diff --git a/src/Analyser/TypeSpecifier.php b/src/Analyser/TypeSpecifier.php
index 9f23cf6472..a5dc7bc6a5 100644
--- a/src/Analyser/TypeSpecifier.php
+++ b/src/Analyser/TypeSpecifier.php
@@ -818,6 +818,60 @@ public function specifyTypesInCondition(
 								$rootExpr,
 							),
 						);
+					} else {
+						$varType = $scope->getType($var->var);
+						if ($varType->isArray()->yes() && !$varType->isIterableAtLeastOnce()->no()) {
+							$varIterableKeyType = $varType->getIterableKeyType();
+
+							if ($varIterableKeyType->isConstantScalarValue()->yes()) {
+								$narrowedKey = TypeCombinator::union(
+									$varIterableKeyType,
+									TypeCombinator::remove($varIterableKeyType->toString(), new ConstantStringType('')),
+								);
+
+								if (!$varType->hasOffsetValueType(new ConstantIntegerType(0))->no()) {
+									$narrowedKey = TypeCombinator::union(
+										$narrowedKey,
+										new ConstantBooleanType(false),
+									);
+								}
+
+								if (!$varType->hasOffsetValueType(new ConstantIntegerType(1))->no()) {
+									$narrowedKey = TypeCombinator::union(
+										$narrowedKey,
+										new ConstantBooleanType(true),
+									);
+								}
+
+								if (!$varType->hasOffsetValueType(new ConstantStringType(''))->no()) {
+									$narrowedKey = TypeCombinator::addNull($narrowedKey);
+								}
+
+								if (!$varIterableKeyType->isNumericString()->no() || !$varIterableKeyType->isInteger()->no()) {
+									$narrowedKey = TypeCombinator::union($narrowedKey, new FloatType());
+								}
+							} else {
+								$narrowedKey = new MixedType(
+									false,
+									new UnionType([
+										new ArrayType(new MixedType(), new MixedType()),
+										new ObjectWithoutClassType(),
+										new ResourceType(),
+									]),
+								);
+							}
+
+							$types = $types->unionWith(
+								$this->create(
+									$var->dim,
+									$narrowedKey,
+									$context,
+									false,
+									$scope,
+									$rootExpr,
+								),
+							);
+						}
 					}
 				}
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-11716.php b/tests/PHPStan/Analyser/nsrt/bug-11716.php
new file mode 100644
index 0000000000..2394e8a175
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-11716.php
@@ -0,0 +1,214 @@
+<?php // lint >= 8.0
+
+namespace Bug11716;
+
+use function PHPStan\Testing\assertType;
+
+class TypeExpression
+{
+	/**
+	 * @return '&'|'|'
+	 */
+	public function parse(string $glue): string
+	{
+		$seenGlues = ['|' => false, '&' => false];
+
+		assertType("array{|: false, &: false}", $seenGlues);
+
+		if ($glue !== '') {
+			assertType('non-empty-string', $glue);
+
+			\assert(isset($seenGlues[$glue]));
+			$seenGlues[$glue] = true;
+
+			assertType("'&'|'|'", $glue);
+			assertType('array{|: bool, &: bool}', $seenGlues);
+		} else {
+			assertType("''", $glue);
+		}
+
+		assertType("''|'&'|'|'", $glue);
+		assertType("array{|: bool, &: bool}", $seenGlues);
+
+		return array_key_first($seenGlues);
+	}
+}
+
+/**
+ * @param array<int, string> $arr
+ */
+function narrowKey($mixed, string $s, int $i, array $generalArr, array $arr): void {
+	if (isset($generalArr[$mixed])) {
+		assertType('mixed~(array|object|resource)', $mixed);
+	} else {
+		assertType('mixed', $mixed);
+	}
+	assertType('mixed', $mixed);
+
+	if (isset($generalArr[$i])) {
+		assertType('int', $i);
+	} else {
+		assertType('int', $i);
+	}
+	assertType('int', $i);
+
+	if (isset($generalArr[$s])) {
+		assertType('string', $s);
+	} else {
+		assertType('string', $s);
+	}
+	assertType('string', $s);
+
+	if (isset($arr[$mixed])) {
+		assertType('mixed~(array|object|resource)', $mixed);
+	} else {
+		assertType('mixed', $mixed);
+	}
+	assertType('mixed', $mixed);
+
+	if (isset($arr[$i])) {
+		assertType('int', $i);
+	} else {
+		assertType('int', $i);
+	}
+	assertType('int', $i);
+
+	if (isset($arr[$s])) {
+		assertType('string', $s);
+	} else {
+		assertType('string', $s);
+	}
+	assertType('string', $s);
