fix(enum): fold enum case property fetches in constant expressions

[yuan-dian]

Summary

Enum::Case->value / ->name are valid constant expressions since PHP 8.2
(php/php-src#8625) and native PHP
accepts them as property defaults. TypePHP's declaration validator already
allowed them, but the stub generator's constant-expression evaluator rejected
Expr_PropertyFetch, aborting compilation. This PR makes the stub-side
evaluator fold enum case ->value/->name, bringing the two evaluators back
in sync.

Repro (before)

enum TextRole: int
{
    case Unset = -1;
    case User = 1;
}

class Session
{
    public int $role = TextRole::Unset->value;   // line 11
}

function main(): void {
    var_dump((new Session())->role);
}

Before:

Fatal error: Uncaught RuntimeException: In program.php: Expression at line 11
must be a global, non-magic constant, Expr_PropertyFetch given
  thrown in src/gen_stub.php on line 204     (exit 255)

The class-constant indirection (private const R = TextRole::Unset->value;
then = self::R) fails identically, since the initializer itself is a
PropertyFetch.

After: compiles and the AOT binary prints -1, matching native PHP 8.4.

Root cause

Two constant-expression evaluators were out of sync:

Path	Handles Enum::Case->value?
Declaration validator (src/Transform/ConstantExpressionValidator.php, Expr\PropertyFetch whitelisted)	✅
Declaration evaluator (src/Resolver/ClassConstantValueTrait.php, resolveCompileTimeEnumCaseBackingValue)	✅
Stub/arginfo generator (src/gen_stub.php, EvaluatedValue::createFromExpression fallback)	❌ threw for anything other than ConstFetch/ClassConstFetch

There was zero test coverage of int $x = Enum::Case->value as a property
default anywhere in the repo, which is why the gap survived.

Changes

• src/gen_stub.php (+25) — in the ConstExprEvaluator fallback closure,
  handle Expr\PropertyFetch / Expr\NullsafePropertyFetch before the
  existing "must be a global, non-magic constant" diagnostic:
  • evaluate the receiver; on an EnumCaseRef, fold ->name to the case name
    and ->value to the backing value;
  • nullsafe receiver null → null (mirrors the declaration evaluator);
  • any other enum-case property throws a deliberate
    Undefined enum case property ... error;
  • non-enum property fetches fall through to the unchanged existing
    diagnostic (single source of truth, no message duplication).
• src/Resolver/ClassConstantValueTrait.php (1 word) —
  resolveCompileTimeEnumCaseBackingValue private → public, so stub
  generation reuses the declaration evaluator's exact resolution rules
  (internal enums via constant(), unit enums rejected with
  "has no backing value") instead of duplicating them.
• phpunit/src/EnumCaseValuePropertyDefaultTest.php (new) — unit coverage:
  int-backed ->value, string-backed ->value, ->name, and a negative case
  asserting non-enum property fetches keep the existing diagnostic.
• tests/compiler/object_property/enum-case-value-property-default.phpt
  (new) — end-to-end runtime coverage (int/string ->value, unit-enum
  ->name, enum-typed bare-case control). The --EXPECT-- section was
  validated against native PHP 8.4.

Behavior preserved

• int $x = SomeClass::NON_ENUM->prop in a constant expression → same
  diagnostic as before.
• Bare enum case → scalar-typed property remains a compile error
  (EnumDeclarationRulesTest).
• Enum-typed property + bare case (TextRole $role = TextRole::Unset;)
  unchanged.

Testing

• RED → GREEN: the new unit tests fail on the parent commit with the
  original Expr_PropertyFetch error (3 errors), and pass with the fix
  — OK (4 tests, 8 assertions).
• Enum regression suites (EnumCaseValuePropertyDefaultTest +
  EnumDeclarationRulesTest): OK (30 tests, 57 assertions).
• Full suite: Tests: 3127 — the 14 failures are pre-existing environment
  issues, reproduced identically with only these production edits reverted
  (bash completion / build stats / linker — no enum overlap).
• Repro: --dry exits 0 (was 255); full AOT build succeeds and the binary
  prints -1.
• phpstan: clean (src/gen_stub.php is excluded by phpstan.neon as
  upstream-derived). php-cs-fixer: no new issues.

Note for reviewers

The new PHPT was not executed via run-tests.php --compiler ./tpc because the
checked-in workflow expects a self-hosted ./tpc, and the locally available
one predates this fix; e2e behavior was verified through a full
bin/tpc.php build of the repro instead. Please re-run the PHPT with a fresh
self-hosted compiler.

Commits

• 3368b5b4 fix(enum): fold enum case property fetches in constant expressions
• 5c18ea64 test(compiler): add phpt coverage for enum case property defaults