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Contextual typing works only with the first element in a union #793

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#1392
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Contextual typing works only with the first element in a union#793
Bug
#1392
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contextual-typingissues related to contextual typing
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V1 Release
@rchen152

Description

@rchen152
rchen152
opened on Jul 31, 2025

Describe the Bug

I was surprised to not see an issue for this yet (unless I just missed it?). Opening one for V1 tracking. Contextual typing doesn't play well with union types, as seen here:

pyrefly/pyrefly/lib/test/contextual.rs

Lines 104 to 116 in 2520a37

testcase!(
bug = "Both assignments should be allowed. When decomposing the contextual hint, we eagerly resolve vars to the 'first' branch of the union. Note: due to the union's sorted representation, the first branch is not necessarily the first in source order.",
test_contextual_typing_against_unions,
r#"
class A: ...
class B: ...
class B2(B): ...
class C: ...
x: list[A] | list[B] = [B2()] # E: `list[B2]` is not assignable to `list[A] | list[B]`
y: list[B] | list[C] = [B2()]
"#,
);

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