[ASDAlexander77]

Problem

boolean && (Class | null) used as a condition failed to compile:

function use(enabled: boolean, handle: Handle | null): number {
    if (enabled && handle) {        // error: can't instantiate function. '___cast'
        return handle.value;        //        not all generic types can be identified
    }
    return 0;
}

Reported in docs/bugs/native-ts-graphics-findings.md, issue 6 (docs/bugs/ntg_and_nullable_class.ts). The problem isn't && itself: any truth test of a union of null and two or more other types failed the same way, e.g. if (x) with x: boolean | string | null. enabled && handle simply has the type boolean | Handle | null.

Cause

Casting such a union to boolean goes through castFromUnion, which generates a __cast<T, U> function with one if (typeof t == '...') return t; per member. For the null member, return t casts null to boolean, which strict-null mode refuses ("can't cast from 'null' to 'boolean'"), so the instantiation fails. (That first error is also reported at a wrong location; that's a separate PR.)

Once that compiled, a second bug showed up: when a Handle | null holding null is converted into boolean | Handle | null, the value keeps the class tag. The class member's if (t instanceof Handle) then read the vtable through the null pointer and crashed.

Fix

In the generated cast, when the target is boolean:

• a null or undefined member returns false (the same thing castFromAny already does for an any);
• with a single class member, the class member returns t. A class tests as non-null, so a null instance is false and nothing is dereferenced.

Not covered: a union with two or more classes plus null still dereferences a null instance in its instanceof chain. That was already the case before this PR, and it needs a different shape of generated code.

Testing

• New 00union_null_truthiness.ts (compile + JIT + rc/none corpus): the issue's function (including handle narrowed inside the block), the reversed handle && enabled, boolean | string | null, !x on boolean | number | null, and e && s ? 1 : 0. Under the current release it doesn't compile.
• docs/bugs/ntg_and_nullable_class.ts now prints 7, done.
• Full ctest: 3919/3919 pass.

🤖 Generated with Claude Code