Fix crash when singledispatch is shadowed by a user function

[TheMuffinMan1320]

Fixes #20339

Root cause

The plugin hook for functools.singledispatch (create_singledispatch_function_callback in mypy/plugins/singledispatch.py) is registered in mypy/plugins/default.py purely by matching the callee's fully qualified name against the string "functools.singledispatch".

If a user's own code produces a function whose fullname happens to be exactly "functools.singledispatch" — e.g. a top-level module literally named functools.py that defines its own def singledispatch(func): ... — the plugin hook still fires for calls to that unrelated function, since the match is name-based rather than tied to the real typeshed definition.

In that case ctx.default_return_type is whatever mypy inferred for the user's own function (not an Instance of functools._SingleDispatchCallable, which is what the real typeshed stub for singledispatch returns), so:

singledispatch_obj = get_proper_type(ctx.default_return_type)
assert isinstance(singledispatch_obj, Instance)

fails its assertion and crashes mypy with an AssertionError / INTERNAL ERROR.

Fix

Following the reproduction and suggestion from @hauntsaninja in the issue thread, when ctx.default_return_type doesn't have the shape the plugin expects, back off and return ctx.default_return_type unchanged instead of asserting — the same pattern already used by the two earlier guard clauses in this function. This lets normal type checking continue rather than crashing.

Testing

Added a regression test in test-data/unit/check-singledispatch.test (testDontCrashWhenSingledispatchIsShadowedByUserFunction) that defines a module named functools.py with its own singledispatch function, matching the issue's exact repro. Verified this test:

• crashes with the pre-fix code (reproduces the reported AssertionError)
• passes cleanly with the fix

Ran the targeted suite: pytest mypy/test/testcheck.py -k singledispatch → 16 passed, 2 xfailed (pre-existing, unrelated).

Also manually verified the original repro script from the issue no longer crashes and reports Success: no issues found in 1 source file.

[github-actions]

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