Fix supertype checks for conditional types

src/Type/ConditionalType.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\Type;
 
+use PHPStan\TrinaryLogic;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 use PHPStan\Type\Traits\LateResolvableTypeTrait;
 use PHPStan\Type\Traits\NonGeneralizableTypeTrait;
@@ -40,6 +41,16 @@ public function isNegated(): bool
 		return $this->negated;
 	}
 
+	public function isSuperTypeOf(Type $type): TrinaryLogic
+	{
+		if ($type instanceof self) {
+			return $this->if->isSuperTypeOf($type->if)
+				->and($this->else->isSuperTypeOf($type->else));
+		}
+
+		return $this->isSuperTypeOfDefault($type);
+	}
+
 	public function getReferencedClasses(): array
 	{
 		return array_merge(

src/Type/ConditionalTypeForParameter.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\Type;
 
+use PHPStan\TrinaryLogic;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 use PHPStan\Type\Traits\LateResolvableTypeTrait;
 use PHPStan\Type\Traits\NonGeneralizableTypeTrait;
@@ -62,6 +63,16 @@ public function toConditional(Type $subject): Type
 		);
 	}
 
+	public function isSuperTypeOf(Type $type): TrinaryLogic
+	{
+		if ($type instanceof self) {
+			return $this->if->isSuperTypeOf($type->if)
+				->and($this->else->isSuperTypeOf($type->else));
+		}
+
+		return $this->isSuperTypeOfDefault($type);
+	}
+
 	public function getReferencedClasses(): array
 	{
 		return array_merge(

src/Type/Traits/LateResolvableTypeTrait.php

@@ -12,6 +12,7 @@
 use PHPStan\Type\BooleanType;
 use PHPStan\Type\CompoundType;
 use PHPStan\Type\Generic\TemplateTypeMap;
+use PHPStan\Type\NeverType;
 use PHPStan\Type\Type;
 
 trait LateResolvableTypeTrait
@@ -26,7 +27,17 @@ public function accepts(Type $type, bool $strictTypes): TrinaryLogic
 
 	public function isSuperTypeOf(Type $type): TrinaryLogic
 	{
-		return $this->getResult()->isSuperTypeOf($type);
+		return $this->isSuperTypeOfDefault($type);
+	}
+
+	private function isSuperTypeOfDefault(Type $type): TrinaryLogic
+	{
+		if ($type instanceof NeverType) {
+			return TrinaryLogic::createYes();
+		}
+
+		return $this->getResult()->isSuperTypeOf($type)
+			->and(TrinaryLogic::createMaybe());
 	}
 
 	public function canAccessProperties(): TrinaryLogic

tests/PHPStan/Analyser/data/conditional-types.php

@@ -155,6 +155,25 @@ public function testMaybeNever(): void
 		assertType('*NEVER*', $this->maybeNever(1));
 		assertType('void', $this->maybeNever(2));
 	}
+
+	/**
+	 * @return ($if is true ? mixed : null)|false
+	 */
+	abstract public function lateConditional1(bool $if);
+
+	/**
+	 * @return ($if is true ? mixed : null)|($if is true ? null : mixed)|false
+	 */
+	abstract public function lateConditional2(bool $if);
+
+	public function testLateConditional(): void
+	{
+		assertType('mixed', $this->lateConditional1(true));
+		assertType('false|null', $this->lateConditional1(false));
+
+		assertType('mixed', $this->lateConditional2(true));
+		assertType('mixed', $this->lateConditional2(false));
+	}
 }
 
 class ParentClassToInherit

tests/PHPStan/Rules/Methods/MethodSignatureRuleTest.php

@@ -373,7 +373,7 @@ public function testOverridenMethodWithConditionalReturnType(): void
 		$this->reportStatic = true;
 		$this->analyse([__DIR__ . '/data/overriden-method-with-conditional-return-type.php'], [
 			[
-				'Return type (($p is int ? stdClass : string)) of method OverridenMethodWithConditionalReturnType\Bar2::doFoo() should be covariant with return type (($p is int ? int : string)) of method OverridenMethodWithConditionalReturnType\Foo::doFoo()',
+				'Return type (($p is int ? stdClass : string)) of method OverridenMethodWithConditionalReturnType\Bar2::doFoo() should be compatible with return type (($p is int ? int : string)) of method OverridenMethodWithConditionalReturnType\Foo::doFoo()',
 				37,
 			],
 		]);