Implement UnionType->looseCompare()

src/Type/UnionType.php

@@ -601,7 +601,21 @@ public function isScalar(): TrinaryLogic
 
 	public function looseCompare(Type $type, PhpVersion $phpVersion): BooleanType
 	{
-		return new BooleanType();
+		$lastResult = null;
+		foreach ($this->types as $innerType) {
+			$result = $innerType->looseCompare($type, $phpVersion);
+			if ($lastResult === null) {
+				$lastResult = $result;
+				continue;
+			}
+			if ($lastResult->equals($result)) {
+				continue;
+			}
+
+			return new BooleanType();
+		}
+
+		return $lastResult ?? new BooleanType();
 	}
 
 	public function isOffsetAccessible(): TrinaryLogic

tests/PHPStan/Analyser/data/loose-comparisons.php

@@ -519,4 +519,37 @@ public function sayEmptyStr(
 		assertType('false', $emptyStr == $phpStr);
 		assertType('true', $emptyStr == $emptyStr);
 	}
+
+	/**
+	 * @param true|1|"1" $looseOne
+	 * @param false|0|"0" $looseZero
+	 * @param false|1 $constMix
+	 */
+	public function sayConstUnion(
+		$looseOne,
+		$looseZero,
+		$constMix
+	): void
+	{
+		assertType('true', $looseOne == 1);
+		assertType('false', $looseOne == 0);
+		assertType('true', $looseOne == true);
+		assertType('false', $looseOne == false);
+		assertType('true', $looseOne == "1");
+		assertType('false', $looseOne == "0");
+
+		assertType('false', $looseZero == 1);
+		assertType('true', $looseZero == 0);
+		assertType('false', $looseZero == true);
+		assertType('true', $looseZero == false);
+		assertType('false', $looseZero == "1");
+		assertType('true', $looseZero == "0");
+
+		assertType('bool', $constMix == 0);
+		assertType('bool', $constMix == 1);
+		assertType('bool', $constMix == true);
+		assertType('bool', $constMix == false);
+		assertType('bool', $constMix == "1");
+		assertType('bool', $constMix == "0");
+	}
 }

tests/PHPStan/Rules/Comparison/ConstantLooseComparisonRuleTest.php

@@ -166,4 +166,16 @@ public function testTreatPhpDocTypesAsCertain(bool $treatPhpDocTypesAsCertain, a
 		$this->analyse([__DIR__ . '/data/loose-comparison-treat-phpdoc-types.php'], $expectedErrors);
 	}
 
+	public function testLooseUnion(): void
+	{
+		$this->checkAlwaysTrueStrictComparison = true;
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/data/loose-comparison-union.php'], [
+			[
+				'Loose comparison using == between 0|1|false and 2 will always evaluate to false.',
+				9,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/loose-comparison-union.php

@@ -0,0 +1,11 @@
+<?php declare(strict_types=1);
+
+namespace LooseCompareUnion;
+
+class HelloWorld
+{
+	public function sayHello(string $s): void
+	{
+		var_dump(preg_match('{[A-Z]}', $s) == 2);
+	}
+}