[ASDAlexander77]

Extends #666 to unions. #666 gives an interface J cast to an interface I it does not extend a vtable laid out for I. J | null given to I | null didn't get that remap, because it was a plain cast of the union. The value kept J's vtable, and I read it by its own slot numbers. This happened under every model, gc included:

const h: Hit | null = pick(true);       // Hit { thing; ray; dist }
const d: HasDist | null = h;            // d.dist read h.thing
const a: A | null = new AC();           // A { f(); g() }
const b: B | null = a;                  // B { g() }: b.g() called f

The same union given to I alone, as I | undefined is through its optional, was a plain cast as well. With a class member (C | null → I | undefined), lowering couldn't turn the class pointer into an interface. It warned invalid cast from '!llvm.ptr' to '!llvm.struct<...>'.

Fix

Both unions are stored as their one member, with null as a null pointer in it. So each is the cast of that member (MLIRGenCast.cpp):

• castToNullableUnionType now accepts an interface member as well as a class member (A null class object, null or undefined given to an interface is a null interface whose methods throw #665). J | null → I | null therefore casts J to I, and An interface cast to one it does not extend gets a vtable laid out for it #666 remaps the vtable.
• castToInterfaceSpecialCases handles such a union by casting its member. A null J becomes a null I, with the throwing stub from A null class object, null or undefined given to an interface is a null interface whose methods throw #665.

Tests

• tests/00interface_union_cast_unrelated.ts covers:

  • Hit | null → HasDist | null, for a value and for null, as a local and as an argument;
  • A | null → B | null, a method;
  • I | undefined from an interface and from a class;
  • <I> of J | null;
  • 100 casts in a loop.

  It runs as a compile test and a JIT test under gc, and in the rc/none corpus.

• Tooth: with the compiler change reverted, gc and rc fail with assertion failed: a field through J | null, along with the invalid-cast warning.

Not covered

• own: own rejects returning the literal from pick (return h) with or without this change, and An interface cast to one it does not extend gets a vtable laid out for it #666's interface-to-interface cast has the same limitation. So the test doesn't run under own.
• This branch is based on main before An interface is { vtable, this }: the type tag is the vtable's first slot #668, which makes an interface { vtable, this }. Whichever lands second needs a rebase. The remap doesn't depend on that layout, because it goes through the An interface cast to one it does not extend gets a vtable laid out for it #666 lowering.

Verification

• Windows: 5414/5414.
• Linux: 5396/5396, and the 19 interface-cast tests pass with MALLOC_PERTURB_=165 MALLOC_CHECK_=3.

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