Do not report assigning a float zero as redundant

src/Analyser/ExprHandler/AssignHandler.php

@@ -3096,6 +3096,11 @@ private static function redundant(ExpressionResult $rhs, Expr $target, Expressio
 		if (count($values) !== 1 || !$values[0]->equals($rhs->getType())) {
 			return null;
 		}
+		// 0.0 and -0.0 are identical (`===`) but distinguishable (`(string)`, `fdiv()`),
+		// and `$f === 0.0` narrows $f to 0.0 while it may still hold -0.0
+		if (in_array(0.0, $values[0]->getConstantScalarValues(), true)) {
+			return null;
+		}
 		$nativeValues = $nativeType->getFiniteTypes();
 		return count($nativeValues) === 1 && $nativeValues[0]->equals($rhs->getNativeType()) ? $rhs->getType() : null;
 	}

tests/PHPStan/Rules/DeadCode/UnusedVariableRuleTest.php

@@ -206,6 +206,14 @@ public function testRedundantAssignment(): void
 		]);
 	}
 
+	public function testRedundantAssignmentOfSignedZero(): void
+	{
+		$this->analyse([__DIR__ . '/data/unused-variable-redundant-signed-zero.php'], [
+			['Value assigned to variable $x is never read before being overwritten.', 12],
+			['Variable $x is assigned value 1.5 but it already has that value.', 50],
+		]);
+	}
+
 	public function testByRefReturn(): void
 	{
 		$this->analyse([__DIR__ . '/data/unused-variable-by-ref-return.php'], [

tests/PHPStan/Rules/DeadCode/data/unused-variable-redundant-signed-zero.php

@@ -0,0 +1,80 @@
+<?php declare(strict_types = 1);
+
+namespace UnusedVariableRedundantSignedZero;
+
+function f(float $f): float {
+	if ($f === 0.0) {
+		$f = 0.0;
+	}
+	return $f;
+}
+function g(): float {
+	$x = -0.0;
+	$x = 0.0;
+	return $x;
+}
+
+/** @param mixed $v */
+function sink($v): void
+{
+}
+
+function negativeZeroNarrowing(float $f): void
+{
+	if ($f === -0.0) {
+		$f = -0.0;
+	}
+	sink($f);
+}
+
+function looseZeroNarrowing(float $f): void
+{
+	if ($f == 0) {
+		$f = 0.0;
+	}
+	sink($f);
+}
+
+function zeroAtOffset(): void
+{
+	$a = ['k' => -0.0];
+	sink($a);
+	$a['k'] = 0.0;
+	sink($a);
+}
+
+function nonZeroFloatIsStillRedundant(): void
+{
+	$x = 1.5;
+	sink($x);
+	$x = 1.5;
+	sink($x);
+}
+
+function zeroAtNestedOffset(): void
+{
+	$a = ['x' => ['y' => -0.0]];
+	sink($a);
+	$a['x']['y'] = 0.0;
+	sink($a);
+}
+
+function repeatedPositiveZeroIsNotReportedEither(): void
+{
+	// the type of $x cannot tell this 0.0 from a narrowed zero of unknown sign
+	$x = 0.0;
+	sink($x);
+	$x = 0.0;
+	sink($x);
+}
+
+// https://3v4l.org/GiSbY
+function h(): void {
+	$f = -0.0;
+	var_dump($f === 0.0); // bool(true)
+	echo $f, "\n";        // -0
+	if ($f === 0.0) {
+		$f = 0.0;         // not redundant: turns -0.0 into 0.0
+	}
+	echo $f, "\n";        // 0
+}

turbo-ext/src/AssignHandler.cpp

@@ -4810,6 +4810,12 @@ class AssignHandler
 			bool equal;
 			AH_OK(ahEquals(zend_hash_index_find(Z_ARRVAL_P(values.raw()), 0), rhsType.raw(), equal));
 			if (!equal) return ahNull();
+			// in_array(0.0, ..., true): matches -0.0 too, never NAN; the sign of a zero is observable
+			AH_VAL(scalarValues, ahTypeOp(zend_hash_index_find(Z_ARRVAL_P(values.raw()), 0), PT_OP_GET_CONSTANT_SCALAR_VALUES, 0, NULL));
+			for (auto entry : zv::TableRef(Z_ARRVAL_P(scalarValues.raw()))) {
+				zval *value = entry.value().deref().raw();
+				if (Z_TYPE_P(value) == IS_DOUBLE && Z_DVAL_P(value) == 0.0) return ahNull();
+			}
 		}
 		AH_VAL(nativeValues, ahCall(nativeType.raw(), PT_LC("getfinitetypes"), 0, NULL));
 		if (zend_hash_num_elements(Z_ARRVAL_P(nativeValues.raw())) != 1) return ahNull();