mypy accepts invalid PEP 604 syntax with forward reference

[Molkree]

Bug Report

PEP 604 introduced the | syntax, however it's not entirely equivalent to the Union syntax. Forward references with | are not supported and result in TypeErrors TypeError: unsupported operand type(s) for |: 'type' and 'str'. Mypy doesn't warn the user about this and accepts | with forward references.

To Reproduce

T = list[int | "T"]

Playground

Expected Behavior

Some kind of error telling me that this syntax is not supported.

Actual Behavior

No errors reported.

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: --strict or no flags, doesn't matter
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11

I originally raised this as a discussion in pyright repo. And since then pyright has implemented microsoft/pyright@6eb8467 to detect this incorrect usage of | (available in release 1.1.285). I have to admit that documentation on this is a bit murky as I was expecting | to be equivalent to Union and I wasn't the only one, see python/cpython#90015. From what I can gather in this CPython issue runtime won't be changed to make this syntax valid and docs should be changed instead (but neither happened yet).

[Molkree]

#11582 might be very close to this and it even was investigated and worked on.