Regression on Promise Type Checking in Latest Insiders

[mjbvz]

From: microsoft/vscode#25215

TypeScript Version: 2.3.1-insiders0420

Code

Using es2015.lib.d.ts:

function foo(x: Promise<boolean>) {}

foo(new Promise(() => true))

Expected behavior:
This compiles fine with TS 2.2.2 and with the older insiders 2.3.1 release.

Actual behavior:
With the current insiders, we get: message: 'Argument of type 'Promise<{}>' is not assignable to parameter of type 'Promise<boolean>'. Type '{}' is not assignable to type 'boolean'.'

This is causing upwards of 230 compiler errors in the vscode codebase

Possible related to #15104

[mhegazy]

This is an intentional change to tighten the promise compatibility checking. in your call back, inference fails, and the result is {}; previously it was allowed to assign Promise<{}> to a Promise<boolean>, which is unsafe.

See https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#covariance-in-callback-parameters for more details

[mjbvz]

Thanks. This change seems to make sense even though it will be a little painful to update our codebase for it. It was not entirely clear to me why the code was compiling in ts 2.2.2 case when I looked at the types and thought more about the problem.

// cc @bpasero