Report a non-bool user-defined TYPE_CHECKING constant#3846
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Type checkers treat `TYPE_CHECKING` as `True` while the runtime sees `False`, so a user-defined module-level `TYPE_CHECKING` (or pyrefly's `TYPE_CHECKING_WITH_PYREFLY`) constant must be a `bool` (conventionally `TYPE_CHECKING = False`). A definition like `TYPE_CHECKING: str = ""` is internally consistent yet wrong, and was previously accepted silently. When solving a module-scope name assignment to a type-checking constant, check that its type is assignable to `bool`, otherwise emit the new `invalid-type-checking-constant` error. The check is restricted to module scope (tracked via a new `NameAssign::is_module_scope` flag) and skips stub files, so imports of `typing.TYPE_CHECKING`, function locals, class attributes that merely share the name, and `.pyi` placeholder values are unaffected. Closes facebook#3756
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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Closes #3756.
Summary
A user-defined module-level
TYPE_CHECKINGconstant is treated asTrueby type checkers andFalseat runtime, so it must be abool(conventionallyTYPE_CHECKING = False). A definition likeTYPE_CHECKING: str = ""is internally consistent yet wrong, and was previously accepted silently. This mirrors ty'sinvalid-type-checking-constant.When solving a module-scope name assignment to a type-checking constant (
TYPE_CHECKINGorTYPE_CHECKING_WITH_PYREFLY), this checks that its type is assignable tobooland otherwise emits a newinvalid-type-checking-constanterror. Imports oftyping.TYPE_CHECKING, function locals, and class attributes that merely share the name are unaffected.Test Plan
New tests in
pyrefly/lib/test/sys_info.rs:TYPE_CHECKING: str = ""/= 1(andTYPE_CHECKING_WITH_PYREFLY) are flagged; the canonical import +if TYPE_CHECKING:,= False,: bool = False, class attributes, and function locals are not.cargo testpasses; full lib test suite is green.