Constrained types is not assignable to constrainer union types, and failed to narrow its type

TypeScript Version:

master

Code

function f<T extends string[] | {}>(a: T) {
  if (a instanceof Array) {
    a = a;
  }
}

Expected behavior:

$ node built/local/tsc.js index.ts

Actual behavior:

$ node built/local/tsc.js index.ts
index.ts(3,5): error TS2322: Type 'any[]' is not assignable to type 'T'.

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