[waw666waw666]

Fixes #16940

T = TypeVar("T", bound=Callable[..., "T"]) crashes mypy with infinite recursion (a segfault when compiled). Unlike e.g. bound=list["T"], which is reported as a cyclic definition, Callable binds the free type variable T itself, so after the second semantic analysis pass the stored bound is def [T <: ...] (...) -> T. Re-analysing that bound on the next pass re-binds T from its own TypeVarExpr, whose bound is that same callable, and anal_var_def() never terminates. The same happens with default=Callable[..., "T"].

This PR makes process_typevar_declaration() detect a bound or default that refers to the type variable being defined, and reports the same Cannot resolve name "T" (possible cyclic definition) error used for the non-Callable case, replacing the bound/default with Any.

Added test cases for both the bound and the default; both crash on master and pass with this change. Bounds that use a different type variable inside a Callable (e.g. bound=Callable[[S], S]) are unaffected.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