Narrow types based on non-strict equality with constant types

src/Analyser/TypeSpecifier.php

@@ -38,6 +38,7 @@
 use PHPStan\Type\ArrayType;
 use PHPStan\Type\BooleanType;
 use PHPStan\Type\ConditionalTypeForParameter;
+use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\Constant\ConstantArrayTypeBuilder;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\Constant\ConstantIntegerType;
@@ -1469,15 +1470,24 @@ private function findTypeExpressionsFromBinaryOperation(Scope $scope, Node\Expr\
 			$leftExpr = $leftExpr->getExpr();
 		}
 
+		$isNativeConstFetch = static function (Expr $expr): bool {
+			if (!$expr instanceof ConstFetch) {
+				return false;
+			}
+
+			$loweredConstName = strtolower($expr->name->toString());
+			return in_array($loweredConstName, ['true', 'false', 'null'], true);
+		};
+
 		if (
 			$leftType instanceof ConstantScalarType
-			&& !$rightExpr instanceof ConstFetch
+			&& !$isNativeConstFetch($rightExpr)
 			&& !$rightExpr instanceof ClassConstFetch
 		) {
 			return [$binaryOperation->right, $leftType];
 		} elseif (
 			$rightType instanceof ConstantScalarType
-			&& !$leftExpr instanceof ConstFetch
+			&& !$isNativeConstFetch($leftExpr)
 			&& !$leftExpr instanceof ClassConstFetch
 		) {
 			return [$binaryOperation->left, $rightType];
@@ -1809,6 +1819,26 @@ public function resolveEqual(Expr\BinaryOp\Equal $expr, Scope $scope, TypeSpecif
 				);
 			}
 
+			$looseyType = new UnionType([
+				new NullType(),
+				new ConstantBooleanType(true),
+				new ConstantBooleanType(false),
+				new ConstantIntegerType(0),
+				new ConstantIntegerType(1),
+				new FloatType(), // equality on floats is not usefull
+				new ConstantStringType(''),
+				new ConstantStringType('0'),
+				new ConstantStringType('1'),
+				new ConstantArrayType([], []),
+			]);
+
+			if ($context->true()
+				&& ($exprNode instanceof Expr\Variable || $exprNode instanceof ConstFetch)
+				&& !$looseyType->isSuperTypeOf($constantType)->yes()
+			) {
+				return $this->specifyTypesInCondition($scope, new Expr\BinaryOp\Identical($expr->left, $expr->right), $context, $rootExpr);
+			}
+
 			if (
 				$exprNode instanceof FuncCall
 				&& $exprNode->name instanceof Name

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1422,6 +1422,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-5961.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10189.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10317.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/loose-equal-consts.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10302-interface-extends.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10302-trait-extends.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10302-trait-implements.php');

tests/PHPStan/Analyser/data/loose-equal-consts.php

@@ -0,0 +1,92 @@
+<?php declare(strict_types=1);
+
+namespace LooseEqualConsts;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	public function vars($mixed): void
+	{
+		if ($mixed === 'backend') {
+			assertType("'backend'", $mixed);
+		}
+
+		if ($mixed == 'backend') {
+			assertType("'backend'", $mixed);
+		}
+		if ($mixed == 500) {
+			assertType("500", $mixed);
+		}
+		if ('backend' == $mixed) {
+			assertType("'backend'", $mixed);
+		}
+		if (500 == $mixed) {
+			assertType("500", $mixed);
+		}
+		if ($mixed == 'backend' || $mixed == 'frontend') {
+			assertType("'backend'|'frontend'", $mixed);
+		}
+
+		if ($mixed == '1') {
+			assertType("mixed", $mixed);
+		}
+		if ($mixed == '0') {
+			assertType("mixed", $mixed);
+		}
+		if ($mixed == 1) {
+			assertType("mixed", $mixed);
+		}
+		if ($mixed == 0) {
+			assertType("mixed", $mixed);
+		}
+
+		// equality on floats is not usefull
+		if ($mixed == 1.0) {
+			assertType("mixed", $mixed);
+		}
+		if ($mixed == 0.0) {
+			assertType("mixed", $mixed);
+		}
+		if ($mixed == 123.0) {
+			assertType("mixed", $mixed);
+		}
+
+		if ($mixed == true) {
+			assertType("mixed~0|0.0|''|'0'|array{}|false|null", $mixed);
+		}
+		if ($mixed == false) {
+			assertType("0|0.0|''|'0'|array{}|false|null", $mixed);
+		}
+		if ($mixed == null) {
+			assertType("0|0.0|''|'0'|array{}|false|null", $mixed);
+		}
+
+		if ($mixed == []) {
+			assertType("mixed", $mixed);
+		}
+
+		if ($mixed !== 'backend') {
+			assertType("mixed~'backend'", $mixed);
+		}
+		if ($mixed != 'backend') {
+			assertType('mixed', $mixed);
+		}
+	}
+
+	public function constants(): void
+	{
+		if (APP_NAME === 'backend') {
+			assertType("'backend'", APP_NAME);
+		}
+		if (APP_NAME == 'backend') {
+			assertType("'backend'", APP_NAME);
+		}
+		if (APP_NAME !== 'backend') {
+			assertType('*ERROR*', APP_NAME);
+		}
+		if (APP_NAME != 'backend') {
+			assertType('*ERROR*', APP_NAME);
+		}
+	}
+}