cmd/gc: add missing conversion from bool to interface in switches.

In switches without an expression, the compiler would not convert the implicit true to an interface, causing codegen errors.

Fixes #3980.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6497147

src/cmd/gc/swt.c

@@ -442,6 +442,10 @@ exprbsw(Case *c0, int ncase, int arg)
 			n = c0->node;
 			lno = setlineno(n);
 
+			if(assignop(n->left->type, exprname->type, nil) == OCONVIFACE ||
+			   assignop(exprname->type, n->left->type, nil) == OCONVIFACE)
+				goto snorm;
+
 			switch(arg) {
 			case Strue:
 				a = nod(OIF, N, N);
@@ -457,6 +461,7 @@ exprbsw(Case *c0, int ncase, int arg)
 				break;
 
 			default:
+			snorm:
 				a = nod(OIF, N, N);
 				a->ntest = nod(OEQ, exprname, n->left);	// if name == val
 				typecheck(&a->ntest, Erv);
@@ -520,6 +525,8 @@ exprswitch(Node *sw)
 		exprname = temp(sw->ntest->type);
 		cas = list1(nod(OAS, exprname, sw->ntest));
 		typechecklist(cas, Etop);
+	} else {
+		exprname = nodbool(arg == Strue);
 	}
 
 	c0 = mkcaselist(sw, arg);

test/switch.go

@@ -294,6 +294,17 @@ func main() {
 		assert(false, `i should be "hello"`)
 	}
 
+	// switch on implicit bool converted to interface
+	// was broken: see issue 3980
+	switch i := interface{}(true); {
+	case i:
+		assert(true, "true")
+	case false:
+		assert(false, "i should be true")
+	default:
+		assert(false, "i should be true")
+	}
+
 	// switch on array.
 	switch ar := [3]int{1, 2, 3}; ar {
 	case [3]int{1,2,3}: