`@deprecated` doesn't work with non-overloaded constructors

[bzoracler]

Bug Report, To Reproduce, & Actual Behaviour

This is similar to #19588, but that was fixed on master for the case when __init__ is overloaded.

In the non-overloaded case, @deprecated still doesn't report anything (mypy Playground):

To Reproduce

from typing import overload
from typing_extensions import deprecated, Self

class Some:
    @deprecated("reason")
    def __new__(cls, x: str) -> Self: ...

Some('test')  # No reports here

class Some2:
    @deprecated("reason")
    def __init__(self, x: str) -> None: ...

Some2('test')  # No reports here

Expected Behavior

Some('test')  # E: function Some.__new__ is deprecated: reason  [deprecated]
Some2('test')  # E: function Some.__init__ is deprecated: reason  [deprecated]

Your Environment

• Mypy version used: 1.18.2, master

[bzoracler]

Oh, oops - I see that mypy expects users to decorate the class rather than the constructor with @deprecated.

This behaves differently to pyright and pyrefly. Pyright issues reports when either classes or constructors are decorated; pyrefly only issues reports when constructors are decorated.

However, mypy is compliant with the typing specs on @deprecated, so I guess this isn't really a bug?

[bzoracler]

Actually, decorating the class isn't the right solution for simply constructors. Doing this will warn on all usages of the class, including subclassing and type annotations.

mypy/test-data/unit/check-deprecated.test

Lines 157 to 167 in b266dd1

	[case testDeprecatedBaseClass]
	# flags: --enable-error-code=deprecated
	from typing_extensions import deprecated
	@deprecated("use C2 instead")
	class C: ...
	class D(C): ... # E: class __main__.C is deprecated: use C2 instead
	class E(D): ...
	class F(D, C): ... # E: class __main__.C is deprecated: use C2 instead

I think @deprecated should work on non-overloaded constructors to indicate that alternative constructors need to be used to build instances, without indicating that the whole class is deprecated.

[A5rocks]

I interpret the spec as needing this:

Any syntax that indirectly triggers a call to the function. For example, if the __add__ method of a class C is deprecated, then the code C() + C() should trigger a diagnostic. Similarly, if the setter of a property is marked deprecated, attempts to set the property should trigger a diagnostic.

i.e. C() indirectly triggers a call to C.__init__() and C.__new__() (and the __call__ for the metaclass, I guess).

[A5rocks]

@JelleZijlstra am I interpreting things correctly?