Support chained assignment for type aliases

[agryman]

I'm running mypy 0.950. I found a problem when I upgraded from SymPy 1.9 to SymPy 1.10.1. Both versions contain the following chained type assignments:

Matrix = MutableMatrix = MutableDenseMatrix

This caused a problem in SymPy 1.10.1 because they enabled mypy type checking in that version.

This problem can easily be recreated in a simple file, see attached.

------- begin ---------
class A:
pass

a: A = A()

B = A

b: B = B()

D = C = A

c: C = C()
d: D = D()
-------- end --------

mypy reports the following errors:

$mypy bug_mypy_chained.py
bug_mypy_chained.py:16: error: Variable "bug_mypy_chained.C" is not valid as a type
bug_mypy_chained.py:16: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
bug_mypy_chained.py:17: error: Variable "bug_mypy_chained.D" is not valid as a type
bug_mypy_chained.py:17: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
Found 2 errors in 1 file (checked 1 source file)

bug_mypy_chained.py.txt

[agryman]

I upgraded to mypy 0.961. I get the same error message.

[JelleZijlstra]

Duplicate of #10101.

We're unlikely to add support for this pattern.

[agryman]

@JelleZijlstra Thanks for the reply. I disagree that this is an exact duplicate of #10101 but they are certainly related. I'll continue the discussion there.