Diagnostics and quick info disagree on type of variable

[ghost]

TypeScript Version: 2.7.0-dev.20171102

Code

declare function f(x: number): string[];
declare function each<T>(array: T[], callback: (a: T) => void): void;
each(f(), s => {
    s.toLowerCase();
});

Hover over s.

Expected behavior:

This test should pass:

/// <reference path='fourslash.ts'/>

////declare function f(x: number): string[];
////declare function each<T>(array: T[], callback: (a: T) => void): void;
////each(f(), /**/s => {
////    s.toLowerCase();
////});

verify.quickInfoAt("", "(parameter) s: string");
verify.numberOfErrorsInCurrentFile(1); // At the call to `f()`

Actual behavior:

The type in quick info is string, but we also get a diagnostic about it being {} instead.
In the real situation f() could be arbitrarily far away from the call to each, such as in a function with no explicit return type, so it's very hard to diagnose such an error when quick info tells you everything is fine!

[mhegazy]

Both now say T. which seems right to me.

[ghost]

When I run the fourslash test in the master branch, I still see a compile-time error Property 'toLowerCase' does not exist on type '{}'. (using debug.printErrorList();). But the quick info still says s is a string, not {}.