+}
+
+/**
+ * @param array<int, array<string, float>> $arr
+ */
+function multiDim($mixed, $mixed2, array $arr) {
+	if (isset($arr[$mixed])) {
+		assertType('mixed~(array|object|resource)', $mixed);
+	} else {
+		assertType('mixed', $mixed);
+	}
+	assertType('mixed', $mixed);
+
+	if (isset($arr[$mixed]) && isset($arr[$mixed][$mixed2])) {
+		assertType('mixed~(array|object|resource)', $mixed);
+		assertType('mixed~(array|object|resource)', $mixed2);
+	} else {
+		assertType('mixed', $mixed);
+	}
+	assertType('mixed', $mixed);
+
+	if (isset($arr[$mixed][$mixed2])) {
+		assertType('mixed~(array|object|resource)', $mixed);
+		assertType('mixed~(array|object|resource)', $mixed2);
+	} else {
+		assertType('mixed', $mixed);
+		assertType('mixed', $mixed2);
+	}
+	assertType('mixed', $mixed);
+	assertType('mixed', $mixed2);
+}
+
+/**
+ * @param array<int, string> $arr
+ */
+function emptyArrr($mixed, array $arr)
+{
+    if (count($arr) !== 0) {
+        return;
+    }
+
+    assertType('array{}', $arr);
+    if (isset($arr[$mixed])) {
+        assertType('mixed', $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+}
+
+function emptyString($mixed)
+{
+    // see https://3v4l.org/XHZdr
+    $arr = ['' => 1, 'a' => 2];
+    if (isset($arr[$mixed])) {
+        assertType("''|'a'|null", $mixed);
+    } else {
+        assertType('mixed', $mixed); // could be mixed~(''|'a'|null)
+    }
+    assertType('mixed', $mixed);
+}
+
+function numericString($mixed, int $i, string $s)
+{
+    $arr = ['1' => 1, '2' => 2];
+    if (isset($arr[$mixed])) {
+        assertType("1|2|'1'|'2'|float|true", $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+
+    $arr = ['0' => 1, '2' => 2];
+    if (isset($arr[$mixed])) {
+        assertType("0|2|'0'|'2'|float|false", $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+
+	$arr = ['1' => 1, '2' => 2];
+	if (isset($arr[$i])) {
+		assertType("1|2", $i);
+	} else {
+		assertType('int', $i);
+	}
+	assertType('int', $i);
+
+	$arr = ['1' => 1, '2' => 2, 3 => 3];
+	if (isset($arr[$s])) {
+		assertType("'1'|'2'|'3'", $s);
+	} else {
+		assertType('string', $s);
+	}
+	assertType('string', $s);
+
+	$arr = ['1' => 1, '2' => 2, 3 => 3];
+	if (isset($arr[substr($s, 10)])) {
+		assertType("string", $s);
+		assertType("'1'|'2'|'3'", substr($s, 10));
+	} else {
+		assertType('string', $s);
+	}
+	assertType('string', $s);
+}
+
+function intKeys($mixed)
+{
+    $arr = [1 => 1, 2 => 2];
+    if (isset($arr[$mixed])) {
+        assertType("1|2|'1'|'2'|float|true", $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+
+    $arr = [0 => 0, 1 => 1, 2 => 2];
+    if (isset($arr[$mixed])) {
+        assertType("0|1|2|'0'|'1'|'2'|bool|float", $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+}
+
+function arrayAccess(\ArrayAccess $arr, $mixed) {
+    if (isset($arr[$mixed])) {
+        assertType("mixed", $mixed);
+    } else {
+        assertType('mixed', $mixed);
+    }
+    assertType('mixed', $mixed);
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-8559.php b/tests/PHPStan/Analyser/nsrt/bug-8559.php
new file mode 100644
index 0000000000..ee68b2fff0
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-8559.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace Bug8559;
+
+use function PHPStan\Testing\assertType;
+
+class X
+{
+	const KEYS = ['a' => 1, 'b' => 2];
+
+	/**
+	 * @phpstan-assert key-of<self::KEYS> $key
+	 * @return value-of<self::KEYS>
+	 */
+	public static function get(string $key): int
+	{
+		assert(isset(self::KEYS[$key]));
+		assertType("'a'|'b'", $key);
+		return self::KEYS[$key];
+	}
+
+	/**
+	 * @phpstan-assert key-of<self::KEYS> $key
+	 * @return value-of<self::KEYS>
+	 */
+	public static function get2(string $key): int
+	{
+		assert(in_array($key, array_keys(self::KEYS), true));
+		assertType("'a'|'b'", $key);
+		return self::KEYS[$key];
+	}
+}
+
+$key = 'x';
+$v = X::get($key);
+assertType("*NEVER*", $key);
+
+$key = 'a';
+$v = X::get($key);
+assertType("'a'", $key);