Inferred return types checked against extended (as opposed to specific) declaration's return type.

[Hashbrown777]

TypeScript Version: typescript@3.4.0-dev.20190223

Search Terms: return types accept extra properties

Code

interface IOnlyBob {bob :string};

//this errors as we expect
//const _errors :IOnlyBob = {bob:'hi', steve:'there'};

//this works fine but should error
let _returnsBob :() => IOnlyBob;
_returnsBob = () => ({bob:'hi', steve:'there'});

//I'd expect this to be the way to do the above, if that's what someone actually wanted
let _returnsNotJustBob :<T extends IOnlyBob>() => T;
_returnsNotJustBob = () => ({bob:'hi', steve:'there'} as any); //have to do any, see https://github.com/Microsoft/TypeScript/issues/28154

Expected behavior:
Line 8 should fail.

Type '{ bob: string; steve: string; }' is not assignable to type 'IOnlyBob'.
  Object literal may only specify known properties, and 'steve' does not exist in type 'IOnlyBob'.

Actual behavior:
Works fine.

Playground Link: here

Related Issues:

[RyanCavanaugh]

Duplicate / root-cause #241

[Hashbrown777]

Oh ta! "Widening", I guess if I knew the lingo I'd've found that.
Thanks, I'll watch that space.