Function invocation pins type of arguments too early

Minimal repro:

def foo[T](a: T, b: T) -> T:
    ...

def test(x: int | None, y: int | None) -> int | None:
    if x is None:
        return foo(x, y)
    return None

Expected: No error
Actual: Argument int | None is not assignable to parameter b with type None in function foo [bad-argument-type]

Sandbox URL: https://pyrefly.org/sandbox/?code=CYUwZgBGD20NoBUC6AKAhgLgggNBARlggJQQC0AfNhgFAT0QB0zNNokALiAM4coAeWAJYA7DhAA+EAHLQRIPAE9hYyTLkhSlCKPFTZ82gx2R+O7usN1jDAE4gOAV1siosAUuLX69py8sgQA

i don't understand the initial example - if x is None coerces the type of x to None and then foo(x, y) could indeed fail typechecking if y is int, so pyrefly is doing the right thing there.

@martindemello

@martindemello If the typevar T for foo gets instantiated into int | None then type checking won't fail. i.e.

def foo(a: int | None, b: int | None) -> int | None:
    ...

def test(x: int | None, y: int | None) -> int | None:
    if x is None:
        return foo(x, y)  # This should type check
    return None

We don't really want to assume the call foo(x, y) forces T to be None just because its first argument x is None -- the point here is that if we wait longer and get to see what the type of y is, we could have guessed better type for T.

If I understand, the following is a even simpler example where the eager pinning of generics can go awry:

def foo[T](a: T, b: T) -> T: ...
foo(0, 1.)   # Argument `float` is not assignable to parameter `b` with type `int` in function `foo`

This issue has someone assigned, but has not had recent activity for more than 2 weeks.

If you are still working on this issue, please add a comment so everyone knows. Otherwise, please unassign yourself and allow someone else to take over.

Thank you for your contributions!

Take upper bounds into account in `solve_bounds`

Check upper bounds before promoting literals in lower bounds

Keep eager pinning for ParamSpec and TypeVarTuple

Check for upper bound violations before adding lower bounds

Collect upper bounds on Quantified and Unwrap

Remove unnecessary `is_partial` variable and check

Extract `add_bound` helper from `add_lower_bound`

Use `add_bound` in `add_upper_bound` (#2941)

Most of the user-facing bugs that were reported on this issue and its duplicates have been closed, so I'm going to close out this issue and open a new one to track the remaining work (edge case bugs, internal solver improvements and cleanups).

Pyrefly: surface secondary annotations as LSP DiagnosticRelatedInform…

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Function invocation pins type of arguments too early #105

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Function invocation pins type of arguments too early #105

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lolpack commented on May 1, 2025

martindemello commented on Jun 29, 2025

grievejia commented on Jul 7, 2025

samwgoldman commented on Jul 7, 2025

github-actions commented on Aug 5, 2025

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rchen152 commented on Apr 30, 2026

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