[ty] avoid unions of generic aliases of the same class in fixpoint

[carljm]

Partially addresses astral-sh/ty#1732
Fixes astral-sh/ty#1800

Summary

At each fixpoint iteration, we union the "previous" and "current" iteration types, to ensure that the type can only widen at each iteration. This prevents oscillation and ensures convergence.

But some unions triggered by this behavior (in particular, unions of differently-specialized generic-aliases of the same class) never simplify, and cause spurious errors. Since we haven't seen examples of oscillating types involving class-literal or generic-alias types, just don't union those.

There may be more thorough/principled ways to avoid undesirable unions in fixpoint iteration, but this narrow change seems like it results in strict improvement.

Test Plan

Removes two false positive unsupported-class-base in mdtests, and several in the ecosystem, without causing other regression.

[astral-sh-bot]

Diagnostic diff on typing conformance tests

No changes detected when running ty on typing conformance tests ✅

[astral-sh-bot]

mypy_primer results

Changes were detected when running on open source projects

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- src/scikit_build_core/_logging.py:153:13: warning[unsupported-base] Unsupported class base with type `<class 'Mapping[str, Style]'> | <class 'Mapping[str, Divergent]'>`
- Found 42 diagnostics
+ Found 41 diagnostics

strawberry (https://github.com/strawberry-graphql/strawberry)
- strawberry/channels/handlers/ws_handler.py:69:5: warning[unsupported-base] Unsupported class base with type `<class 'AsyncBaseHTTPView[GraphQLWSConsumer, GraphQLWSConsumer, GraphQLWSConsumer, GraphQLWSConsumer, GraphQLWSConsumer, Context@GraphQLWSConsumer, RootValue@GraphQLWSConsumer]'> | <class 'AsyncBaseHTTPView[GraphQLWSConsumer[None, None], GraphQLWSConsumer[None, None], GraphQLWSConsumer[None, None], GraphQLWSConsumer[None, None], GraphQLWSConsumer[None, None], Context@GraphQLWSConsumer, RootValue@GraphQLWSConsumer]'>`
- Found 402 diagnostics
+ Found 401 diagnostics

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ pandas-stubs/_typing.pyi:1217:16: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- Found 5122 diagnostics
+ Found 5123 diagnostics

No memory usage changes detected ✅

[carljm]

@mtshiba I can't request your review, but would be happy for you to take a look at this.

[AlexWaygood]

Have we actually seen any issues reported due to unions of class-literal types from our fixpoint machinery? Or has it just been generic-alias types?

I'm not sure how we ever would get a spurious union of class-literal types from our fixpoint machinery. Though if we did, I agree it could cause similar issues to the ones we've seen in the ecosystem (and in issue reports) regarding generic-alias types

[MichaReiser]

Seems worth a try and, at the very least, would help us learn where our assumption is off