose their ideal-ness
-		if t == idealbool {
-			t = Types[TBOOL]
+			evconst(n)
 		}
 
 		if et == TSTRING {
@@ -1751,7 +1741,7 @@ OpSwitch:
 
 		switch n.Op {
 		case OCONVNOP:
-			if n.Left.Op == OLITERAL && n.Type != Types[TBOOL] {
+			if n.Left.Op == OLITERAL {
 				r := Nod(OXXX, nil, nil)
 				n.Op = OCONV
 				n.Orig = r

test/fixedbugs/issue13821.go

+// compile
+
+// Copyright 2015 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.
+
+// Issue 13821.  Compiler rejected "bool(true)" as not a constant.
+
+package p
+
+const (
+	A = true
+	B = bool(A)
+	C = bool(true)
+)

test/fixedbugs/issue13821b.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Issue 13821.  Additional regress tests.
+
+package p
+
+type B bool
+type B2 bool
+
+var b B
+var b2 B2
+var x1 = b && 1 < 2 // x1 has type B, not ideal bool
+var x2 = 1 < 2 && b // x2 has type B, not ideal bool
+var x3 = b && b2    // ERROR "mismatched types B and B2"
+var x4 = x1 && b2   // ERROR "mismatched types B and B2"
+var x5 = x2 && b2   // ERROR "mismatched types B and B2"
+var x6 = b2 && x1   // ERROR "mismatched types B2 and B"
+var x7 = b2 && x2   // ERROR "mismatched types B2 and B"