For min/max functions a default value changes the key function type

[kaste]

Related but maybe not the same #3354

Given

from typing import List, Tuple
l: List[Tuple[int, str]] = []
v = max(l, key=lambda item: item[0], default=None)
reveal_type(v)

mypy complains about the item[0] usage in the key function:

Value of type "Optional[Tuple[int, str]]" is not indexable
Revealed type is 'Union[Tuple[builtins.int, builtins.str], None]'

Also interesting that it shows two different representations for the inferred type.
For user ergonomics the revealed type should be 'Optional[Tuple[int, str]]' as well, no?

[gvanrossum]

Maybe should this be closed here and opened in typeshed instead? @srittau.

Agreed on using the same representation for types; can you file that separately? Or send in a PR if you can.

[kaste]

Well, changing reveal_type is probably easy but then we get approx. 1.000.000 test failures. E.g.

Expected:
  main:10: error: Revealed type is 'builtins.int' (diff)
  main:11: error: Revealed type is 'Tuple[builtins.int, builtins.int]' (diff)
  main:13: error: Revealed type is 'Literal[1]' (diff)
  main:14: error: Argument 1 to "force2" has incompatible type "Tuple[int, in...
  main:14: error: Revealed type is 'Tuple[builtins.int, builtins.int]' (diff)
Actual:
  main:10: error: Revealed type is "int"        (diff)
  main:11: error: Revealed type is "Tuple[int, int]" (diff)
  main:13: error: Revealed type is "Literal[1]" (diff)
  main:14: error: Argument 1 to "force2" has incompatible type "Tuple[int, in...
  main:14: error: Revealed type is "Tuple[int, int]" (diff)

Say, I find my regex-ninja in me, the diff of the PR would be 💣.

[srittau]

Original issue was fixed in python/typeshed#2833.

[kaste]

I think a 8 lines diff still counts as contribution to python core. 🙌