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Fix type variables leaking from inference #7113

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Jul 1, 2019
Merged

Fix type variables leaking from inference #7113

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5 changes: 4 additions & 1 deletion mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2622,7 +2622,10 @@ def set_inference_error_fallback_type(self, var: Var, lvalue: Lvalue, type: Type

We implement this here by giving x a valid type (replacing inferred <nothing> with Any).
"""
self.set_inferred_type(var, lvalue, type.accept(SetNothingToAny()))
fallback = type.accept(SetNothingToAny())
# Type variables may leak from inference, see https://github.com/python/mypy/issues/5738,
# we therefore need to erase them.
self.set_inferred_type(var, lvalue, erase_typevars(fallback))

def check_simple_assignment(self, lvalue_type: Optional[Type], rvalue: Expression,
context: Context,
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9 changes: 9 additions & 0 deletions test-data/unit/python2eval.test
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Expand Up @@ -419,3 +419,12 @@ if MYPY:
[file lib.pyi]
x = b'abc'
[out]

[case testNestedGenericFailedInference]
from collections import defaultdict
def foo() -> None:
x = defaultdict(list) # type: ignore
x['lol'].append(10)
reveal_type(x)
[out]
_testNestedGenericFailedInference.py:5: note: Revealed type is 'collections.defaultdict[Any, builtins.list[Any]]'