[Johnx-w]

Fixes #22377

Summary

Binary operators now use metaclasses to determine dispatch order for both direct class objects (A * B) and class objects passed as type[A] / type[B] parameters.

 check_op_reversible
- compare constructor callable types or TypeType wrappers
+ compare the class objects' runtime metaclasses

An overriding right-metaclass __rmul__ takes priority. Inherited or unrelated reflected methods keep left-method priority, and operands with the same metaclass do not try the reflected method.

Evidence

• Before: The original regression inferred int instead of str. Additional cases failed for typed parameters, bounded type variables, and direct class operands sharing a metaclass.
  After: All regression cases pass.
• Related suites (check-classes, check-expressions, check-overloading, check-generics, check-protocols): 1,502 passed, 4 skipped, 1 xfailed.
• Self-check: no issues in 349 source files. Black and Ruff passed.

Merge Danger

Door: two-way

Blast Radius: inference

Affects binary-operator result typing for class objects and parameters annotated with type[...].

[chrikrah]

Mergeable from my side; reverting the fix fails the new test.

• non-blocking: type[A] * type[B] still infers int, runtime str; TypeType isn't unwrapped.

$ python -m mypy probe.py   # A*B, A*C, A*D, SubA*SubB, A*SubB, A*RightMeta(...); f(x: type[A], y: type[B]) reveals x*y
base 55472c0: Found 4 errors in 1 file   (Expression is of type "int", not "str"); probe.py:26: note: Revealed type is "int"
head 1903600: Success: no issues found in 1 source file; probe.py:26: note: Revealed type is "int"
$ python probe.py
A*B, A*C, A*D: str int int
f(A, B): str
$ pytest -n 4 -q -k testReverseOperatorOrderingForClassObjects mypy/test/testcheck.py
head: 1 passed    checkexpr.py reverted to base: 1 failed (main:20: Expression is of type "int", not "str")
$ pytest -n 4 -q mypy/test/testcheck.py -k "check-classes or check-expressions or check-overloading or check-generics or check-protocols"
1502 passed, 4 skipped, 1 xfailed

Here or in a follow-up?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[Johnx-w]

Thanks for the review! I addressed the type[A] / type[B] case in this PR in 29c567e, with regression tests for typed parameters and bounded type variables. I also corrected dispatch for operands sharing a metaclass. All CI checks are green.

[chrikrah]

29c567e covers type[A] * type[B]; still mergeable from my side.

$ python -m mypy probe.py   # a: type[A], b: type[B], sub: type[SubA], a: type[T bound A]
head 29c567e: a*b "str", sub*b "int", T-bound a*b "str"   # runtime: str, int, str
checkexpr.py from 1903600: "int", "int", "int"
$ pytest -q -n 0 mypy/test/testcheck.py -k testReverseOperatorOrderingForClassObjects
head: 1 passed    checkexpr.py from 1903600: 1 failed (main:25, 29, 31, 39)
$ pytest -q -n 0 mypy/test/testcheck.py -k "check-classes.test or check-expressions.test or check-generics.test"
1019 passed, 2 skipped, 1 xfailed