Prevent intersections between impossible interface methods

[bdsl]

As metnioned in comments of #1686. Code at https://psalm.dev/r/20aa96387e :

<?php

interface IFoo {
    function foo() : string;
}

interface IBar {
    function foo() : int;
}

/** @param IFoo&IBar $i */
function iFooFirst1($i) : int {
    return $i->foo();
}

/** @param IFoo&IBar $i */
function iFooFirst2($i) : string {
    return $i->foo();
}

Psalm output (using commit d51ee03):

ERROR: InvalidReturnStatement - 18:12 - The type 'int' does not match the declared return type 'string' for iFooFirst2

ERROR: InvalidReturnType - 17:27 - The declared return type 'string' for iFooFirst2 is incorrect, got 'int'

Psalm reports issues about iFooFirst2, but nothing about iFooFirst1. There should be issues reported for both iFooFirst1 and iFooFirst2, since no class can implement both interfaces. If the interface types were docbloc types then the class could exist, but it's foo function wouldn't be able return without violating LSP, so again anything expecting a return value from it is likely wrong.

[muglug]

I think Psalm should just prevent the intersection in the first place.

[AndrolGenhald]

This is improved now to show mixed as the return type since the interfaces don't agree, but it would still be nice to show a warning for the impossible intersection as well. I've added a test for this to my type comparison rewrite.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/7d06648934

<?php

interface IFoo {
    function foo(): string;
}

interface IBar {
    function foo(): int;
}

/** @param IFoo&IBar $i */
function iFooFirst1($i): mixed {
    return $i->foo();
}

Psalm output (using commit 2e2099e):

No issues!