TypedArray constructor overload behaves incorrectly(?)

[alexburner]

TypeScript Version: 2.6.1-insiders.20171019

Search Terms:
TypedArray Float32Array ArrayBufferView constructor overload

Code

class Broken {
  length: number;
  array: Float32Array;
  constructor(arg: number | Float32Array) { 
    this.array = new Float32Array(arg) // arg throws error
    this.length = this.array.length
  }
}

class Works {
  length: number;
  array: Float32Array;
  constructor(arg: number | Float32Array) { 
    if (typeof arg === 'number') {
      this.array = new Float32Array(arg)
      this.length = this.array.length
    } else {
      this.array = new Float32Array(arg)
      this.length = this.array.length
    }
  }
}

Expected behavior:
I'd expect the constructor for class Broken to not throw an error, as TypedArray can accept both numbers or TypedArray instances. From the MDN docs:

new TypedArray(); // new in ES2017
new TypedArray(length);
new TypedArray(typedArray);
new TypedArray(object);
new TypedArray(buffer [, byteOffset [, length]]);

where TypedArray() is one of:

Int8Array();
Uint8Array();
Uint8ClampedArray();
Int16Array();
Uint16Array();
Int32Array();
Uint32Array();
Float32Array();
Float64Array();

Actual behavior:
The constructor for class Broken throws a type error:

Argument of type 'number | Float32Array' is not assignable to parameter of type 'ArrayLike<number> | ArrayBuffer'.
  Type 'number' is not assignable to type 'ArrayLike<number> | ArrayBuffer'.
(parameter) arg: number | Float32Array

This type error can be circumvented by wrapping duplicate versions of the same code in a typeguard, as shown in the second class Works example.

Playground Link: https://www.typescriptlang.org/play/index.html#src=class%20Broken%20%7B%0D%0A%20%20length%3A%20number%3B%0D%0A%20%20array%3A%20Float32Array%3B%0D%0A%20%20constructor(arg%3A%20number%20%7C%20Float32Array)%20%7B%20%0D%0A%20%20%20%20this.array%20%3D%20new%20Float32Array(arg)%20%2F%2F%20arg%20throws%20error%0D%0A%20%20%20%20this.length%20%3D%20this.array.length%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Aclass%20Works%20%7B%0D%0A%20%20length%3A%20number%3B%0D%0A%20%20array%3A%20Float32Array%3B%0D%0A%20%20constructor(arg%3A%20number%20%7C%20Float32Array)%20%7B%20%0D%0A%20%20%20%20if%20(typeof%20arg%20%3D%3D%3D%20'number')%20%7B%0D%0A%20%20%20%20%20%20this.array%20%3D%20new%20Float32Array(arg)%0D%0A%20%20%20%20%20%20this.length%20%3D%20this.array.length%0D%0A%20%20%20%20%7D%20else%20%7B%0D%0A%20%20%20%20%20%20this.array%20%3D%20new%20Float32Array(arg)%0D%0A%20%20%20%20%20%20this.length%20%3D%20this.array.length%0D%0A%20%20%20%20%7D%0D%0A%20%20%7D%0D%0A%7D

Related Issues:
Suggestion: a built-in TypedArray interface
Empty constructors for TypedArrays are not allowed
TypedArrays missing signature

[DanielRosenwasser]

Looks like a duplicate of #14107, where you want to see if a union type can satisfy each overload.

[alexburner]

Whoops, you are correct, I got caught up in the TypedArray and missed the deeper "overload + union arguments" angle.