cmd/gc: accept cases with same value but different types in switch.

Fixes #4781.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7365056

src/cmd/gc/swt.c

@@ -117,12 +117,15 @@ exprcmp(Case *c1, Case *c2)
 	n1 = c1->node->left;
 	n2 = c2->node->left;
 
+	// sort by type (for switches on interface)
 	ct = n1->val.ctype;
-	if(ct != n2->val.ctype) {
-		// invalid program, but return a sort
-		// order so that we can give a better
-		// error later.
+	if(ct != n2->val.ctype)
 		return ct - n2->val.ctype;
+	if(!eqtype(n1->type, n2->type)) {
+		if(n1->type->vargen > n2->type->vargen)
+			return +1;
+		else
+			return -1;
 	}
 
 	// sort by constant value
@@ -379,6 +382,7 @@ mkcaselist(Node *sw, int arg)
 		case Strue:
 		case Sfalse:
 			c->type = Texprvar;
+			c->hash = typehash(n->left->type);
 			switch(consttype(n->left)) {
 			case CTFLT:
 			case CTINT:

test/switch.go

@@ -305,6 +305,35 @@ func main() {
 		assert(false, "i should be true")
 	}
 
+	// switch on interface with constant cases differing by type.
+	// was rejected by compiler: see issue 4781
+	type T int
+	type B bool
+	type F float64
+	type S string
+	switch i := interface{}(float64(1.0)); i {
+	case nil:
+		assert(false, "i should be float64(1.0)")
+	case (*int)(nil):
+		assert(false, "i should be float64(1.0)")
+	case 1:
+		assert(false, "i should be float64(1.0)")
+	case T(1):
+		assert(false, "i should be float64(1.0)")
+	case F(1.0):
+		assert(false, "i should be float64(1.0)")
+	case 1.0:
+		assert(true, "true")
+	case "hello":
+		assert(false, "i should be float64(1.0)")
+	case S("hello"):
+		assert(false, "i should be float64(1.0)")
+	case true, B(false):
+		assert(false, "i should be float64(1.0)")
+	case false, B(true):
+		assert(false, "i should be float64(1.0)")
+	}
+
 	// switch on array.
 	switch ar := [3]int{1, 2, 3}; ar {
 	case [3]int{1, 2, 3}: