Do not let placeholder constants of intersection members override declared constants

`IntersectionType::getConstant()` returned the constant of the first member
answering `hasConstant()` with yes. `T of mixed` does so for every name with
a `DummyClassConstantReflection` declared on `stdClass`, so on `T&Foo` the
result depended on the order of the members. Placeholder constants are now
used only when no member declares the constant.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NBzALCg92yf3LXFxxNBzoY

Author: ondrejmirtes

src/Type/IntersectionType.php

@@ -12,6 +12,7 @@
 use PHPStan\Reflection\Callables\CallableParametersAcceptor;
 use PHPStan\Reflection\ClassConstantReflection;
 use PHPStan\Reflection\ClassMemberAccessAnswerer;
+use PHPStan\Reflection\Dummy\DummyClassConstantReflection;
 use PHPStan\Reflection\Dummy\DummyMethodReflection;
 use PHPStan\Reflection\Dummy\DummyPropertyReflection;
 use PHPStan\Reflection\ExtendedMethodReflection;
@@ -804,10 +805,26 @@ public function hasConstant(string $constantName): TrinaryLogic
 
 	public function getConstant(string $constantName): ClassConstantReflection
 	{
+		$dummyConstant = null;
 		foreach ($this->types as $type) {
-			if ($type->hasConstant($constantName)->yes()) {
-				return $type->getConstant($constantName);
+			if (!$type->hasConstant($constantName)->yes()) {
+				continue;
+			}
+
+			$constant = $type->getConstant($constantName);
+
+			// a member like T of mixed has every constant only as a placeholder,
+			// it must not override the constant declared by another member
+			if ($constant instanceof DummyClassConstantReflection) {
+				$dummyConstant ??= $constant;
+				continue;
 			}
+
+			return $constant;
+		}
+
+		if ($dummyConstant !== null) {
+			return $dummyConstant;
 		}
 
 		throw new MissingConstantFromReflectionException($this->describe(VerbosityLevel::typeOnly()), $constantName);

tests/PHPStan/Type/IntersectionTypeTest.php

@@ -3,6 +3,7 @@
 namespace PHPStan\Type;
 
 use DateTimeImmutable;
+use DateTimeInterface;
 use DoctrineIntersectionTypeIsSupertypeOf\Collection;
 use Iterator;
 use ObjectTypeEnums\FooEnum;
@@ -1008,4 +1009,11 @@ public function testMethodOfIntersectionWithTemplateMixed(IntersectionType $type
 		$this->assertSame('modifier', $variant->getParameters()[0]->getName());
 	}
 
+	#[DataProvider('dataIntersectionWithTemplateMixed')]
+	public function testConstantOfIntersectionWithTemplateMixed(IntersectionType $type): void
+	{
+		$constant = $type->getConstant('ATOM');
+		$this->assertSame(DateTimeInterface::class, $constant->getDeclaringClass()->getName());
+	}
+
 }