[ASDAlexander77]

Summary

class C { fn?: () => int } seen as interface I { fn?: () => int } was a deliberate compile error since #614 ("field type not matching"). The interface slot points at a method cell shared by every instance. That cell holds a thunk calling this.fn(...), so an unset field read as present and was called.

• Per-instance presence: every method cell is now {method, flag offset}, with the method first so the slot is still a ref to it. Methods and non-optional fields store -1, meaning always present. An optional class field stores the offset from this of its optional's flag: &(null)->fn + SizeOf(T), which is 32 on x64 and 16 on i686 for the test class.
• Lowering: on an optional read, InterfaceSymbolRefOpLowering reads that offset. It loads the instance's flag only when there is one, and gives a null address (undefined) while the flag is clear. Under rc the thunk is never called on an unset field.
• Still an error: a class's optional field cannot fill a required interface field.
• Read-only interface function fields: writing one (m.fn = f) stays "saving to constant object" for every implementer. A read binds this, and a write could not be checked against the object behind the interface. The comments that claimed a write changes the shared cell are corrected, and test-compile-interface-function-field-write pins the error.

Tests

• New 00interface_optional_function_typed_field_class_field.ts (AOT, JIT, rc/none corpus). It covers set, unset, assigned later, assigned undefined, two instances sharing the cell, a field between other fields, a parameter, an inherited field, and a mixed array at one access site.
• RED before: the compile error, then (with only the guard dropped) JIT printed 21 0 instead of 21 -1.
• test-compile-interface-optional-class-function-field and interface-errors/optional_class_function_field.ts are removed.
• Full Release ctest -C Release -j 16 --timeout 300: 4736/4736.

Note: under rc some test closures capture the instance they are stored in. That cycle leaks, which the corpus does not check. Bound methods do not hold their receiver yet (fix-rc-bound-this-more), so the tests keep instances in locals.

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