callable(T): T doesn't preserve argument types on return types after invocation

[mpyw]

Bug report

The following simple snippet works:

interface Iface {
}
class Impl implements Iface {}

/**
  * @template T of Iface
  * @param T $value
  * @return T
  */
function check($value) {
    return $value;
}

\phpstan\dumpType(new Impl);         // Dumped type: Impl
\phpstan\dumpType(check(new Impl));  // Dumped type: Impl <-- Correct

But the following doesn't work:

interface Iface {
}
class Impl implements Iface {}

/**
  * @template T of Iface
  * @param callable(T): T $func
  */
function check($func): void {
    \phpstan\dumpType(new Impl);        // Dumped type: Impl
    \phpstan\dumpType($func(new Impl)); // Dumped type: T of Iface (function check(), argument) <-- Wrong
                                        // Parameter #1 $ of callable callable(T): T expects T of Iface, Impl given.
}

Code snippet that reproduces the problem

https://phpstan.org/r/552156a3-7933-43b9-9d56-076043d96034

Expected output

// Dumped type: Impl
// Dumped type: Impl

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[mpyw]

After the review, it looks to be not a bug, thanks

[mpyw]

But are there any nice workaround for this?

[mpyw]

Possibly equivalent in TypeScript:

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAmPJyDeyBQzkwD2hAXMiAK4C2ARtDgL44IA2cAzu8gJJUAOLVPxYQqEcFzSIUmPMj4UaLYAgLFkAXmQBGANyMcOGBRAIwwQiGQIAFhAQBrABQwQZADwAVZBAAekEAATSXQIAD4nUAUwMk8ASk0w5HiyADdCYECsOQRLdjBUEGjNcggAdx5hfXwagHpa5AA9AH4cvILCCjBirVdIoq646prkeqbWpiA

interface iface { 
  foo: number
}
class Impl implements iface {
  public foo = 1;
}

function check(fn: <T extends iface>(input: T) => T): void {
  const input = new Impl;
     // ^? const input: Impl
  const output = fn(input);
     // ^? const output: Impl
}

The point of view is Generic parameters for callable

[ondrejmirtes]

With this signature:

/**
  * @template T of Iface
  * @param callable(T): T $func
  */
function check($func): void {

You're making the function check generic, but you're not making the callable generic. Here's a feature request: #8964

[github-actions]

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