better static checks on initializing CheckedDict variables with dictionary literals

[LuKuangChen]

a4a71ae 2021-12-20

What program did you run?

from __static__ import CheckedDict

class B: pass

def testfunc():
x: CheckedDict[B, int] = {object():42}
return x

This program is found in cinder/3.8/Lib/test/test_compiler/test_static/tests.py. This test is labeled test_compile_checked_dict_with_annotation_wrong_key_type. There is a similar test labeled test_compile_checked_dict_with_annotation_wrong_value_type.

What happened?

compiler.errors.TypedSyntaxError: type mismatch: Exact[chkdict[object, Literal[42]]] cannot be assigned to Exact[chkdict[__main__.B, int]]

What should have happened?

We expected a compile-time error complaining that object cannot be assigned to B. In general, we expect that everytime the type checker sees x: CheckedDict[T1, T2] = { e1: e2, ... }, it checks every key expression e1 against T1, and every e2 against T2.

[carljm]

@DinoV did https://github.com/facebookincubator/cinder/commit/b69cb743622412d4df79ef63667ce92f027103ee also fix this issue? It looks like it might have, but it didn't explicitly add a test with literal RHS.