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Reject Self in PEP 695 type parameter bounds #21961

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Reject Self in PEP 695 type parameter bounds #21961

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12 changes: 10 additions & 2 deletions mypy/semanal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1888,7 +1888,11 @@ def analyze_type_param(
) -> TypeVarLikeExpr | None:
fullname = self.qualified_name(type_param.name)
if type_param.upper_bound:
upper_bound = self.anal_type(type_param.upper_bound, allow_placeholder=True)
upper_bound = self.anal_type(
type_param.upper_bound,
allow_placeholder=True,
prohibit_self_type="a type parameter bound",
)
# TODO: we should validate the upper bound is valid for a given kind.
if upper_bound is None:
# This and below copies special-casing for old-style type variables, that
Expand Down Expand Up @@ -1929,7 +1933,11 @@ def analyze_type_param(
values: list[Type] = []
if type_param.values:
for value in type_param.values:
analyzed = self.anal_type(value, allow_placeholder=True)
analyzed = self.anal_type(
value,
allow_placeholder=True,
prohibit_self_type="a type parameter constraint",

@EmmanuelNiyonshuti EmmanuelNiyonshuti Sep 9, 2026

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I think mypy already rejects Self in constraints. Why is this needed, would you elaborate?

@EmmanuelNiyonshuti EmmanuelNiyonshuti Sep 9, 2026

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I think mypy already rejects Self in constraints. Why is this needed, would you elaborate?

class Foo:
    # TypeVar constraint type cannot be parametrized by type variables
    def constrained[T: (Foo, Self)](self: T) -> None: pass

)
if analyzed is None:
analyzed = PlaceholderType(None, [], context.line)
if has_type_vars(analyzed):
Expand Down
8 changes: 8 additions & 0 deletions test-data/unit/check-python312.test
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Expand Up @@ -1446,6 +1446,14 @@ reveal_type(F[str]().m()) # N: Revealed type is "__main__.F[builtins.str]"
reveal_type(F[str]().mm(b'x')) # N: Revealed type is "tuple[__main__.F[builtins.str], builtins.bytes]"
[builtins fixtures/tuple.pyi]

[case testTypingSelfInvalidLocationsTypeParams]
# flags: --python-version 3.12
from typing import Self

class C:
def bounded[T: Self](self: T) -> None: ... # E: Self type cannot be used in a type parameter bound
def constrained[T: (C, Self)](self: T) -> None: ... # E: Self type cannot be used in a type parameter constraint

[case testPEP695CallAlias]
class C:
def __init__(self, x: str) -> None: ...
Expand Down
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