cmd/gc: check duplicate keys in maps with interface{} key type

Fixes #7214

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews, minux.ma
https://golang.org/cl/82080044

src/cmd/gc/typecheck.c

@@ -2304,10 +2304,13 @@ keydup(Node *n, Node *hash[], ulong nhash)
 	ulong b;
 	double d;
 	int i;
-	Node *a;
+	Node *a, *orign;
 	Node cmp;
 	char *s;
 
+	orign = n;
+	if(n->op == OCONVIFACE)
+		n = n->left;
 	evconst(n);
 	if(n->op != OLITERAL)
 		return;	// we dont check variables
@@ -2340,17 +2343,29 @@ keydup(Node *n, Node *hash[], ulong nhash)
 	for(a=hash[h]; a!=N; a=a->ntest) {
 		cmp.op = OEQ;
 		cmp.left = n;
-		cmp.right = a;
-		evconst(&cmp);
-		b = cmp.val.u.bval;
+		if(a->op == OCONVIFACE && orign->op == OCONVIFACE) {
+			if(a->left->type == n->type) {
+				cmp.right = a->left;
+				evconst(&cmp);
+				b = cmp.val.u.bval;
+			}
+			else {
+				b = 0;
+			}
+		}
+		else {
+			cmp.right = a;
+			evconst(&cmp);
+			b = cmp.val.u.bval;
+		}
 		if(b) {
 			// too lazy to print the literal
 			yyerror("duplicate key %N in map literal", n);
 			return;
 		}
 	}
-	n->ntest = hash[h];
-	hash[h] = n;
+	orign->ntest = hash[h];
+	hash[h] = orign;
 }
 
 static void

test/fixedbugs/issue7214.go

+// errorcheck
+
+// Copyright 2014 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 7214: No duplicate key error for maps with interface{} key type
+
+package p
+
+var _ = map[interface{}]int{2: 1, 2: 1} // ERROR "duplicate key"
+var _ = map[interface{}]int{int(2): 1, int16(2): 1}
+var _ = map[interface{}]int{int16(2): 1, int16(2): 1} // ERROR "duplicate key"
+
+type S string
+
+var _ = map[interface{}]int{"a": 1, "a": 1} // ERROR "duplicate key"
+var _ = map[interface{}]int{"a": 1, S("a"): 1}
+var _ = map[interface{}]int{S("a"): 1, S("a"): 1} // ERROR "duplicate key"
+
+type I interface {
+	f()
+}
+
+type N int
+
+func (N) f() {}
+
+var _ = map[I]int{N(0): 1, N(2): 1}
+var _ = map[I]int{N(2): 1, N(2): 1} // ERROR "duplicate key"