gc: implement new comparison rule

The new comparison rule was added to the spec by

	changeset:   5605:33abb649cb63
	user:        Robert Griesemer <gri@golang.org>
	date:        Thu Jun 03 16:55:50 2010 -0700
	files:       doc/go_spec.html
	description:
	go spec: Base comparison compatibility on assignment compatibility.

	Specifically:
	- Simplified definition of comparison compatibility and folded into
	  section on comparison operators since it's only used there.

	This is a small language change/cleanup. As a consequence:
	- An interface value may now be compared against a non-interface value.
	- Channels with opposite directions cannot be compared directly anymore
	  (per discussion with rsc).

	R=rsc, r, iant, ken2
	CC=golang-dev
	https://golang.org/cl/1462041

but never implemented.

Fixes #1070.

R=ken2
CC=golang-dev
https://golang.org/cl/2116047

src/cmd/gc/typecheck.c

@@ -63,7 +63,7 @@ typechecklist(NodeList *l, int top)
 Node*
 typecheck(Node **np, int top)
 {
-	int et, op, ptr;
+	int et, aop, op, ptr;
 	Node *n, *l, *r;
 	NodeList *args;
 	int lno, ok, ntop;
@@ -350,6 +350,26 @@ reswitch:
 		et = t->etype;
 		if(et == TIDEAL)
 			et = TINT;
+		if(iscmp[n->op] && t->etype != TIDEAL && !eqtype(l->type, r->type)) {
+			// comparison is okay as long as one side is
+			// assignable to the other.  convert so they have
+			// the same type.  (the only conversion that isn't
+			// a no-op is concrete == interface.)
+			if(r->type->etype != TBLANK && (aop = assignop(l->type, r->type, nil)) != 0) {
+				l = nod(aop, l, N);
+				l->type = r->type;
+				l->typecheck = 1;
+				n->left = l;
+				t = l->type;
+			} else if(l->type->etype != TBLANK && (aop = assignop(r->type, l->type, nil)) != 0) {
+				r = nod(aop, r, N);
+				r->type = l->type;
+				r->typecheck = 1;
+				n->right = r;
+				t = r->type;
+			}
+			et = t->etype;
+		}
 		if(t->etype != TIDEAL && !eqtype(l->type, r->type)) {
 		badbinary:
 			defaultlit2(&l, &r, 1);

test/cmp1.go

@@ -26,6 +26,8 @@ func istrue(b bool) {
 	}
 }
 
+type T *int
+
 func main() {
 	var a []int
 	var b map[string]int
@@ -55,6 +57,24 @@ func main() {
 	isfalse(ib == id)
 	istrue(ic == id)
 	istrue(ie == ie)
+	
+	// these are okay because one side of the
+	// comparison need only be assignable to the other.
+	isfalse(a == ib)
+	isfalse(a == ic)
+	isfalse(a == id)
+	isfalse(b == ic)
+	isfalse(b == id)
+	istrue(c == id)
+	istrue(e == ie)
+
+	isfalse(ia == b)
+	isfalse(ia == c)
+	isfalse(ia == d)
+	isfalse(ib == c)
+	isfalse(ib == d)
+	istrue(ic == d)
+	istrue(ie == e)
 
 	// 6g used to let this go through as true.
 	var g uint64 = 123
@@ -73,4 +93,38 @@ func main() {
 		println("m[ic] = ", m[ic])
 		panic("bad m[ic]")
 	}
+	
+	// non-interface comparisons
+	{
+		c := make(chan int)
+		c1 := (<-chan int)(c)
+		c2 := (chan<- int)(c)
+		istrue(c == c1)
+		istrue(c == c2)
+		istrue(c1 == c)
+		istrue(c2 == c)
+		
+		d := make(chan int)
+		isfalse(c == d)
+		isfalse(d == c)
+		isfalse(d == c1)
+		isfalse(d == c2)
+		isfalse(c1 == d)
+		isfalse(c2 == d)
+	}
+
+	// named types vs not
+	{
+		var x = new(int)
+		var y T
+		var z T = x
+		
+		isfalse(x == y)
+		istrue(x == z)
+		isfalse(y == z)
+
+		isfalse(y == x)
+		istrue(z == x)
+		isfalse(z == y)
+	}
 }

test/cmp6.go

+// errchk $G -e $D/$F.go
+
+// Copyright 2010 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 main
+
+func use(bool) {}
+
+type T1 *int
+type T2 *int
+
+func main() {
+	// Arguments to comparison must be
+	// assignable one to the other (or vice versa)
+	// so chan int can be compared against
+	// directional channels but channel of different
+	// direction cannot be compared against each other.
+	var c1 chan <-int
+	var c2 <-chan int
+	var c3 chan int
+	
+	use(c1 == c2)	// ERROR "invalid operation"
+	use(c2 == c1)	// ERROR "invalid operation"
+	use(c1 == c3)
+	use(c2 == c2)
+	use(c3 == c1)
+	use(c3 == c2)
+
+	// Same applies to named types.
+	var p1 T1
+	var p2 T2
+	var p3 *int
+	
+	use(p1 == p2)	// ERROR "invalid operation"
+	use(p2 == p1)	// ERROR "invalid operation"
+	use(p1 == p3)
+	use(p2 == p2)
+	use(p3 == p1)
+	use(p3 == p2)
+}