Basic support for decorated overloads

[ilevkivskyi]

Fixes #15737
Fixes #12844
Fixes #12716

My goal was to fix the ParamSpec issues, but it turns out decorated overloads were not supported at all. Namely:

• Decorators on overload items were ignored, caller would see original undecorated item types
• Overload item overlap checks were performed for original types, while arguably we should use decorated types
• Overload items completeness w.r.t. to implementation was checked with decorated implementation, and undecorated items

Here I add basic support using same logic as for regular decorated functions: initially set type to None and defer callers until definition is type-checked. Note this results in few more Cannot determine type in case of other errors, but I think it is fine.

Note I also add special-casing for "inline" applications of generic functions to overload arguments. This use case was mentioned few times alongside overloads. The general fix would be tricky, and my special-casing should cover typical use cases.

[ilevkivskyi]

Just to confirm: new errors in asynq are correct, they use a decorator like Callable[..., T] -> Callable[..., T] which obviously erased overload item signatures.

[hauntsaninja]

Looks great!

[hauntsaninja]

nit: check_call is pretty hot, should we move this inside if isinstance(callee, CallableType): on L1513?

[ilevkivskyi]

Yes, good idea!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Expression (https://github.com/cognitedata/Expression)
- expression/extra/parser.py:81: error: No overload variant of "starmap" matches argument type "Callable[..., Any]"  [call-overload]
- expression/extra/parser.py:81: note: Possible overload variants:
- expression/extra/parser.py:81: note:     def [_A, _B, _C] starmap(mapper: Callable[[_A, _B], _C], parser: Parser[tuple[_A, _B]]) -> Parser[_C]
- expression/extra/parser.py:81: note:     def [_A, _B, _C, _D] starmap(mapper: Callable[[_A, _B, _C], _D], parser: Parser[tuple[_A, _B, _C]]) -> Parser[_D]
- expression/extra/parser.py:219: error: Overloaded function implementation does not accept all possible arguments of signature 1  [misc]
- expression/extra/parser.py:219: error: Overloaded function implementation does not accept all possible arguments of signature 2  [misc]
+ expression/extra/parser.py:435: error: Cannot infer type argument 3 of "pipe"  [misc]
+ expression/extra/parser.py:440: error: Argument 1 to "starmap" has incompatible type "Callable[[Some[str] | Nothing_[str], Block[str]], int]"; expected "Callable[[_A, _B], _C]"  [arg-type]
- expression/extra/parser.py:440: error: No overload variant of "starmap" matches argument type "Callable[[Some[str] | Nothing_[str], Block[str]], int]"  [call-overload]
- expression/extra/parser.py:440: note: Possible overload variants:
- expression/extra/parser.py:440: note:     def [_A, _B, _C] starmap(mapper: Callable[[_A, _B], _C], parser: Parser[tuple[_A, _B]]) -> Parser[_C]
- expression/extra/parser.py:440: note:     def [_A, _B, _C, _D] starmap(mapper: Callable[[_A, _B, _C], _D], parser: Parser[tuple[_A, _B, _C]]) -> Parser[_D]
+ expression/extra/parser.py:469: error: Cannot infer type argument 2 of "pipe"  [misc]
+ expression/extra/parser.py:480: error: Argument 1 to "starmap" has incompatible type "Callable[[tuple[Some[str] | Nothing_[str], Block[str]], Some[Block[str]] | Nothing_[Block[str]]], float]"; expected "Callable[[_A, _B], _C]"  [arg-type]
- expression/extra/parser.py:480: error: No overload variant of "starmap" matches argument type "Callable[[tuple[Some[str] | Nothing_[str], Block[str]], Some[Block[str]] | Nothing_[Block[str]]], float]"  [call-overload]
- expression/extra/parser.py:480: note: Possible overload variants:
- expression/extra/parser.py:480: note:     def [_A, _B, _C] starmap(mapper: Callable[[_A, _B], _C], parser: Parser[tuple[_A, _B]]) -> Parser[_C]
- expression/extra/parser.py:480: note:     def [_A, _B, _C, _D] starmap(mapper: Callable[[_A, _B, _C], _D], parser: Parser[tuple[_A, _B, _C]]) -> Parser[_D]

asynq (https://github.com/quora/asynq)
+ asynq/tools.pyi:42: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]
+ asynq/tools.pyi:48: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]

[maffoo]

I'm eager to use this feature, since it resolves #15737. Any estimate on when it might be included in a release version of mypy, e.g. a 1.5.2 release?

[AlexWaygood]

I'm eager to use this feature, since it resolves #15737. Any estimate on when it might be included in a release version of mypy, e.g. a 1.5.2 release?

Follow #15919 for updates on the next feature release.