`mypy` does not infer that a string `Literal` is a `str` in the context of a `dict` or `Mapping`

[msto]

Bug Report

Mypy reports an arg-type error when passing a dictionary with Literal keys to a function typed to receive a dictionary with str keys, even when all such literals are strings.

To Reproduce

from collections.abc import Mapping
from typing import Literal


def foo(mapping: dict[str, str]) -> None:
    pass


def bar(mapping: Mapping[str, str]) -> None:
    pass


mapping: dict[Literal["key"], str] = {"key": "value"}

foo(mapping)  # E: Argument 1 to "foo" has incompatible type "dict[Literal['key'], str]"; expected "dict[str, str]"  [arg-type]
bar(mapping)  # E: Argument 1 to "bar" has incompatible type "dict[Literal['key'], str]"; expected "Mapping[str, str]"  [arg-type]

Expected Behavior

Since the literal "key" is a string, I expected mypy to report no error here.

Actual Behavior

Mypy reported the arg-type errors described in the above reprex.

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):

   strict_optional             = true
   strict_equality             = true
   check_untyped_defs          = true
   disallow_incomplete_defs    = true
   disallow_untyped_calls      = true
   disallow_untyped_decorators = true
   disallow_untyped_defs       = true
   no_implicit_optional        = true
   warn_no_return              = true
   warn_redundant_casts        = true
   warn_return_any             = true
   warn_unreachable            = true
   warn_unused_configs         = true
   warn_unused_ignores         = true
   enable_error_code           = "ignore-without-code"
  

• Python version used: 3.12.8

[sterliakov]

Mapping is invariant in key type, so Mapping[Literal['a'], str] can't be assigned to Mapping[str, str].

See, for example, an old python/typing#273 for discussion of this limitation.