Address review feedback

- Use a temp variable for the swap normalization. Tuple-unpack form
  (lhs, rhs = rhs, lhs) interacted badly with mypy's narrowing in
  mypyc-compiled mypy, producing a runtime IndexExpr-vs-StrExpr cast
  failure (mypy#21586). Workaround per @p-sawicki on PR #21579.

- Drop test_any_dispatch_uses_generic_path. The 'Any' dispatch still
  calls the mypyc-compiled eq_comma, which has the specialization, so
  this test was not exercising the unspecialized path as claimed. The
  IR golden pins the specialized lowering, and eq_two_chars / eq_empty
  cover the fall-through behavior.

Author: VaggelisD

mypyc/irbuild/expression.py

@@ -992,7 +992,8 @@ def try_specialize_str_index_compare(
     """
     # Normalize so the IndexExpr is on the left.
     if isinstance(rhs, IndexExpr) and not isinstance(lhs, IndexExpr):
-        lhs, rhs = rhs, lhs
+        tmp = lhs
+        lhs, rhs = rhs, tmp
     # Shape: s[i] {==, !=} "x" where "x" is exactly one codepoint.
     if (
         not isinstance(lhs, IndexExpr)

mypyc/test-data/run-strings.test

@@ -1414,8 +1414,6 @@ def test_isdigit_strings() -> None:
     assert not "\U0001d7ce!".isdigit()
 
 [case testStrIndexEqLiteralSpecialize]
-from typing import Any
-
 from testutil import assertRaises
 
 # The specializer fires on the AST shape `IndexExpr == StrLiteral` (or the
@@ -1475,11 +1473,3 @@ def test_out_of_range_raises_indexerror() -> None:
         eq_comma(s, 3)
     with assertRaises(IndexError):
         eq_comma(s, -4)
-
-def test_any_dispatch_uses_generic_path() -> None:
-    # Going through `Any` routes through the interpreted wrapper, which
-    # uses the unspecialized lowering. Confirms the str surface still
-    # works for callers that bypass the specializer.
-    f: Any = eq_comma
-    assert f("hello,world", 5) is True
-    assert f("hello", 0) is False