DI: `T | null` constructor param types not narrowed for token resolution

[brandonroberts]

A constructor parameter typed as a class unioned with null (MyService | null) fails token resolution. OXC emits ɵɵinvalidFactoryDep, which throws at instantiation. This is a token-resolution bug — independent of optionality.

Repro (both fail)

// canonical optional-DI pattern — still broken
@Component({ selector: 'x', template: '' })
export class X { constructor(@Optional() private svc: MyService | null) {} }

// no decorator — also broken
export class Y { constructor(private svc: MyService | null) {} }

Actual

ɵfac = function X_Factory(t) { return new (t||X)(i0.ɵɵinvalidFactoryDep(0)); };
// setClassMetadata emits { type: undefined, decorators: [{ type: Optional }] }

Expected

The reference compiler filters null literal type nodes out of the union; if exactly one type remains, it becomes the token. Token resolution and @Optional() are independent — @Optional() only sets injection flag 8.

• @Optional() svc: MyService | null → ɵɵdirectiveInject(MyService, 8)
• svc: MyService | null → ɵɵdirectiveInject(MyService)

Reference: getConstructorParameters() in compiler-cli/src/ngtsc/reflection/src/typescript.ts — filters SyntaxKind.NullKeyword literal type nodes, narrows to the sole remaining type. undefined is intentionally not stripped.

Scope note: Genuinely ambiguous unions (A | B, two class types) should produce a compile diagnostic ("not supported as injection token"), not a silent runtime failure — tracked separately under diagnostics.

Impact

Hard runtime crash. Breaks the standard @Optional() x: T | null optional-DI pattern.

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Researched and drafted with Claude Code, cross-checked against the Angular reference compiler source.