interp: apply type conversion on untyped variable at run

Fixes #938.

interp/interp_eval_test.go

@@ -77,6 +77,7 @@ func TestEvalShift(t *testing.T) {
 		{src: "a, b, m := uint32(1), uint32(2), uint32(0); m = a + (1 << b)", res: "5"},
 		{src: "c := uint(1); d := uint(+(-(1 << c)))", res: "18446744073709551614"},
 		{src: "e, f := uint32(0), uint32(0); f = 1 << -(e * 2)", res: "1"},
+		{src: "p := uint(0xdead); byte((1 << (p & 7)) - 1)", res: "31"},
 		{pre: func() { eval(t, i, "const k uint = 1 << 17") }, src: "int(k)", res: "131072"},
 	})
 }

interp/typecheck.go

@@ -634,16 +634,13 @@ func (check typecheck) conversion(n *node, typ *itype) error {
 	if !ok {
 		return n.cfgErrorf("cannot convert expression of type %s to type %s", n.typ.id(), typ.id())
 	}
-
-	if n.typ.untyped {
-		if isInterface(typ) || c != nil && !isConstType(typ) {
-			typ = n.typ.defaultType()
-		}
-		if err := check.convertUntyped(n, typ); err != nil {
-			return err
-		}
+	if !n.typ.untyped || c == nil {
+		return nil
 	}
-	return nil
+	if isInterface(typ) || !isConstType(typ) {
+		typ = n.typ.defaultType()
+	}
+	return check.convertUntyped(n, typ)
 }
 
 type param struct {