Mypyc crashes on conditionally-defined class

[nickdrozd]

To Reproduce

if True:
    class A:
        pass

(This example is silly, but a real use case would be defining a protocol inside a TYPE_CHECKING block.)

Actual Behavior

  File "mypy/mypy/nodes.py", line 1141, in accept
    return visitor.visit_class_def(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "mypy/mypyc/irbuild/visitor.py", line 167, in visit_class_def
    transform_class_def(self.builder, cdef)
  File "mypy/mypyc/irbuild/classdef.py", line 88, in transform_class_def
    ir = builder.mapper.type_to_ir[cdef.info]
         ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
asdf.py:2: KeyError: <TypeInfo asdf.A>

mypyc --version
mypy 1.6.0+dev.89ad125fa5a31a7c82f267e957d3c94da6b52f61
(compiled: no)

[nickdrozd]

Similar in spirit to #13613

[nickdrozd]

The stack trace makes it look like the problem is just with building the class mapping incorrectly. That can be fixed easily, but beyond that there is also a codegen problem that is not so straightforward.

[guywilsonjr]

For anyone following According to https://mypyc.readthedocs.io/en/latest/differences_from_python.html#conditional-functions-or-classes:

Function and class definitions guarded by an if-statement are not supported.

[JelleZijlstra]

Even if it's unsupported, mypy should show a clear error message instead of crashing. A crash is always a bug.