result of string.__add__ has to be str

Author: tomasstupka

graalpython/com.oracle.graal.python.test/src/tests/test_string.py

@@ -1155,3 +1155,21 @@ def test_replace_count():
     s = "1 2 3 1 2 3 1 2 3 1 2 3"
     s = s.replace("1", "1 _", s.count("1"))
     assert s == "1 _ 2 3 1 _ 2 3 1 _ 2 3 1 _ 2 3"
+
+def test_str_add_result_type():
+    class S(str): pass
+
+    assert (S('') + S('a')).__class__ == str
+    assert (S('a') + S('')).__class__ == str
+    assert (S('') + S('')).__class__ == str
+    assert (S('a') + S('a')).__class__ == str
+
+    assert ('' + S('')).__class__ == str
+    assert ('a' + S('')).__class__ == str
+    assert (S('') + '').__class__ == str
+    assert (S('') + 'a').__class__ == str
+
+    assert ('' + '').__class__ == str
+    assert ('' + 'a').__class__ == str
+    assert ('a' + '').__class__ == str
+    assert ('a' + 'a').__class__ == str

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -405,27 +405,29 @@ String doSSSimple(String self, String other) {
         }
 
         @Specialization(guards = "!concatGuard(self, other.getCharSequence())")
-        Object doSSSimple(String self, PString other) {
+        String doSSSimple(String self, PString other) {
             if (LazyString.length(self, leftProfile1, leftProfile2) == 0) {
-                return other;
+                // result type has to be str
+                return toString(other.getCharSequence());
             }
             return self;
         }
 
         @Specialization(guards = "!concatGuard(self.getCharSequence(), other)")
-        Object doSSSimple(PString self, String other) {
+        String doSSSimple(PString self, String other) {
             if (LazyString.length(self.getCharSequence(), leftProfile1, leftProfile2) == 0) {
                 return other;
             }
-            return self;
+            // result type has to be str
+            return toString(self.getCharSequence());
         }
 
         @Specialization(guards = "!concatGuard(self.getCharSequence(), other.getCharSequence())")
-        PString doSSSimple(PString self, PString other) {
+        String doSSSimple(PString self, PString other) {
             if (LazyString.length(self.getCharSequence(), leftProfile1, leftProfile2) == 0) {
-                return other;
+                return toString(other.getCharSequence());
             }
-            return self;
+            return toString(self.getCharSequence());
         }
 
         @Specialization(guards = "concatGuard(self.getCharSequence(), other)")
@@ -474,6 +476,11 @@ private static String stringConcat(CharSequence left, CharSequence right) {
             return left.toString() + right.toString();
         }
 
+        @TruffleBoundary
+        private static String toString(CharSequence cs) {
+            return cs.toString();
+        }
+
         @Specialization(guards = "isString(self)")
         Object doSNative(VirtualFrame frame, Object self, PythonAbstractNativeObject other,
                         @Cached CastToJavaStringNode cast,