[ASDAlexander77]

The last ray tracer cost: Intersection | null. Two commits:

1. The null test reads the tag's descriptor kind (83659cd, carried over from Ray tracer: parameters captured by copy, direct calls to methods no class overrides #759, which was merged before it was pushed). A tagged union compared with null did a strcmp of its tag with "null". The tag points into its type's descriptor, whose kind (TYPE_KIND_NULL) answers with one load and one compare.
2. A @value type holding an interface or a class needs no tag for its null (63be294). An interface is {vtable, this} and is null when this is; a class is null when its pointer is. So a @value T with such a field holds its own null. MLIRTypeHelper::getValueNullNiche finds the first such field, looking into nested @value fields too, and isUnionTypeNeedsTag answers no, with base type T.
   • Null converted into the union is T zeroed, and == null reads that pointer. typeof and boxing into any check the same pointer. Undefined is still the empty optional, so null and undefined stay apart.
   • -mm=own: a cast between T and its own T | null is a view. A cast into a tagged union becomes CreateUnionInstanceOp, but this one stays a cast.
   • A @value type with only numeric fields keeps its tag.

Ray tracer

Median of 7 runs, ms; every build prints the same checksum.

Build	main	null by kind	+ null in the reference	vs Rust idiomatic (55)
gc	79	76	72	1.3x
rc	120	117	111	2.0x
none	79	76	70	1.3x
own	86	83	77	1.4x

Under none the benchmark now runs as fast as the variant that removes the union by hand (69.9 ms).

Testing

• ctest 6540/6540 (Windows Release).
• Fuzz seeds 82000-82599 clean.
• DefaultLib lib.ts compiles under own, rc, gc and none.
• New: value/value_missing_niche.ts, which is value_missing.ts with an interface and a class in the @value types, runs under gc, rc and none. It covers every spelling and state, typeof, and boxing into any. It doesn't run under own, which rejects that program's records holding a borrowed global; own_raytrace covers own.

Behavior change

Under // @strict-null false, a @value value built with its interface or class field null ({ thing: null, dist: 1 }) now reads as null. This is documented in the spec's "Missing values" section.

Found along the way, already on main

• rc: function mark(d): Mark | null { return d < 0 ? null : { at: d, owner: new Owner(d) }; } (a @value holding a new class instance, returned from a ?: with null) crashes at run time.
• (find(-1) ?? find(3)).dist with find(): Hit | null fails to compile: the ?? result type is a union nested in a union.
• print(find(-1)) of a @value T | null crashes the compiler.

🤖 Generated with Claude Code