Reconcile diamond-inherited generic bases with variance

[nitishagar]

Fixes #4842

Problem

When two base paths reach the same generic ancestor, pyrefly compared the two specializations with plain type-argument equality:

class Base(Generic[ValueT_contra, ValueT_co]): ...
class RangeBase(Base[ValueT_contra, ValueT_co], Generic[ValueT_contra, ValueT_co]): ...
class WideParam(Base[SupportsInt | SupportsIndex, int]): ...

class NarrowRange(RangeBase[int, int], WideParam): ...
# ERROR: Class `NarrowRange` has inconsistent type arguments for base class `Base`:
# `Base[int, int]` and `Base[SupportsIndex | SupportsInt, int]`

Base is contravariant in its first parameter, so Base[SupportsIndex | SupportsInt, int] is a subtype of Base[int, int] and the diamond is consistent — pyright accepts it (and this pattern occurs in the wild, e.g. click's ParamType hierarchy).

Change

The conflict predicate inside the C3-linearization check now delegates to a variance-aware comparison callback (constructed where solver access exists):

• positions are compared under the ancestor class's variance (declared keywords or inferred, via the same VarianceMap the subset check uses);
• a covariant or contravariant position is compatible when either specialization is assignable to the other (the first-wins walk order must not decide the outcome);
• invariant and bivariant positions still require two-way consistency, so distinct type variables and unrelated invariant arguments error exactly as before, with the same message;
• Any remains gradual; MRO construction (C3) and the first-wins ancestor selection are unchanged.

One pre-existing expectation is updated deliberately: the Iterable[int] vs Iterable[int | str] diamond in tuple.rs is a covariant-legal pair under the adopted semantics (pyright-compatible), so it no longer errors.

Test plan

• New tests: the issue's contravariant diamond (clean), a covariant diamond (clean), int vs str in a contravariant position (still errors, same message), an Any diamond (no-regression pin)
• All previously pinned inconsistent-diamond tests (generic_basic.rs, tuple.rs minus the covariant-legal case above) pass unedited
• cargo test -p pyrefly --lib -- --skip test::lsp: 7744 passed, 0 failed (the two missing-source LSP interaction failures also fail on unmodified main)
• Formatting and linting clean

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D123340876. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

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SUMMARY
============================================================
Total: +0 new errors, -9 fixed errors
By preset: +0/-9 (default), +0/-9 (strict)

Projects with changes (2):
  steam.py: +0 -8
  meson: +0 -1
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FULL DIFF DETAILS
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steam.py (https://github.com/Gobot1234/steam.py)
- ERROR steam/ext/commands/converters.py:273:7-20: Class `UserConverter` has inconsistent type arguments for base class `ConverterBase`: `ConverterBase[PartialUser]` and `ConverterBase[User]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:273:7-20: Class `UserConverter` has inconsistent type arguments for base class `Converter`: `Converter[PartialUser]` and `Converter[User]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:294:7-29: Class `PartialMemberConverter` has inconsistent type arguments for base class `ConverterBase`: `ConverterBase[PartialUser]` and `ConverterBase[PartialMember]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:294:7-29: Class `PartialMemberConverter` has inconsistent type arguments for base class `Converter`: `Converter[PartialUser]` and `Converter[PartialMember]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:302:7-22: Class `MemberConverter` has inconsistent type arguments for base class `ConverterBase`: `ConverterBase[PartialUser]` and `ConverterBase[Member]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:302:7-22: Class `MemberConverter` has inconsistent type arguments for base class `Converter`: `Converter[PartialUser]` and `Converter[Member]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:389:7-26: Class `PartialAppConverter` has inconsistent type arguments for base class `ConverterBase`: `ConverterBase[App]` and `ConverterBase[PartialApp[str]]` [invalid-inheritance]
- ERROR steam/ext/commands/converters.py:389:7-26: Class `PartialAppConverter` has inconsistent type arguments for base class `Converter`: `Converter[App]` and `Converter[PartialApp[str]]` [invalid-inheritance]

meson (https://github.com/mesonbuild/meson)
- ERROR mesonbuild/utils/universal.py:716:7-33: Class `PerThreeMachineDefaultable` has inconsistent type arguments for base class `PerMachine`: `PerMachine[_T | None | None]` and `PerMachine[_T | None]` [invalid-inheritance]