Preserve types of functions registered with singledispatch #10756
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Add a test that makes sure that we preserve the type of registered functions and we correctly type check errors when registered functions are called directly.
Make mypy keep track of the declared type of registered functions, which allows properly type checking code that calls those registered functions directly, without going through the singledispatch function.
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msullivan
reviewed
Jul 2, 2021
Instead of deciding what to return in register_function, returning ctx.default_return_type when there's an error with the register call, just return the function directly from the callback. That allows calls to registered functions to be type checked properly even when registered incorrectly, and makes the code simpler and easier to test.
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Diff from mypy_primer, showing the effect of this PR on open source code: edgedb (https://github.com/edgedb/edgedb.git)
- edb/ir/ast.py:759:12: error: "None" not callable
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Description
When calling a function that has been registered as an implementation for a singledispatch function, currently, mypy doesn't type check arguments to those functions when those functions are called directly (instead of through the main singledispatch function). This fixes that by preserving the type of the registered function even after the register decorator has been used.
Test Plan
I added a test that made sure that we report errors when using registered functions directly.