#429 polyfills should not override reflected internal PHP symbols

[Ocramius]

Fixes #429

As discovered in #429 by @weirdan, the Stringable interface is not added to symbols, when
the interface is declared by a vendor/ package.

This leads to the AncestorRemoved BC checker (correctly) kicking in, and reporting BC breaks:

Live example: https://github.com/vimeo/psalm/runs/5059860222?check_suite_focus=true

Steps to reproduce:

1. Add `roave/backward-compatibility-check:6.1` to a project that didn't use `symfony/polyfill-php80` and that also has some classes with `__toString()` method.

2. Commit `composer.json` (and `composer.lock` if it's versioned),

3. Run `vendor/bin/roave-backward-compatibility-check --from="HEAD^"`.

Expected

No backward compatibility breaks should be reported as no project files have changed.

Actual

....
[BC] REMOVED: These ancestors of Psalm\Storage\Assertion\InArray have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Storage\FunctionLikeStorage have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Storage\Assertion have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Node\VirtualIdentifier have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Node\VirtualName have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Node\VirtualVarLikeIdentifier have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Node\Name\VirtualRelative have been removed: ["Stringable"]
[BC] REMOVED: These ancestors of Psalm\Node\Name\VirtualFullyQualified have been removed: ["Stringable"]
125 backwards-incompatible changes detected

I also confirmed that it's triggered by symfony/polyfill-php80 by installing BCC into a separate environment and running vendor/bin/roave-backward-compatibility-check --from="HEAD^" with the HEAD commit having just this:

commit 4cc0571866e8405658117613a8106f880116decb (HEAD -> add-bcc-2)
Author: Bruce Weirdan <[email protected]>
Date:   Fri Feb 4 03:39:44 2022 +0200

    Add polyfill

diff --git a/composer.json b/composer.json
index f06872480..567e5d275 100644
--- a/composer.json
+++ b/composer.json
@@ -35,7 +35,8 @@
         "openlss/lib-array2xml": "^1.0",
         "sebastian/diff": "^4.0",
         "symfony/console": "^3.4.17 || ^4.1.6 || ^5.0 || ^6.0",
-        "symfony/filesystem": "^5.4 || ^6.0"
+        "symfony/filesystem": "^5.4 || ^6.0",
+        "symfony/polyfill-php80": "^1.24"
     },
     "provide": {
         "psalm/psalm": "self.version"

As discovered by @Ocramius ( #429 (comment) )
the problem is that symfony/polyfill-php80 declares a non-internal Stringable interface, and BetterReflection
rightfully does NOT use it when deciding whether to add Stringable to a class implementing __toString(), because
only an internal PHP class would be a valid candidate.

Aha, found it. In roave/better-reflection, we only add the Stringable interface to the ancestors in a ReflectionClass of our sources if it is an internal class ( https://github.com/Roave/BetterReflection/blob/d5c9a1afe2ed031c1c86aae58d22f70edb586c67/src/Reflection/ReflectionClass.php#L1113-L1144 )

                    $stringableInterfaceReflection = $this->reflectClassForNamedNode(new Node\Name($stringableClassName));

                    if ($stringableInterfaceReflection->isInternal()) {
                        $interfaces[$stringableClassName] = $stringableInterfaceReflection;
                    }

Obviously, the symfony/polyfill-php80 is not an internal class, and therefore it is skipped. The trouble is that Reflector#reflectClass() finds the one under symfony/polyfill-php80 before looking at the internal source locators.

This could potentially be fixed by sorting the source locators differently (currently trying that).

NOTE: it is OK to ignore Stringable if it didn't come from an internal source locator - that's because nobody knows what nightmarish contraption people may have invented before PHP 7.x.

With this commit, we put the example in a reproducible test scenario