Declared return type does not widen literal template arguments inside a closure with a body (unresolvedTemplateArguments) #15415

Bug report

With bleedingEdge on 2.3.0, a generic call inside a closure with a body keeps its literal template arguments. The declared return type does not widen them. In an arrow function, it does.

In the snippet, keyBy() infers TNewKey as 'negative'|'positive'. The method declares @return Coll<string, int>, and TKey is invariant:

• direct() returns the call: no error.
• arrowFunction() returns it through run(fn (): Coll => ...): no error.
• blockClosure() returns it through run(function () use ($c): Coll { return ...; }): return.type, because the result is Coll<'negative'|'positive', int>.

There is no error without bleedingEdge, with featureToggles.unresolvedTemplateArguments: false, or on 2.2.17. It still happens on 2.3.x at f9a915969a9710d6e7f4c1ae3e9249b015115753.

Laravel's Collection has an invariant TKey too. I found this with keyBy() inside once(function () { ... }), and with a mapWithKeys() callback with a body that returns an inner collection.

Code snippet that reproduces the problem

https://phpstan.org/r/a58bbb26-3d4f-44a3-a9d3-6faf8973390b

Expected output

No error for blockClosure(), the same as for arrowFunction().