Ignore `str-format` error when object implements `__format__()` with custom type

[pmhahn]

Bug Report

I have inherited a project, where one class implements __format__(self, format_spec: str) -> str to implement a custom type for formatting. Running mypy on that code throws a false-positive error:

mypy-format.py:5 error: Unsupported format character "l" [str-format]

To Reproduce

class MyType:
    def __format__(self, format_spec: str) -> str:
        return "a" if format_spec == "l" else str(self)

print("{:l}".format(MyType()))

Expected Behavior

For a type implementing __format__ mypy/checkstrformat.py should not output any error.

Actual Behavior

mypy-format.py:5 error: Unsupported format character "l"  [str-format]

Your Environment

• mypy 1.11.2 (compiled: yes)
• Python 3.11.2

Links

• #17714
• #11723
• How does type-checking work for datetime.datetime, which implements a rich set of custom formatters?
• string - Format Mini Language