[ty] Allow subtypes of LiteralString to be narrowed using equality checks#23794
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AlexWaygood merged 2 commits intomainfrom Mar 9, 2026
Merged
[ty] Allow subtypes of LiteralString to be narrowed using equality checks#23794AlexWaygood merged 2 commits intomainfrom
LiteralString to be narrowed using equality checks#23794AlexWaygood merged 2 commits intomainfrom
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Typing conformance resultsNo changes detected ✅Current numbersThe percentage of diagnostics emitted that were expected errors held steady at 85.05%. The percentage of expected errors that received a diagnostic held steady at 78.05%. The number of fully passing files held steady at 63/132. |
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Memory usage reportMemory usage unchanged ✅ |
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Clearly this is a feature our users are desperate for |
dcreager
approved these changes
Mar 9, 2026
| // literal, in which case (given that it is single-valued), LiteralString | ||
| // cannot compare equal to it. | ||
| Type::LiteralValue(literal) if literal.is_literal_string() => true, | ||
| ty if ty.is_subtype_of(db, Type::literal_string()) => true, |
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I'm guessing there are other callers of Type::is_literal_string since clippy didn't flag a dead code warning. Are those callers legit? Should they be doing subtype checks as well? (This might also deserve a comment over at the method definition calling out that if you're worried about type relations, you probably want to do this kind of subtype check instead)
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You were correct, Type::is_literal_string() was a bug magnet! We now also support this feature (just as crucial as the original one -- our users will be overjoyed):
from typing import LiteralString
def f(literal_a: LiteralString, literal_b: LiteralString):
if literal_a != "foo":
reveal_type(literal_a) # revealed: LiteralString & ~Literal["foo"]
# on `main`: `str`
# this branch: `LiteralString`!
reveal_type(f"{literal_a}")
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Summary
Our current narrowing machinery has some very specific special casing for
LiteralStringexactly, which produces odd results inif/elifchains:The second
reveal_typethere should clearly beLiteral["ABORTED"].This PR extends our narrowing behaviour for
LiteralStringto all subtypes ofLiteralString.Test Plan
mdtests extended