Inconsistent type inference

[que-etc]

TypeScript Version: 2.8.0-dev.20180308

Search Terms:

• inferred type
• narrow

Code

const fn = (options: {foo: 'BAR'}) => {};

const fnOptions = {foo: 'BAR'};

// TS tells that "fnOptions" is not assignable to the "options" parameter.
fn(fnOptions);

// Although following invocation doesn't raise errors.
fn({foo: 'BAR'});

Expected behavior:
When passing fnOptions to fn function inferred type of its foo property should be narrowed to "BAR".

Actual behavior:
At the time of fn invocation inferred type of fnOptions is {foo: string} which renders it not assignable to the options parameter.

Playground Link: link

Related Issues: #17943

[RyanCavanaugh]

There's no guarantee at any given fn(fnOptions) call that someone hasn't written fn.foo = 'baz'; after the object was initialized. Search for "string literal widening" to see other issues on this.