fix(vm): only promote to int64/uint64 when both operands match signedness

Avoid mixed-sign regression where int64 + uint64 would incorrectly
promote to uint64, causing wrong results for negative values.

Author: eyupcanakman

expr_test.go

@@ -805,11 +805,11 @@ func TestExpr(t *testing.T) {
 		},
 		{
 			`Uint64 + 0`,
-			uint64(0),
+			0,
 		},
 		{
 			`Uint64 + Int64`,
-			uint64(0),
+			0,
 		},
 		{
 			`Int32 + Int64`,

vm/runtime/helpers/main.go

@@ -72,10 +72,10 @@ func cases(op string, xs ...[]string) string {
 		echo(`switch y := b.(type) {`)
 		for _, b := range types {
 			t := "int"
-			if isInt64(a) || isInt64(b) {
+			if (isInt64(a) || isInt64(b)) && isSigned(a) && isSigned(b) {
 				t = "int64"
 			}
-			if isUint64(a) || isUint64(b) {
+			if (isUint64(a) || isUint64(b)) && isUnsigned(a) && isUnsigned(b) {
 				t = "uint64"
 			}
 			if isDuration(a) || isDuration(b) {
@@ -147,6 +147,14 @@ func isUint64(t string) bool {
 	return t == "uint64"
 }
 
+func isSigned(t string) bool {
+	return t == "int" || t == "int8" || t == "int16" || t == "int32" || t == "int64"
+}
+
+func isUnsigned(t string) bool {
+	return t == "uint" || t == "uint8" || t == "uint16" || t == "uint32" || t == "uint64"
+}
+
 func isDuration(t string) bool {
 	return t == "time.Duration"
 }