Erroneous "Alternative patterns bind different names" in match statement

[zach-waggoner]

Bug Report

In a match statement, mypy complains that a case block binds different names in alternative patterns, but this is not the case.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.10&gist=68e7e816223c64a1e05a12d5935930c1

subject = ""

match subject:
    case [str(s)] | str(s):
        pass

Expected Behavior

Mypy finds no errors since each alternative pattern binds the same variable (s).

Actual Behavior

main.py:4: error: Alternative patterns bind different names
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.981
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10

[cdce8p]

The issue with the example above is that [str(s)] will never match a subject of type str. This was explicitly mentioned in the pattern matching PEPs IIRC. Thus mypy just skips evaluating that branch and therefore never binds s internally.

Yes it would be possible to still evaluate the inner pattern, I'm just not sure it's worth it. The "solution" here would be to adjust the subject type.

Going to close this as not planned.

[Hnasar]

Can someone reopen this bug? There's a valid repro.
@cdce8p you're correct about str, but can you reopen this bug? There’s another repro of this "Alternative patterns bind different names" bug that seems to be new in 1.18 (before it was silently Any).

d1: dict[str, tuple[str, ...]] = {}
d2: dict[str, str] = {}

match (d1, d2):
    case ({"Symbol": (sym,)}, _) | (_, {"Symbol": sym}):
        reveal_type(sym)

playground

# main.py:5: error: Alternative patterns bind different names [misc]

I added a breakpoint to the source.
This looks like an issue in checkpattern.visit_or_pattern.
Debugging shows one alternative is given Never instead of a tuple type::

> mypy/mypy/checkpattern.py(187)visit_or_pattern()
-> self.msg.fail(message_registry.OR_PATTERN_ALTERNATIVE_NAMES, o.patterns[i])
(Pdb) p pattern_types[0]
PatternType(type=tuple[builtins.dict[builtins.str, tuple[builtins.str]], builtins.dict[builtins.str, builtins.str]], rest_type=tuple[builtins.dict[builtins.str, tuple[builtins.str]], Never], captures={<mypy.nodes.NameExpr object at 0x7f50c2c2b9a0>: builtins.str})
(Pdb) p pattern_types[1]
PatternType(type=Never, rest_type=tuple[Never, Never], captures={})

So mypy concludes the alternatives bind differently, even though both capture sym.

[cdce8p]

@Hnasar This is actually a separate issue. Opened #19993 with a fix.

[A5rocks]

Created #19995 so that we actually have something tracking that and not an already-closed issue!