Typing spec update for constructors

[erictraut]

I'd like to request that the TC consider adoption of a new chapter in the typing spec that spells out type checking behaviors related to constructor calls.

Links to PR & Discussion The PR can be found here. The latest draft incorporates feedback from PR reviews and the discussion.

The discussion can be found here.

TC Sign-off

• [x] Eric
• [x] Guido
• [x] Jelle
• [x] Rebecca
• [x] Shantanu

Current Type Checker Behaviors None of the type checkers fully comply with the proposed behavior.

I haven't written a full test suite, but I created a partial suite to test current behaviors. Here are the results.

Area	mypy	pyright	pyre	pytype
Honors metaclass __call__	no	yes	yes	no
__new__ basic specialization	yes	yes	yes	partial
Honors __new__ return type	no	partial	no	no
Skips __init__ based on __new__ return type	no	yes	no	no
Validates binding of __new__ cls	no	partial	no	no
__init__ basic specialization	yes	yes	yes	partial
Validates binding of __init__ self	no	partial	no	no
Honors __init__ self annotation (basic)	partial	yes	yes	no
Honors __init__ self annotation (func TypeVar)	no	no	yes	no
Rejects __init__ self annotation (class TypeVar)	no	no	no	no
Constructor specialization defaults	partial	yes	no	no
Converting constructor to callable	partial	partial	partial	partial

Controversial Issues ~There is one potentially controversial issue related to the handling of a metaclass __call__ that has a return type (implicit or explicit) of Any or a union that includes Any. If you're interested in the details, refer to the discussion thread.~

EDIT: We've resolved this issue through a compromise suggested by Rebecca. This is reflected in the latest draft.

[gvanrossum]

I've signed off. Thanks for this, Eric!