[ASDAlexander77]

Problem

class C { v?: string } stored a plain string. An unassigned c.v read as null (0 for a number), and c.v === undefined was false. The same was true for a constructor parameter field, constructor(public m?: string), so new Error().message === undefined was false.

Type literals were fixed in #427, but classes were left out. An interface's v?: T reads the field at an offset stored in the vtable, using the interface's type. The type-literal fix patches a copy of the vtable for each object, which works because the object is copied. A class instance is shared, so that approach doesn't apply.

Fix

• Storage. A class field v?: T, and a public m?: T constructor parameter field, is stored as optional<T>, i.e. {T, flag} with the value first.
• Class → interface. A conditional interface member v?: T accepts optional<T> storage. The class's interface vtable marks that slot with INTERFACE_OPTIONAL_STORAGE_TAG added to the offset.
• Lowering (InterfaceSymbolRefOpLowering):
  • a -1 slot is absent, as before;
  • a tagged slot reads the field's flag, and an unset flag reads as undefined;
  • a write through the interface sets the flag, and = undefined clears it. mlirGenSaveLogicOneItem marks the access a store goes to with INTERFACE_OPTIONAL_WRITE_ATTR.
• InterfaceSymbolRefOp now reads memory (MemoryEffects<[MemRead]>, previously Pure). A flag read before a write must not stand in for one after it: CSE merged them, and LICM could hoist one out of a loop.

Needs DefaultLib #20

DefaultLib's declarations have to match its compiled library. Its RegExp classes declare ? fields that the implementation stores plain; DefaultLib #20 drops those ?. Error's message? now matches its public message? constructor fields. A DefaultLib built before this PR has the old layouts, so it needs a rebuild: I:\tslang, which your environment points DEFAULT_LIB_PATH at, will fail test-compile-gc-defaultlib-collector until it's rebuilt.

Tests

• New 00class_optional_fields.ts, run as compile and jit. It covers:

  • unassigned fields, directly and through an interface;
  • writing a value and writing undefined through an interface;
  • an inherited field;
  • an initializer;
  • a generic class;
  • a loop through an interface under --opt;
  • a class passed as a type literal;
  • constructor parameter fields.

  It passes under gc, rc and none, with and without --opt.

• Release suite: 3351/3351, against a DefaultLib built from DefaultLib [Question] Speed difference #20 with this compiler.

• That DefaultLib: 159/159 compiled and 159/159 under JIT (release).

• rc: a 2M-iteration loop that writes and clears ? fields directly and through an interface gives the right result. Its memory matches the same loop without ? fields (226 MB vs 235 MB). That loop grows under rc either way, and the growth isn't from this change.

• Not run: Debug builds, x86, -mm=own.

🤖 Generated with Claude Code