Add regression test for #12820 (#6836)

Author: Amoifr

tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php

@@ -4560,6 +4560,15 @@ public function testBug15251(): void
 		$this->analyse([__DIR__ . '/data/bug-15251.php'], []);
 	}
 
+	#[RequiresPhp('>= 8.0.0')]
+	public function testBug12820(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkNullables = true;
+		$this->checkUnionTypes = true;
+		$this->analyse([__DIR__ . '/data/bug-12820.php'], []);
+	}
+
 	#[RequiresPhp('>= 8.1.0')]
 	public function testClassConstantWildcardArguments(): void
 	{

tests/PHPStan/Rules/Methods/data/bug-12820.php

@@ -0,0 +1,49 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug12820;
+
+class A
+{
+
+	public function a(): void
+	{
+	}
+
+}
+
+class B
+{
+
+}
+
+/** @template-covariant T of A|B = A|B */
+interface FooInterface
+{
+
+	/**
+	 * @phpstan-assert-if-true static<A> $this
+	 */
+	public function isA(): bool;
+
+	/** @return T */
+	public function get(): A|B;
+
+}
+
+/**
+ * @template-covariant T of A|B = A|B
+ * @implements FooInterface<T>
+ */
+abstract class Foo implements FooInterface
+{
+
+	public function other(): void
+	{
+		if ($this->isA()) {
+			$this->get()->a();
+		}
+	}
+
+}