Structural pattern matching data corruption bug

[JohnLockwood]

Bug report

Your environment

• CPython versions tested on: 3.10.2, 3.11.0a6
• Operating system and architecture: macOS Monterey, Version 12.3.1. Mac mini running M1, Apple silicon.

Steps to reproduce:

Run the code in bug.py, attached.

bug.zip

The output on my Mac mini is:

% python bug.py
Cheese Sandwich

Search for 'match' in the code. Notice that one of the terms matches
the only str in the list, "Cheese Sandwich". Later we create a dataclass
of type PersonHeight. On the last line of the file, we print the type
of one of the fields in a PersonHeight object. Note that it is not <class 'str'> as it should be,
but "Cheese Sandwich"

If we comment out the match expression, we get the correct behavior
(see comment below in code)

[JelleZijlstra]

The code in the sample boils down to this:

match "Cheese Sandwich":
    case str as s: pass

print(str)

The case str as s: pattern binds the "Cheese Sandwich" object to both the name str and the name s. You likely wanted to write case str() as s: instead, which matches to instances of the class str.

This is behaving as expected, but the behavior is indeed surprising. Perhaps we could give a SyntaxWarning for an as-pattern where the LHS is a name-pattern, or this could be handled in linters. (cc @brandtbucher for pattern matching.)