Detect metaclass conflicts, refs #13563

[sobolevn]

Two questions:

1. Is copying cpython's TypeError message is good enough?
2. Should I ignore .pyi files from check? Or should it say consistent with .py files?

[sobolevn]

Without known exceptions it generates a lot of false-positives. Let's wait for the latest mypy-primer report.

[sobolevn]

I need to fix sympy's case, right now we handle it incorrectly.

[sobolevn]

Ok, looks like I found the problem:

• Usually metaclass is set here:

  mypy/mypy/semanal.py

  Line 2096 in 9393fa1

  defn.info.metaclass_type=defn.info.calculate_metaclass_type()

• But, in case of sympy for some reason metaclass= points to a PlaceholderNode, so it does not calculate .metaclass_type and returns early:

  mypy/mypy/semanal.py

  Lines 2070 to 2072 in 9393fa1

  	ifisinstance(sym.node, PlaceholderNode):
  	self.defer(defn)
  	return

• .metaclass_type is never recalculated

I will address this in a separate PR.

[sobolevn]

So, basically we have a bug in mypy that was revealed by this.

[sobolevn]

Proof, I added these lines to sympy on master branches (both tools):

reveal_type(AssocOp)
reveal_type(type(AssocOp))
reveal_type(AssocOp.__lt__)

sympy/core/operations.py:460: note: Revealed type is "Any"
sympy/core/operations.py:461: note: Revealed type is "Type[Any]"
sympy/core/operations.py:462: note: Revealed type is "Any"

Link: https://github.com/sympy/sympy/blob/c07abcc94e5b1521062c84d6d0a21b96dedfe07a/sympy/core/operations.py#L459

This project has lots of circular imports. They are everywhere:

• https://github.com/sympy/sympy/blob/master/sympy/core/basic.py#L2173
• https://github.com/sympy/sympy/blob/c07abcc94e5b1521062c84d6d0a21b96dedfe07a/sympy/core/sympify.py#L635
• etc

Some of them are causing this.

[sobolevn]

Blocked by: #13579

[sobolevn]

Going to open a new one, since this PR was affected by several other commits.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