[PHP 8.6] ReflectionParameter::getDocComment() support

[lisachenko]

Refs #221 (part of epic #219).

What

PHP 8.6 allows doc comments on function/method parameters and exposes them through the new native ReflectionParameter::getDocComment(): string|false. This PR implements the same method statically from the AST, so it is available on every supported PHP version, not only on 8.6.

Changes

• src/ReflectionParameter.php — new getDocComment(): string|false (signature-compatible with the PHP 8.6 parent; on 8.5 it is simply an extra method, the 8.5 parent has none).
• tests/Stub/FileWithParameters86.php — new stub with the doc-comment matrix. It contains only ordinary comments, so it parses and loads on every supported PHP version and is included by the test to allow native comparison.
• tests/ReflectionParameterTest.php — dedicated testGetDocComment() parity test, two tests pinning the known trailing-comment gap, plus getDocComment added to the generic getter parity matrix when running on PHP >= 8.6.
• docs/reflection_parameter.md — documents the method and the known limitation.

Resolution strategy

PHP-Parser attaches a doc comment to the Param node itself only when the comment directly precedes the parameter declaration. When the comment sits inside the declaration it ends up on a nested node instead — verified with the bundled nikic/php-parser v5.8.0:

source	node that receives the comment
/** doc */ string $a	Param
/** doc */ #[Attr] string $a	Param
#[Attr] /** doc */ string $a	Identifier (the type)
#[A] /** doc */ #[B] string $a	the second AttributeGroup
int $a = /** doc */ 5	the default value expression

Therefore the implementation scans the whole parameter sub-tree and picks the last doc comment in source order, which is exactly how PHP itself resolves a parameter's doc comment (its lexer remembers the most recent doc-comment token). Non-doc comments (/* … */, // …) never reset it, and the last of several doc comments wins — both verified against the 8.6 binary.

Verified parity against PHP 8.6.0beta1

Every parameter in the new stub returns byte-for-byte the same value as native reflection: leading doc comments, variadic, by-reference, nullable, union-typed, defaults (scalar/array), promoted constructor properties (public/protected), static methods, doc comment before and after attributes, several consecutive doc comments, doc comment followed by a block comment, plus the negative cases (no comment, block comment, line comment). Function/method doc comments correctly do not leak into the first parameter.

Also checked: native var_dump()/(array) cast of ReflectionParameter on 8.6 still exposes only name, so __debugInfo() / InternalPropertiesEmulationTrait needed no change. testCoverAllMethods for ReflectionParameter now passes on 8.6 (it would otherwise report getDocComment as missing).

Known parity gap

A doc comment written after the parameter it documents is reported by native reflection as belonging to that parameter, because PHP records the last doc-comment token seen while reducing the parameter rule. PHP-Parser attaches every comment to the node that follows it, and a comment sitting between the end of a parameter and the separating comma is discarded entirely, so such a trailing comment never reaches the AST at all:

function lastParameter(string $a /** doc */) {}
// native: $a => '/** doc */'                 engine: $a => false

function nextParameter(string $a /** doc */, string $b) {}
// native: $a => '/** doc */', $b => false    engine: $a => false, $b => false

Note that the comment is not mis-attributed to the following parameter — it is simply lost. This position is pathological and cannot be recovered without access to the token stream (the comma position is not available on the AST). It is documented in docs/reflection_parameter.md, in the stub, and pinned from both sides by testTrailingDocCommentIsAKnownParityGap() and testTrailingDocCommentIsLostWhenAnotherParameterFollows(), which also assert the divergent native behaviour on 8.6 so a future change in php-src or PHP-Parser is noticed.

Local results

check	result
vendor/bin/phpunit (PHP 8.5.9)	OK — 13764 tests, 15417 assertions, 133 skipped, 2 incomplete (pre-existing, ReflectionPropertyTest::testCoverAllMethods)
php8.6 vendor/bin/phpunit (PHP 8.6.0beta1)	OK — 13892 tests, 15567 assertions, 133 skipped, 2 incomplete (same pre-existing one)
vendor/bin/phpstan analyse src --no-progress (level 10)	[OK] No errors — no phpstan.neon changes needed

Baseline on master was 13743 tests on 8.5; the new tests add 21 cases on 8.5 and 149 on 8.6 (the extra ones come from getDocComment joining the generic getter parity matrix).

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[lisachenko]

@claude need to rebase, or merge master here to update checks in CI pipeline naming

[lisachenko]

Done — merged master into this branch (merge commit, no rebase), so the renamed CI checks now apply. The full local gate is green after the merge: PHPUnit on 8.5 (13,770 tests) and 8.6.0beta1 (15,001 tests), PHPStan level 10 clean.

Also brought master into the other five open epic branches (#228–#232) for the same reason — each validated the same way before pushing.

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