[mypyc] Add note about using non-native class to subclass built-in types (#19236)

Without the note, it's not clear what's the easiest way forward. Also
add a doc link.

Test that subclassing a built-in exception type actually works as
suggested by the note.

Author: JukkaL

mypyc/irbuild/prepare.py

@@ -298,6 +298,16 @@ def prepare_class_def(
                 errors.error(
                     "Inheriting from most builtin types is unimplemented", path, cdef.line
                 )
+                errors.note(
+                    "Potential workaround: @mypy_extensions.mypyc_attr(native_class=False)",
+                    path,
+                    cdef.line,
+                )
+                errors.note(
+                    "https://mypyc.readthedocs.io/en/stable/native_classes.html#defining-non-native-classes",
+                    path,
+                    cdef.line,
+                )
 
     # Set up the parent class
     bases = [mapper.type_to_ir[base.type] for base in info.bases if base.type in mapper.type_to_ir]

mypyc/test-data/commandline.test

@@ -138,7 +138,9 @@ Foo.lol = 50  # E: Only class variables defined as ClassVar can be assigned to
 def decorator(x: Any) -> Any:
     return x
 
-class NeverMetaclass(type):  # E: Inheriting from most builtin types is unimplemented
+class NeverMetaclass(type):  # E: Inheriting from most builtin types is unimplemented \
+                             # N: Potential workaround: @mypy_extensions.mypyc_attr(native_class=False) \
+                             # N: https://mypyc.readthedocs.io/en/stable/native_classes.html#defining-non-native-classes
     pass
 
 class Concrete1:

mypyc/test-data/irbuild-classes.test

@@ -1375,7 +1375,9 @@ class BadUse():  # E: native_class must be used with True or False only
 from mypy_extensions import mypyc_attr
 
 @mypyc_attr(native_class=True)
-class M(type):  # E: Inheriting from most builtin types is unimplemented
+class M(type):  # E: Inheriting from most builtin types is unimplemented \
+                # N: Potential workaround: @mypy_extensions.mypyc_attr(native_class=False) \
+                # N: https://mypyc.readthedocs.io/en/stable/native_classes.html#defining-non-native-classes
     pass
 
 @mypyc_attr(native_class=True)

mypyc/test-data/run-classes.test

@@ -934,6 +934,53 @@ def welp() -> int:
 from native import welp
 assert welp() == 35
 
+[case testSubclassUnsupportedException]
+from mypy_extensions import mypyc_attr
+
+@mypyc_attr(native_class=False)
+class MyError(ZeroDivisionError):
+    pass
+
+@mypyc_attr(native_class=False)
+class MyError2(ZeroDivisionError):
+    def __init__(self, s: str) -> None:
+        super().__init__(s + "!")
+        self.x = s.upper()
+
+def f() -> None:
+    raise MyError("foobar")
+
+def test_non_native_exception_subclass_basics() -> None:
+    e = MyError()
+    assert isinstance(e, MyError)
+    assert isinstance(e, ZeroDivisionError)
+    assert isinstance(e, Exception)
+
+    e = MyError("x")
+    assert repr(e) == "MyError('x')"
+
+    e2 = MyError2("ab")
+    assert repr(e2) == "MyError2('ab!')", repr(e2)
+    assert e2.x == "AB"
+
+def test_raise_non_native_exception_subclass_1() -> None:
+    try:
+        f()
+    except MyError:
+        x = True
+    else:
+        assert False
+    assert x
+
+def test_raise_non_native_exception_subclass_2() -> None:
+    try:
+        f()
+    except ZeroDivisionError:
+        x = True
+    else:
+        assert False
+    assert x
+
 [case testSubclassPy]
 from b import B, V
 class A(B):