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Dataclass field defaults cannot be expressions

Today

@dataclass
class P:
    x: int = 2 + 3



error: a dataclass field default must be a scalar literal,
field(default_factory=list/dict) or a zero-arg lambda — an expression default is
not evaluated, and silently using only its first token would be worse than
refusing it

Until that check landed this compiled, and P().x was 2: the scalar path claimed the first token and the rest of the line was skipped in silence. The refusal is not the goal, it is the honest interim.

Why the plain-class route already works and this one does not

A non-literal class attribute on an ordinary class is evaluated ONCE at the class statement into a hidden $clsattr.<Class>.<name> global, and construction copies it in. A dataclass default cannot reuse that: it travels as a FOLDED CONSTANT in the PyDc* tables (PyDcKind / PyDcIVal / PyDcSOff), which is what PyEmitDataclassCtor reads to build the generated constructor. There is no slot for "an expression evaluated at class-definition time".

Shape of the fix

The same hidden-global mechanism, plus a way for the PyDc tables to name it:

  1. a PyDcSym parallel entry holding the hidden global's symbol (-1 = none), the way ProcParamDefaultSym was added for parameter defaults
  2. PyParseClass evaluates the expression at the class statement — it already does exactly this for a plain class attribute (PyEmitClassAttrExpr)
  3. PyEmitDataclassCtor emits a read of that global instead of the constant

Python's own semantics fall out of this and are worth stating, because they are the reason a fresh-per-construction shortcut would be wrong: a dataclass default is evaluated ONCE, at class definition — which is exactly why field(default_factory=...) exists and why a bare mutable default is an error in CPython.

Gate

A .npy diffed against CPython: an int expression default, a string concatenation default, a default naming a module global, all read from several constructions to prove the value is shared and evaluated once; plus the existing field(default_factory=...) and scalar-literal tests green, and test/test_nilpy_dataclass_expr_default_fail.npy flipped from a refusal to a value when this lands.