Conditional + annotation causes erroneous typing

TypeScript Version: 33.9.4 & 4.0.0@beta

Search Terms: Type annotation wrongly overrides generic with errors

Code

const createMatrix = <D extends number, T>(
    dimensions: D,
    initialValues: T | null = null
): Matrix<D, T> => {
    const currentDimensionLength = dimensions;
    const remainingDimensions = dimensions - 1;
    const needsRecursion = remainingDimensions > 0;

    const currentMatrix = Array(currentDimensionLength).fill(initialValues);

    const finalMatrix = needsRecursion
        ? currentMatrix.map(() =>
              createMatrix(remainingDimensions, initialValues)
          )
        : currentMatrix;

    return finalMatrix as Matrix<D, T>;
};

type Matrix<D extends number, T> = D extends 1
    ? T[]
    : D extends 2
    ? T[][]
    : D extends 3
    ? T[][][]
    : any[][][][];

Expected behavior:
Adding type annotations to variables should be compatible with the functions return signature, causing no type errors

Actual behavior:
Assigning an annotated variable to the return value of the function causes an error. The generic T is wrongly inferred to be any of array where depth < D where it should be inferred to only the base type of the nested array. Removing the annotation solves the problem but removes type safety when variable assignment happens after creation.

The problem seems to be that (when annotated) the type is inferred to be the union of each type in Matrix that's preceding the conditional that returns true.

const n1: number[] = createMatrix(1); // <- works - generic T is set to `number`
const n2: number[][] = createMatrix(2); // <- doesn't work - generic T is set to `number | number[]`
const n3: number[][][] = createMatrix(3); // <- doesn't work - generic T is set to `number | number[] | number[][]`

const n4 = createMatrix(2); // <- regular inference works - generic T is `unknown`, return type is unknown[][]
const n5: number[][] = createMatrix(2, 0); // <- works when - initialValues is set - generic T is `number`, return type is number[][]
const n6: unknown[][] = createMatrix(2); // <- works fine, as type T is unknown. A more specific annotation should constrain `unknown` to `number`

Playground Link: Link

Related Issues:
None

Usa la reproducción vinculada de Playground con TypeScript 3.9.4 y 4.0.0-beta, centrándote en las llamadas n2 y n3 anotadas frente a los casos no anotados y explícitamente inicializados. Rastrea la inferencia de tipos y el comportamiento del tipo de retorno condicional hasta identificar el origen de la unión acumulada; se considera terminado cuando las llamadas anotadas producen los tipos de array anidados esperados sin errores.