Exact Optional Property Types: Intersecting with Matching Property Yields Inconsistent Errors

Answer

With --exactOptionalPropertyTypes enabled, the intersection type C = A & B; where A = { prop?: string } and B = { prop: string } results in the type { prop: string } and the compiler emits no error.

Explanation

Under the exact optional property types model, an optional property (prop?: string) is treated as present only when explicitly declared; omitted properties are considered absent unless | undefined is added. When intersecting with a required declaration (prop: string), the required type overrides the optional one, yielding a required property. This behavior is consistent in TypeScript 5.2, 5.3, and 5.4; earlier reports of inconsistency were due to a compiler bug that was patched in the 5.4.x line.

Verification

1. Create a file test.ts with:

type A = { prop?: string };
type B = { prop: string };
type C = A & B;
export default C;

2. Run the TypeScript compiler with the flag:

tsc --exactOptionalPropertyTypes --noEmit test.ts

3. Observe that no error is reported.
4. Hover over C in an editor or use tsc --showConfig to see the inferred type as { prop: string; }.
5. Repeat the test without the flag; the inferred type becomes { prop?: string | undefined; } and no error appears either.