[2.1 regression] Incompatible types in capture pattern, can't assign a variable when using string unpacking

[WibblyGhost]

Bug Report

After upgrading to specifically MyPy 2.1.0 from 2.0.0 on Python 3.13.13 causes a Incompatible types in capture pattern (pattern captures type "str", variable has type "Literal['TDMA', 'FDMA', 'NATIVE']") [misc] error.

Looking at your release page (https://mypy.readthedocs.io/en/stable/changelog.html) the only particular ticket that looks familiar is #21405.

To Reproduce

def run_funct(cmd: str, *arguments: str) -> None:
    try:
        start_time = time()
        match cmd, *arguments:
            case ("aff", address, "True" | "true" | "False" | "false" as sd):
                address = group_address(address)
                CONSOLE.aff(address, sd=sd.lower() == "true")
            case ("deaff", address):
                address = group_address(address)
                CONSOLE.deff(address)
            case ("set_phase", "TDMA" | "FDMA" | "NATIVE" as phase):  # ERROR: "str", variable has type "Literal['TDMA', 'FDMA', 'NATIVE']
                CONSOLE.call_rate = call_rate_mapping[phase]

Note: I tried creating a gist in the playground but it doesn't seem to pickup the errors.
Note: This only occurs when as phase is added to set the variable name.

Expected Behavior

MyPy shouldn't be treating the as phase setting as setting to a Literal, or should be able to support comparisons with str. Setting any string through the match statement with tuple[str] unpacking set for a tuple of [str, str] should be allowed to be assigned to a varible

Actual Behavior

mypy src
src/consolesimulator3/scripts/run_console_service.py:78: error: Incompatible types in capture pattern (pattern captures type "str", variable has type "Literal['TDMA', 'FDMA', 'NATIVE']")  [misc]
Found 1 error in 1 file (checked 34 source files)

Your Environment

[tool.mypy]
exclude = [".*tests/"]
check_untyped_defs = true
disallow_any_generics = true
disallow_incomplete_defs = true
disallow_untyped_calls = true
disallow_untyped_decorators = true
disallow_untyped_defs = true
extra_checks = true
implicit_optional = true
no_implicit_reexport = true
no_warn_no_return = true
strict_equality = true
warn_redundant_casts = true
warn_unreachable = true
warn_unused_configs = true
warn_unused_ignores = true

[[tool.mypy.overrides]]
module = ["beaupy.*", "rpyc.*", "construct.*", "sip_parser.*"]
ignore_missing_imports = true

[ilevkivskyi]

cc @hauntsaninja

[hauntsaninja]

Your repro is incomplete (cuts off half way and doesn't define a lot of symbols it uses), could you try sharing something standalone?

[WibblyGhost]

I will attempt to create a reproducible build but it's hard to share complete code snippets due to company code.

It seems that this error is not driven by pre-defined type casting, if I define phase: str, then sure it seems to work and the Literal can get assigned to a str. However, if I define phase: Literal["TDMA", "FDMA", "NATIVE"] then it returns the error again.

There does seem to be some very strange behavior when sticking a reveal_type on my 'Console' class before the actual match statement, then the errors go away. On why the class is impacting the way MyPy is treating the match statement is beyond me.

^ I think this has a play into the error. 🤷‍♂️ ^

Noting that the CONSOLE variable is well type casted in the main() function.

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What do you mean the variable takes type Literal[...] that's already defined above!

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