(🎁) What if unsupported features reported a warning instead of silently(or unexpectedly) failing

[KotlinIsland]

Full ParamSpec support is on the way(#8645) but until that time, completely valid usages of ParamSpec and Concatenate are met with perplexing error messages that most people would misinterpret as misusing the feature.

I believe it would be at least 10 times more understandable if mypy showed a warning in these circumstances:

from typing import type_check_only, ParamSpec, Concatenate, Callable

@type_check_only  # no warning about unsupported functionality
def foo() -> None: ...

P = ParamSpec("P")
def bar(func:  # error: The first argument to Callable must be a list of types or "..."
    Callable[
      Concatenate[P, int],
      None,
    ]
) -> None:  # 
    ...

Proposed behavior:

> mypy test.py
test.py:3: warning: Unsupported typing feature: "type_check_only"
test.py:9: warning: Unsupported typing feature: "Concatenate"
Success 0 errors in 1 file

Idk if it's possible to determine what would constitute a warning, next best thing would be to show the current error with an info message.

[henryiii]

I was trying to implement Concatenate for a while before realizing it was unsupported, since I thought I was simply using it incorrectly. This would be very nice to have! 👍 Another example of a as-yet unsupported feature: Self.

This is the current result of using Concatenate, FYI:

...: error: The first argument to Callable must be a list of types or "..."  [misc]
...: error: Incompatible return value type (got "Callable[P, None]", expected "Callable[P, None]")  [return-value]

The last one is quite, um, interesting. :)

[KotlinIsland]

Related #9531

[hauntsaninja]

Closing, since we support PEP 612. We'll try to keep in mind for future type system features, e.g. we currently do issue a warning about PEP 695