KOS_OP_MATCH_CLASS_VAR / KOS_OP_MATCH_CLASS_FAST

• Style: EXTRA (LABEL, VAR, RAW) on MATCH_CLASS_VAR, (LABEL, FASTVAR, RAW) on MATCH_CLASS_FAST: label, class, positional field count.

• Stack: [..., e] -> [..., e]; jump to a0 on a miss, e left either way.

• Semantics: e read through Datum must be an instance of the class in the variable a1, read by name on the _VAR form and by fast index on the _FAST form; a non-instance is a miss. The class’s __match_args__ must then hold at least a2 names: a class with no __match_args__, a __match_args__ that is not a tuple, or one shorter than a2 raises TypeError before any attribute of e is read. On Python 3.9 a dataclass or named tuple without __match_args__ reads the equivalent tuple Python 3.10 would generate.

• Compiler form: Emitted by the match compiler for the instance check of a class pattern with positional fields, a2 their count; the _VAR form pre-compile, the _FAST twin once convert_fast has lowered the variable.