TypeScript 5.0 Conditional Type Inference Changes for Union Predicates

Goal

Determine how TypeScript 5.0 modifies the inference behavior of distributive conditional types when the predicate is a union that includes a generic type parameter.

Constraints & Uncertainty

• In TS 4.9 a conditional type such as T extends U ? X : Y distributes over each member of U when U is a union, and inference on T can propagate constraints that sometimes resolve to never.
• TS 5.0 introduces stricter control‑flow analysis and altered overload resolution ordering, which may affect which branch is selected when inference yields never.
• Documentation notes a “compatibility boundary” for conditional type inference but does not enumerate the exact version‑to‑version differences.

Open Questions

1. Does TS 5.0 change the distribution of conditional types over union predicates when a generic appears in the predicate?
2. How does overload resolution prioritize branches when inference produces never under the new version?
3. Are there documented migration steps for code that relied on the pre‑5.0 inference behavior?