diff --git a/src/Analyser/ExprHandler/AssignHandler.php b/src/Analyser/ExprHandler/AssignHandler.php
index 62ee67ed27b..fa52e2b2a49 100644
--- a/src/Analyser/ExprHandler/AssignHandler.php
+++ b/src/Analyser/ExprHandler/AssignHandler.php
@@ -2170,8 +2170,10 @@ private function currentTypeForConditionalHolder(NodeScopeResolver $nodeScopeRes
 		// them from the walk's own storage (the scope's storage stack misses it
 		// on loop-convergence passes); synthetic terms narrowing extensions built
 		// (@phpstan-assert property fetches etc.) answer from scope state or a walk
+		// - unless the walk read them on a narrowed branch (the right operand of
+		// `$a['k'] === null || is_int($a['k'])`), then they are re-priced here
 		$storedResult = $storage->findExpressionResult($expr);
-		if ($storedResult !== null) {
+		if ($storedResult !== null && $storedResult->answersOnScope($scope, $scope->nativeTypesPromoted)) {
 			return $storedResult->getTypeOnScope($scope, $scope->nativeTypesPromoted);
 		}
 
diff --git a/src/Analyser/ExpressionResult.php b/src/Analyser/ExpressionResult.php
index 9f69558d9b1..4b6c7d89d52 100644
--- a/src/Analyser/ExpressionResult.php
+++ b/src/Analyser/ExpressionResult.php
@@ -699,7 +699,19 @@ public function answersOnScope(MutatingScope $scope, bool $useNativeTypes): bool
 		$useNativeTypes = $useNativeTypes || $scope->nativeTypesPromoted;
 		$readScope = $useNativeTypes ? $scope->doNotTreatPhpDocTypesAsCertain() : $scope;
 
-		return $this->isScopeAuthoritative($readScope) || $this->askScopeVariableStateMatches($scope, $useNativeTypes);
+		if ($this->isScopeAuthoritative($readScope)) {
+			return true;
+		}
+
+		// the walk position narrowed this very expression (e.g. the right
+		// operand of `$a['k'] === null || is_int($a['k'])`) - a position not
+		// carrying that narrowing must not see it through the memoized type
+		$positionScope = $useNativeTypes ? $this->beforeScope->doNotTreatPhpDocTypesAsCertain() : $this->beforeScope;
+		if ($this->isScopeAuthoritative($positionScope)) {
+			return false;
+		}
+
+		return $this->askScopeVariableStateMatches($scope, $useNativeTypes);
 	}
 
