Compiled: default/optional params on class members (#705 #723 #737 #739 #738)

[nickna]

Closes the compiled-mode default/optional-parameter cluster for class members. The interpreter was already correct; this brings compiled mode to parity. All run in both modes to pin interp/compiler parity.

Fixes #705, #723, #737, #739, #738.

What was broken

In compiled mode, class-member default/optional parameters were mishandled: value-type defaults never fired on methods, generator/async-generator methods ran no default prologue at all, omitted optionals read as CLR null (so typeof was "object"), and a --ref-asm function-expression value-call shifted its arguments.

Changes

#723 / #705 — value-type defaults fire on instance & static methods. ParameterTypeResolver widens a value-type-defaulted param to an object slot so the entry prologue can observe the $Undefined sentinel (a double/bool slot cannot — it coerces to NaN/false).

• Static methods widen directly (non-virtual → always safe).
• Instance methods widen hierarchy-consistently: a position is widened across the whole override group (root declaration + every override) when any member makes it an optional value-type param. Widening the whole group keeps every override’s CLR signature identical, so a derived override that adds a default still lands in the base’s vtable slot — virtual dispatch is preserved (guarded by Override_DerivedAddsDefault_StillOverrides). Reference-type defaults are left alone (they already fire via null-padding without a slot change). Adds a TypeMap.ClassTypes accessor for the override-group walk.

#737 — generator & async-generator method defaults. GeneratorMoveNextEmitter and AsyncGeneratorMoveNextEmitter now run a default-parameter prologue (operating on the hoisted state-machine fields), placed right after the state-dispatch switch so it runs exactly once on initial entry (resume states jump past it; no guard field needed).

#739 — omitted optional reads undefined, not null. A direct instance/static method or constructor call pads an omitted trailing optional param with the $Undefined sentinel instead of CLR null. Centralized as a shared EmitOmittedArgument(slot) helper (object slot → $Undefined, else CLR default) applied across the call-emission sites (sync + async/generator + call handlers).

#738 — --ref-asm value-call no longer shifts arguments. Root cause: the reference-assembly rewrite (external NickNa.PEPacker) strips parameter names, breaking the runtime params[0].Name == "__this" receiver-slot check in $TSFunction, so the synthetic __this slot was miscounted as a real argument. Adds a $ExpectsThis marker attribute (custom attributes survive the rewrite) applied at the function-expression/arrow __this sites; EmitComputeExpectsThis now OR-s IsDefined($ExpectsThis) with the name check — identical behavior under --compile, fixed under --ref-asm.

Bonus fix — generator/async ?? ignored the $Undefined sentinel. The correct #739 padding surfaced a latent state-machine bug: StateMachineEmitHelpers.EmitNullishCoalescing only tested CLR null (brfalse) and missed $Undefined (a non-null reference), so x ?? rhs returned undefined inside a generator/async body. In the real yaml parser this was an infinite loop (*pop(error) { error ?? stack.pop() } with an omitted error never popped the stack). Now mirrors the sync ILEmitter override (also tests isinst $Undefined).

Testing

• Full unit suite green: 13173 passed, 0 failed (incl. the real-package smoke tests — yaml/lodash/minimatch — which exercise the compiled generator/parser paths).
• New cases in ClassMethodDefaultParameterTests (instance/static/generator/async-generator value-type defaults, omitted-optional typeof, the override omit-fires-derived-default case, the generator ?? regression, and an IL-verification guard).
• New ReferenceAssemblyTests.RefAsm_FunctionExpressionValueCall_DoesNotShiftArguments (runtime).
• Conformance: changes are compilation/runtime only (no type-checker changes); validated against the interpreter as the reference across all new cases. The standalone-Test262/TSConformance suites were not re-baselined here.

Follow-up filed

• Compiled: static generator methods (static *m()) fail with "Yield not supported in this context" #778 — compiled static generator methods (static *m()) fail with “Yield not supported in this context” (pre-existing; EmitStaticMethodBody has no IsGenerator routing). Once routed, they’ll pick up the Compiled: value-type default parameters on (virtual) instance & static methods are not applied #737 prologue automatically.