cmd/gc: update ideal bool rules to match latest spec

Fixes #3915.
Fixes #3923.

R=ken2
CC=golang-dev
https://golang.org/cl/7281044

src/cmd/gc/const.c

diff --git a/src/cmd/gc/const.c b/src/cmd/gc/const.c
index 83e62bd..92b87c0 100644
--- a/src/cmd/gc/const.c
+++ b/src/cmd/gc/const.c
@@ -78,7 +78,6 @@
 	if(!explicit && !isideal(n->type))
 		return;
 
-	
 	if(n->op == OLITERAL) {
 		nn = nod(OXXX, N, N);
 		*nn = *n;
@@ -88,8 +87,12 @@
 
 	switch(n->op) {
 	default:
-		if(n->type == idealbool)
-			n->type = types[TBOOL];
+		if(n->type == idealbool) {
+			if(t->etype == TBOOL)
+				n->type = t;
+			else
+				n->type = types[TBOOL];
+		}
 		if(n->type->etype == TIDEAL) {
 			convlit(&n->left, t);
 			convlit(&n->right, t);

test/const6.go

diff --git a/test/const6.go b/test/const6.go
new file mode 100644
index 0000000..c04435d
--- /dev/null
+++ b/test/const6.go
@@ -0,0 +1,30 @@
+// errorcheck
+
+// Copyright 2013 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.
+
+// Ideal vs non-ideal bool. See issue 3915, 3923.
+
+package p
+
+type mybool bool
+type mybool1 bool
+
+var (
+	x, y int = 1, 2
+	c1 bool = x < y
+	c2 mybool = x < y
+	c3 mybool = c2 == (x < y)
+	c4 mybool = c2 == (1 < 2)
+	c5 mybool = 1 < 2
+	c6 mybool1 = x < y
+	c7 = c1 == c2 // ERROR "mismatched types"
+	c8 = c2 == c6 // ERROR "mismatched types"
+	c9 = c1 == c6 // ERROR "mismatched types"
+	_ = c2 && (x < y)
+	_ = c2 && (1 < 2)
+	_ = c1 && c2 // ERROR "mismatched types"
+	_ = c2 && c6 // ERROR "mismatched types"
+	_ = c1 && c6 // ERROR "mismatched types"
+)