diff --git a/src/Analyser/MutatingScope.php b/src/Analyser/MutatingScope.php
index 85867200a20..7b25ba91b18 100644
--- a/src/Analyser/MutatingScope.php
+++ b/src/Analyser/MutatingScope.php
@@ -4091,11 +4091,22 @@ public function applySpecifiedTypes(SpecifiedTypes $specifiedTypes): self
 					$trackedType = $currentTypes[0];
 					$trackedNativeType ??= $currentTypes[1];
 				}
-			} elseif (!$specifiedTypes->shouldOverwrite() && $scope->isComplexUnionType($trackedType)) {
-				// mirrors addTypeToExpression()/removeTypeFromExpression(): narrowing
-				// a combinatorially-grown offset union doubles it with every isset()-
-				// style check and gets skipped (overwrites assign, they never narrow)
-				continue;
+			} else {
+				if (!$specifiedTypes->shouldOverwrite() && $scope->isComplexUnionType($trackedType)) {
+					// mirrors addTypeToExpression()/removeTypeFromExpression(): narrowing
+					// a combinatorially-grown offset union doubles it with every isset()-
+					// style check and gets skipped (overwrites assign, they never narrow)
+					continue;
+				}
+				if ($trackedNativeType === null) {
+					// a phpdoc-only holder (e.g. one a conditional expression put in
+					// place) - the native base comes from the native state, not from
+					// the phpdoc-derived specified type
+					$currentTypes = $scope->getCurrentTypesOfSpecifiedExpr($expr);
+					if ($currentTypes !== null) {
+						$trackedNativeType = $currentTypes[1];
+					}
+				}
 			}
 
 			if (isset($typeSpecification['terms'])) {
diff --git a/tests/PHPStan/Analyser/nsrt/bug-15404.php b/tests/PHPStan/Analyser/nsrt/bug-15404.php
new file mode 100644
index 00000000000..f25c306779e
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-15404.php
@@ -0,0 +1,106 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15404;
+
+use function PHPStan\Testing\assertNativeType;
+use function PHPStan\Testing\assertType;
+
+/** @param array{projects?: list<string>} $s */
+function a(array $s): void
+{
+	$has = isset($s['projects']);
+	if (!$has) {
+		return;
+	}
+	foreach ($s['projects'] as $p) {
+		assertType('string', $p);
+		assertNativeType('mixed', $p);
+		if (!is_string($p)) {
+			throw new \InvalidArgumentException();
+		}
+	}
+}
+
+/** @param array{projects?: list<string>} $s */
+function inlineIsset(array $s): void
+{
+	if (!isset($s['projects'])) {
+		return;
+	}
+	foreach ($s['projects'] as $p) {
+		assertType('string', $p);
+		assertNativeType('mixed', $p);
+	}
+}
+
+/** @param array{projects?: list<string>} $s */
+function arrayKeyExists(array $s): void
+{
+	$has = array_key_exists('projects', $s);
+	if (!$has) {
+		return;
+	}
+	foreach ($s['projects'] as $p) {
+		assertType('string', $p);
+		assertNativeType('mixed', $p);
+	}
+}
+
+/** @param array{projects?: list<string>} $s */
+function notEmptyArray(array $s): void
+{
+	$has = isset($s['projects']);
+	if (!$has) {
+		return;
+	}
+	if ($s['projects'] !== []) {
+		assertType('non-empty-list<string>', $s['projects']);
+		assertNativeType('mixed~array{}', $s['projects']);
+	}
+}
+
+/** @param array{projects?: list<string>} $s */
+function isArrayCheck(array $s): void
+{
+	$has = isset($s['projects']);
+	if (!$has) {
+		return;
+	}
+	if (is_array($s['projects'])) {
+		assertType('list<string>', $s['projects']);
+		assertNativeType('array<mixed, mixed>', $s['projects']);
+	}
+}
+
+/** @param array{projects?: list<string>} $s */
+function notIsArrayCheck(array $s): void
+{
+	$has = isset($s['projects']);
+	if (!$has) {
+		return;
+	}
+	if (!is_int($s['projects'])) {
+		assertType('list<string>', $s['projects']);
+		assertNativeType('mixed~int', $s['projects']);
+	}
+}
+
+class Foo
+{
+
+	/** @var list<string>|null */
+	public $projects;
+
+	public function doFoo(): void
+	{
+		$has = $this->projects !== null;
+		if (!$has) {
+			return;
+		}
+		foreach ($this->projects as $p) {
+			assertType('string', $p);
+			assertNativeType('mixed', $p);
+		}
+	}
+
+}
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleWithUncertainPhpDocTypesTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleWithUncertainPhpDocTypesTest.php
index 3fff9077bce..8d59006a411 100644
--- a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleWithUncertainPhpDocTypesTest.php
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleWithUncertainPhpDocTypesTest.php
@@ -271,4 +271,9 @@ public function testBug15250(): void
 		$this->analyse([__DIR__ . '/data/bug-15250.php'], []);
 	}
 
+	public function testBug15404(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-15404.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Comparison/data/bug-15404.php b/tests/PHPStan/Rules/Comparison/data/bug-15404.php
new file mode 100644
index 00000000000..1dd5291a8cb
--- /dev/null
+++ b/tests/PHPStan/Rules/Comparison/data/bug-15404.php
@@ -0,0 +1,17 @@
+<?php declare(strict_types = 1);
+
+namespace Bug15404Rule;
+
+/** @param array{projects?: list<string>} $s */
+function a(array $s): void
+{
+	$has = isset($s['projects']);
+	if (!$has) {
+		return;
+	}
+	foreach ($s['projects'] as $p) {
+		if (!is_string($p)) {
+			throw new \InvalidArgumentException();
+		}
+	}
+}
diff --git a/turbo-ext/src/MutatingScope.cpp b/turbo-ext/src/MutatingScope.cpp
index 1138b46ed9a..01d45327736 100644
--- a/turbo-ext/src/MutatingScope.cpp
+++ b/turbo-ext/src/MutatingScope.cpp
@@ -7826,6 +7826,29 @@ class MutatingScope
 						skipSpecification = true;
 					}
 				}
+				if (!skipSpecification && !hasTrackedNative) {
+					/* a phpdoc-only holder - the native base comes from the
+					 * native state, not from the phpdoc-derived specified type */
+					zval exprZv;
+					ZVAL_OBJ(&exprZv, expr);
+					zv::Val currentTypes = otherPrivate(scopeObject, PT_LC("getcurrenttypesofspecifiedexpr"), 1, &exprZv, [&](MutatingScope &other) {
+						return other.getCurrentTypesOfSpecifiedExpr(expr);
+					});
+					if (UNEXPECTED(currentTypes.isUndef())) return zv::Val();
+					if (Z_TYPE_P(currentTypes.raw()) != IS_NULL) {
+						if (UNEXPECTED(Z_TYPE_P(currentTypes.raw()) != IS_ARRAY)) {
+							zend_throw_error(NULL, "phpstan_turbo: getCurrentTypesOfSpecifiedExpr() answered an unexpected shape");
+							return zv::Val();
+						}
+						zval *native = zend_hash_index_find(Z_ARRVAL_P(currentTypes.raw()), 1);
+						if (UNEXPECTED(native == NULL)) {
+							zend_throw_error(NULL, "phpstan_turbo: getCurrentTypesOfSpecifiedExpr() answered an unexpected shape");
+							return zv::Val();
+						}
+						trackedNativeType = zv::Val::copyOf(zv::Ref(native));
+						hasTrackedNative = true;
+					}
+				}
 			}
 			if (skipSpecification) continue;