Bug Report

When using nested type aliases that overlap, the deduplication of union items seems to happen before type aliases are resolved, resulting in unions such as str | str being shown. I encountered this while using the sentence_transformers library, hence the type alias names below.

To Reproduce

https://mypy-play.net/?gist=4286145388e11efedb4115a82230b38b

from typing import TypeAlias


TextInput: TypeAlias = str
ImageInput: TypeAlias = str | None

SingleInput: TypeAlias = TextInput | ImageInput


def f(si: SingleInput) -> None:
    print(si)


f(1)

Expected Behavior

The two str union elements are collapsed into one, showing the error message:

main.py:14: error: Argument 1 to "f" has incompatible type "int"; expected "str | None"  [arg-type]

Actual Behavior

The duplication is not resolved, showing

main.py:14: error: Argument 1 to "f" has incompatible type "int"; expected "str | str | None"  [arg-type]

Your Environment

• Mypy version used: 2.4.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.12, 3.14

Additional Details

I believe this is a bug since ignore_last_known_values explicitly says "This will avoid types like str | str in error messages."

This behaviour happens both with TypeAlias and type.