Add basic type narrowing in the loose equality with the empty string

src/Analyser/TypeSpecifier.php

@@ -43,6 +43,7 @@
 use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\Constant\ConstantArrayTypeBuilder;
 use PHPStan\Type\Constant\ConstantBooleanType;
+use PHPStan\Type\Constant\ConstantFloatType;
 use PHPStan\Type\Constant\ConstantIntegerType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\ConstantScalarType;
@@ -1949,7 +1950,20 @@ public function resolveEqual(Expr\BinaryOp\Equal $expr, Scope $scope, TypeSpecif
 		if ($expressions !== null) {
 			$exprNode = $expressions[0];
 			$constantType = $expressions[1];
-			if (!$context->null() && ($constantType->getValue() === false || $constantType->getValue() === null)) {
+
+			if (!$context->null() && $constantType->getValue() === null) {
+				$trueTypes = [
+					new NullType(),
+					new ConstantBooleanType(false),
+					new ConstantIntegerType(0),
+					new ConstantFloatType(0.0),
+					new ConstantStringType(''),
+					new ConstantArrayType([], []),
+				];
+				return $this->create($exprNode, new UnionType($trueTypes), $context, false, $scope, $rootExpr);
+			}
+
+			if (!$context->null() && $constantType->getValue() === false) {
 				return $this->specifyTypesInCondition(
 					$scope,
 					$exprNode,
@@ -1967,6 +1981,28 @@ public function resolveEqual(Expr\BinaryOp\Equal $expr, Scope $scope, TypeSpecif
 				);
 			}
 
+			if (!$context->null() && $constantType->getValue() === '') {
+				/* There is a difference between php 7.x and 8.x on the equality
+				 * behavior between zero and the empty string, so to be conservative
+				 * we leave it untouched regardless of the language version */
+				if ($context->true()) {
+					$trueTypes = [
+						new NullType(),
+						new ConstantBooleanType(false),
+						new ConstantIntegerType(0),
+						new ConstantFloatType(0.0),
+						new ConstantStringType(''),
+					];
+				} else {
+					$trueTypes = [
+						new NullType(),
+						new ConstantBooleanType(false),
+						new ConstantStringType(''),
+					];
+				}
+				return $this->create($exprNode, new UnionType($trueTypes), $context, false, $scope, $rootExpr);
+			}
+
 			if (
 				$exprNode instanceof FuncCall
 				&& $exprNode->name instanceof Name

tests/PHPStan/Analyser/nsrt/equal-narrow.php

@@ -0,0 +1,148 @@
+<?php declare(strict_types = 1);
+
+// phpcs:disable SlevomatCodingStandard.ControlStructures.DisallowYodaComparison.DisallowedYodaComparison
+// phpcs:disable SlevomatCodingStandard.Operators.DisallowEqualOperators.DisallowedEqualOperator
+// phpcs:disable SlevomatCodingStandard.Operators.DisallowEqualOperators.DisallowedNotEqualOperator
+// phpcs:disable Squiz.Functions.GlobalFunction.Found
+// phpcs:disable Squiz.Strings.DoubleQuoteUsage.NotRequired
+
+namespace EqualTypeNarrowing;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param 0|0.0|1|''|'0'|'x'|array{}|bool|object|null $x
+ * @param int|string|null $y
+ * @param mixed $z
+ */
+function doNull($x, $y, $z): void
+{
+	if ($x == null) {
+		assertType("0|0.0|''|array{}|false|null", $x);
+	} else {
+		assertType("1|'0'|'x'|object|true", $x);
+	}
+	if (null != $x) {
+		assertType("1|'0'|'x'|object|true", $x);
+	} else {
+		assertType("0|0.0|''|array{}|false|null", $x);
+	}
+
+	if ($y == null) {
+		assertType("0|''|null", $y);
+	} else {
+		assertType("int<min, -1>|int<1, max>|non-empty-string", $y);
+	}
+
+	if ($z == null) {
+		assertType("0|0.0|''|array{}|false|null", $z);
+	} else {
+		assertType("mixed~0|0.0|''|array{}|false|null", $z);
+	}
+}
+
+/**
+ * @param 0|0.0|1|''|'0'|'x'|array{}|bool|object|null $x
+ * @param int|string|null $y
+ * @param mixed $z
+ */
+function doFalse($x, $y, $z): void
+{
+	if ($x == false) {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	} else {
+		assertType("1|'x'|object|true", $x);
+	}
+	if (false != $x) {
+		assertType("1|'x'|object|true", $x);
+	} else {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	}
+
+	if (!$x) {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	} else {
+		assertType("1|'x'|object|true", $x);
+	}
+
+	if ($y == false) {
+		assertType("0|''|'0'|null", $y);
+	} else {
+		assertType("int<min, -1>|int<1, max>|non-falsy-string", $y);
+	}
+
+	if ($z == false) {
+		assertType("0|0.0|''|'0'|array{}|false|null", $z);
+	} else {
+		assertType("mixed~0|0.0|''|'0'|array{}|false|null", $z);
+	}
+}
+
+/**
+ * @param 0|0.0|1|''|'0'|'x'|array{}|bool|object|null $x
+ * @param int|string|null $y
+ * @param mixed $z
+ */
+function doTrue($x, $y, $z): void
+{
+	if ($x == true) {
+		assertType("1|'x'|object|true", $x);
+	} else {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	}
+	if (true != $x) {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	} else {
+		assertType("1|'x'|object|true", $x);
+	}
+
+	if ($x) {
+		assertType("1|'x'|object|true", $x);
+	} else {
+		assertType("0|0.0|''|'0'|array{}|false|null", $x);
+	}
+
+	if ($y == true) {
+		assertType("int<min, -1>|int<1, max>|non-falsy-string", $y);
+	} else {
+		assertType("0|''|'0'|null", $y);
+	}
+
+	if ($z == true) {
+		assertType("mixed~0|0.0|''|'0'|array{}|false|null", $z);
+	} else {
+		assertType("0|0.0|''|'0'|array{}|false|null", $z);
+	}
+}
+
+/**
+ * @param 0|0.0|1|''|'0'|'x'|array{}|bool|object|null $x
+ * @param int|string|null $y
+ * @param mixed $z
+ */
+function doEmptyString($x, $y, $z): void
+{
+	// PHP 7.x/8.x compatibility: Keep zero in both cases
+	if ($x == '') {
+		assertType("0|0.0|''|false|null", $x);
+	} else {
+		assertType("0|0.0|1|'0'|'x'|array{}|object|true", $x);
+	}
+	if ('' != $x) {
+		assertType("0|0.0|1|'0'|'x'|array{}|object|true", $x);
+	} else {
+		assertType("0|0.0|''|false|null", $x);
+	}
+
+	if ($y == '') {
+		assertType("0|''|null", $y);
+	} else {
+		assertType("int|non-empty-string", $y);
+	}
+
+	if ($z == '') {
+		assertType("0|0.0|''|false|null", $z);
+	} else {
+		assertType("mixed~''|false|null", $z);
+	}
+}