Fix IndexError when unpacking empty tuple in type alias#20931
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bxff wants to merge 1 commit intopython:masterfrom
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Fix IndexError when unpacking empty tuple in type alias#20931bxff wants to merge 1 commit intopython:masterfrom
bxff wants to merge 1 commit intopython:masterfrom
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When expanding type arguments for a builtins.tuple instance, the normalization code accessed args[0] without checking if args was non-empty. This caused an IndexError when Unpack[tuple[()]] was used in a type alias, since the empty tuple expansion produces zero args. Fixes python#20913
Contributor
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
A5rocks
reviewed
Mar 1, 2026
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| type T[T, *Ts] = C[*Ts] | ||
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| x: T[bool, Unpack[tuple[()]]] |
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Can you add a reveal type to make sure the type is being preserved?
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Crash
mypy crashes with
IndexError: list index out of rangewhen usingUnpack[tuple[()]]in a PEP 695 type alias:Root Cause
In
ExpandTypeVisitor.visit_instance(), when handlingbuiltins.tupleinstances, the normalization code at line 228 accessesargs[0]without checking ifargsis non-empty.Unpack[tuple[()]]expands to zero items (empty tuple), leavingargsas an empty list afterexpand_type_tuple_with_unpack().Fix
Added a
len(args) > 0guard to thebuiltins.tuplenormalization condition.Test Plan
Added
testUnpackEmptyTupleInTypeAliasNoCrashincheck-python312.testreproducing the exact crash from the issue. Also verified all 191 existing Unpack/TypeVarTuple tests pass.Fixes #20913