Comparing an Opaque with Opaque | null no longer overflows the stack

[ASDAlexander77]

Problem

Comparing an Opaque with an Opaque | null crashed the compiler with a stack overflow (0xC00000FD) and printed nothing:

class State {
    current: Opaque | null;
    set(next: Opaque): boolean {
        if (next != this.current) { ... }   // tslang: 0xC00000FD, no diagnostic

Reported in docs/bugs/native-ts-graphics-findings.md, issue 1 (docs/bugs/ntg_opaque_nullable_compare.ts).

Cause

LogicOp (UnaryBinLogicalOrHelper.h), in the branch for a nullable left operand (isNullableTypeNoUnion) and a union on the right that needs no tag, called itself again with left, baseType, right, rightType: the union's base type went in as the left type, and the right type was still the union. That reaches the same branch again, forever. A WinDbg stack of the debug build showed LogicOp<CmpIOp, ..., ne, ...> calling itself until the stack ran out.

The minimal trigger is any reachable opaque != (opaque | null). A function that is never called is dropped before lowering, so it doesn't crash.

Fix

Pass the base type as the right type: left, leftType, right, baseType. The mirrored branch for a union on the left already did it this way.

Testing

• New 00opaque_nullable_compare.ts (compile + JIT): the issue's class, Opaque vs Opaque | null with the union on either side, and a class vs Class | null with ===/!==. The current release crashes with 0xC00000FD.
• Full ctest: 3915/3915 pass.

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