A cast between unrelated classes needs their fields to fit

[ASDAlexander77]

class A { constructor(public v: number) {} }
class B { constructor(public v: string) {} }
const b: B = new A(1);   // compiled without complaint
print(b.v);              // read a number as a string: 0xC0000005

A cast between two classes where neither extends the other became a plain ts.Cast. #389 covers two specializations of one generic; this covers unrelated classes.

Fix (verifyUnrelatedClassCast, MLIRGenCast.cpp): two unrelated classes are compatible as in TypeScript, by their members. One side's data fields, inherited ones included, must all be found in the other side with types that extend them. Either direction is enough, as for a type assertion, because cast() serves both assignments and <T>x. Otherwise it's an error:

error: type A is not assignable to type B: 'v' is number, not string

The check covers fields only, skipping internal ones such as .vtbl and the storage a derived class embeds for its base. A field read at the wrong type is what crashed.

Not affected:

• Classes related by inheritance, in either direction.
• Two specializations of one generic (A cast between two specializations of one generic is checked #389).
• A type guard narrowing to an unrelated class (isB(a) on an A): the value is checked at run time, and TypeScript narrows it to A & B. addSafeCastStatement keeps its plain cast for that case; 00type_guard_function.ts needs it.

Tests:

• class-cast/unrelated.ts: an error test matched on the message.
• 00class_cast_structural.ts: a same-shape unrelated class and a type guard between classes without fields. Registered for compile, jit and the corpus.

Results:

• The suite passes locally, 3018/3018.
• I rebuilt DefaultLib (release, gc) with this compiler: it builds without errors, and its tests pass 156/156 in both jit and compile modes. The local DefaultLib build was restored afterwards.

Limitation: a cast TypeScript allows between classes whose layouts differ, such as a class to one with a subset of its fields in another order, still compiles to a plain cast of the storage.

🤖 Generated with Claude Code