[reginaldalfret]

Summary

Fixes #1217.

In Python's ast module, keyword argument names in Call(func, args, keywords=[keyword(arg="kwarg", value=...)]) are plain identifiers (str), not expression nodes, and do not carry an expr_context (ast.Load, ast.Store, etc.).

In LibCST, Arg.keyword is an Optional[Name]. Previously, ExpressionContextVisitor did not implement visit_Arg. Because default traversal visits all child fields, visit_Name was invoked on Arg.keyword and mistakenly assigned ExpressionContext.LOAD to the argument name. This produced unexpected results for codemods and linters inspecting unqualified variable loads (e.g. interpreting kwarg=1 as a load of kwarg).

Changes

1. libcst/metadata/expression_context_provider.py:
   • Added visit_Arg(self, node: cst.Arg) -> bool to ExpressionContextVisitor, ensuring only node.value is visited for expression context resolution while leaving node.keyword uncontexted (matching the rationale for Attribute.attr already documented in ExpressionContextProvider).
2. libcst/metadata/tests/test_expression_context_provider.py:
   • Added unit test test_call_keyword_argument verifying that in fn(kwarg=x, other=1), fn and x receive ExpressionContext.LOAD, whereas kwarg and other do not receive an expression context.

Verification

• Ran unit tests:

  python -m unittest libcst.metadata.tests.test_expression_context_provider
  python -m unittest discover -s libcst.metadata.tests
  python -m unittest discover -s libcst.matchers.tests

  All 23 tests in test_expression_context_provider, 177 tests in libcst.metadata.tests, and 144 tests in libcst.matchers.tests passed cleanly.
• Verified formatting using ufmt check.