 	/**
diff --git a/tests/PHPStan/Analyser/nsrt/bug-15479.php b/tests/PHPStan/Analyser/nsrt/bug-15479.php
new file mode 100644
index 00000000000..ea2285df247
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-15479.php
@@ -0,0 +1,71 @@
+<?php // lint >= 8.0
+
+namespace Bug15479;
+
+use function PHPStan\Testing\assertType;
+
+/** @param array<string, mixed> $a */
+function f(array $a): void {
+	$ok = array_key_exists('k', $a) && ($a['k'] === null || is_int($a['k']));
+	if ($ok) { assertType('int|null', $a['k']); }
+}
+
+/** @param array<string, mixed> $a */
+function g(array $a): void {
+	$ok = ($a['k'] === null || is_int($a['k']));
+	if ($ok) { assertType('int|null', $a['k']); }
+}
+
+/** @param array<string, mixed> $a */
+function h(array $a): void {
+	if (array_key_exists('k', $a) && ($a['k'] === null || is_int($a['k']))) { assertType('int|null', $a['k']); }
+}
+
+function i(mixed $v): void {
+	$ok = $v === null || is_int($v);
+	if ($ok) { assertType('int|null', $v); }
+}
+
+/** @param array<string, mixed> $a */
+function negated(array $a): void {
+	$ok = ($a['k'] !== null && !is_int($a['k']));
+	if (!$ok) {
+		assertType('int|null', $a['k']);
+	} else {
+		assertType('mixed~(int|null)', $a['k']);
+	}
+}
+
+/** @param array<string, mixed> $a */
+function ternary(array $a): void {
+	$ok = $a['k'] === null || is_int($a['k']) ? 1 : 0;
+	if ($ok === 1) { assertType('int|null', $a['k']); }
+}
+
+/** @param array<string, array<string, mixed>> $a */
+function nested(array $a): void {
+	$ok = ($a['k']['j'] === null || is_int($a['k']['j']));
+	if ($ok) { assertType('int|null', $a['k']['j']); }
+}
+
+/** @param array<string, mixed> $a */
+function typeChecks(array $a): void {
+	$ok = (is_string($a['k']) || is_int($a['k']));
+	if ($ok) { assertType('int|string', $a['k']); }
+}
+
+/** @param array<string, mixed> $a */
+function constantOrType(array $a): void {
+	$ok = ($a['k'] === 1 || is_string($a['k']));
+	if ($ok) { assertType('1|string', $a['k']); }
+}
+
+/** @param array<string, mixed> $a */
+function elseBranch(array $a): void {
+	$ok = ($a['k'] === null || is_string($a['k']));
+	if ($ok) {
+		assertType('string|null', $a['k']);
+	} else {
+		assertType('mixed~(string|null)', $a['k']);
+	}
+}
diff --git a/turbo-ext/src/AssignHandler.cpp b/turbo-ext/src/AssignHandler.cpp
index e249818c5d4..f5dd909b880 100644
--- a/turbo-ext/src/AssignHandler.cpp
+++ b/turbo-ext/src/AssignHandler.cpp
@@ -2489,8 +2489,16 @@ class AssignHandler
 		// them from the walk's own storage (the scope's storage stack misses it
 		// on loop-convergence passes); synthetic terms narrowing extensions built
 		// (@phpstan-assert property fetches etc.) answer from scope state or a walk
+		// - unless the walk read them on a narrowed branch (the right operand of
+		// `$a['k'] === null || is_int($a['k'])`), then they are re-priced here
 		AH_VAL(storedResult, pt_expression_result_storage_find(storage, expr));
-		if (!storedResult.isNull()) return ahTypeOnScope(storedResult.raw(), scope);
+		if (!storedResult.isNull()) {
+			bool promoted;
+			AH_OK(ahPromoted(scope, promoted));
+			bool answers;
+			AH_OK(pt_expression_result_answers_on_scope(storedResult.raw(), scope, promoted, answers));
+			if (answers) return ahTypeOnScope(storedResult.raw(), scope);
+		}
 
 		return nsrReadScopeStateOrSyntheticType(nsr, expr, scope);
 	}
diff --git a/turbo-ext/src/ExpressionResult.cpp b/turbo-ext/src/ExpressionResult.cpp
index c72c7984242..7d2a5b9a9cd 100644
--- a/turbo-ext/src/ExpressionResult.cpp
+++ b/turbo-ext/src/ExpressionResult.cpp
@@ -600,6 +600,21 @@ class ExpressionResult
 
 		zend_long authoritative = isScopeAuthoritative(readScope);
 		if (authoritative != 0) return authoritative;
+
+		// the walk position narrowed this very expression (e.g. the right
+		// operand of `$a['k'] === null || is_int($a['k'])`) - a position not
+		// carrying that narrowing must not see it through the memoized type
+		zv::Val positionScopeHolder;
+		zval *positionScope = slot(slots::beforeScope);
+		if (useNativeTypes) {
+			positionScopeHolder = scopeNativeView(positionScope);
+			if (UNEXPECTED(positionScopeHolder.isUndef())) return -1;
+			positionScope = positionScopeHolder.raw();
+		}
+		zend_long positionAuthoritative = isScopeAuthoritative(positionScope);
+		if (positionAuthoritative < 0) return -1;
+		if (positionAuthoritative == 1) return 0;
+
 		return askScopeVariableStateMatches(scope, useNativeTypes, false);
 	}