More precise array_keys return type

src/Type/Accessory/AccessoryArrayListType.php

@@ -172,6 +172,11 @@ public function unsetOffset(Type $offsetType): Type
 		return new ErrorType();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this;

src/Type/Accessory/HasOffsetType.php

@@ -343,6 +343,11 @@ public function looseCompare(Type $type, PhpVersion $phpVersion): BooleanType
 		return new BooleanType();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return new NonEmptyArrayType();

src/Type/Accessory/HasOffsetValueType.php

@@ -195,6 +195,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this;
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return new NonEmptyArrayType();

src/Type/Accessory/NonEmptyArrayType.php

@@ -159,6 +159,11 @@ public function unsetOffset(Type $offsetType): Type
 		return new ErrorType();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this;

src/Type/Accessory/OversizedArrayType.php

@@ -154,6 +154,11 @@ public function unsetOffset(Type $offsetType): Type
 		return new ErrorType();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this;

src/Type/ArrayType.php

@@ -170,6 +170,11 @@ public function generalizeValues(): self
 		return new self($this->keyType, $this->itemType->generalize(GeneralizePrecision::lessSpecific()));
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return TypeCombinator::intersect(new self(new IntegerType(), $this->getIterableKeyType()), new AccessoryArrayListType());

src/Type/Constant/ConstantArrayType.php

@@ -35,6 +35,7 @@
 use PHPStan\Type\Generic\TemplateTypeMap;
 use PHPStan\Type\Generic\TemplateTypeVariance;
 use PHPStan\Type\IntegerRangeType;
+use PHPStan\Type\IntegerType;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\IsSuperTypeOfResult;
 use PHPStan\Type\MixedType;
@@ -1250,6 +1251,19 @@ public function generalizeValues(): self
 		return new self($this->keyTypes, $valueTypes, $this->nextAutoIndexes, $this->optionalKeys, $this->isList);
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		$keysArray = $this->getKeysOrValuesArray($this->keyTypes);
+
+		return TypeCombinator::intersect(
+			new ArrayType(
+				new IntegerType(),
+				$keysArray->getIterableValueType(),
+			),
+			new AccessoryArrayListType(),
+		);
+	}
+
 	public function getKeysArray(): self
 	{
 		return $this->getKeysOrValuesArray($this->keyTypes);

src/Type/IntersectionType.php

@@ -799,6 +799,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this->intersectTypes(static fn (Type $type): Type => $type->unsetOffset($offsetType));
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->intersectTypes(static fn (Type $type): Type => $type->getKeysArrayFiltered($filterValueType, $strict));
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this->intersectTypes(static fn (Type $type): Type => $type->getKeysArray());

src/Type/MixedType.php

@@ -179,6 +179,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this;
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		if ($this->isArray()->no()) {

src/Type/NeverType.php

@@ -273,6 +273,11 @@ public function unsetOffset(Type $offsetType): Type
 		return new NeverType();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return new NeverType();

src/Type/Php/ArrayKeysFunctionDynamicReturnTypeExtension.php

@@ -6,6 +6,7 @@
 use PHPStan\Analyser\Scope;
 use PHPStan\Php\PhpVersion;
 use PHPStan\Reflection\FunctionReflection;
+use PHPStan\TrinaryLogic;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
 use PHPStan\Type\NeverType;
 use PHPStan\Type\NullType;
@@ -27,15 +28,27 @@ public function isFunctionSupported(FunctionReflection $functionReflection): boo
 
 	public function getTypeFromFunctionCall(FunctionReflection $functionReflection, FuncCall $functionCall, Scope $scope): ?Type
 	{
-		if (count($functionCall->getArgs()) !== 1) {
+		$args = $functionCall->getArgs();
+		if (count($args) < 1) {
 			return null;
 		}
 
-		$arrayType = $scope->getType($functionCall->getArgs()[0]->value);
+		$arrayType = $scope->getType($args[0]->value);
 		if ($arrayType->isArray()->no()) {
 			return $this->phpVersion->arrayFunctionsReturnNullWithNonArray() ? new NullType() : new NeverType();
 		}
 
+		if (count($args) >= 2) {
+			$filterType = $scope->getType($args[1]->value);
+
+			$strict = TrinaryLogic::createNo();
+			if (count($args) >= 3) {
+				$strict = $scope->getType($args[2]->value)->isTrue();
+			}
+
+			return $arrayType->getKeysArrayFiltered($filterType, $strict);
+		}
+
 		return $arrayType->getKeysArray();
 	}
 

src/Type/StaticType.php

@@ -396,6 +396,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this->getStaticObjectType()->unsetOffset($offsetType);
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getStaticObjectType()->getKeysArrayFiltered($filterValueType, $strict);
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this->getStaticObjectType()->getKeysArray();

src/Type/Traits/LateResolvableTypeTrait.php

@@ -248,6 +248,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this->resolve()->unsetOffset($offsetType);
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->resolve()->getKeysArrayFiltered($filterValueType, $strict);
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this->resolve()->getKeysArray();

src/Type/Traits/MaybeArrayTypeTrait.php

@@ -39,6 +39,11 @@ public function isList(): TrinaryLogic
 		return TrinaryLogic::createMaybe();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return new ErrorType();

src/Type/Traits/NonArrayTypeTrait.php

@@ -39,6 +39,11 @@ public function isList(): TrinaryLogic
 		return TrinaryLogic::createNo();
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->getKeysArray();
+	}
+
 	public function getKeysArray(): Type
 	{
 		return new ErrorType();

src/Type/Type.php

@@ -134,6 +134,8 @@ public function setExistingOffsetValueType(Type $offsetType, Type $valueType): T
 
 	public function unsetOffset(Type $offsetType): Type;
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type;
+
 	public function getKeysArray(): Type;
 
 	public function getValuesArray(): Type;

src/Type/UnionType.php

@@ -730,6 +730,11 @@ public function unsetOffset(Type $offsetType): Type
 		return $this->unionTypes(static fn (Type $type): Type => $type->unsetOffset($offsetType));
 	}
 
+	public function getKeysArrayFiltered(Type $filterValueType, TrinaryLogic $strict): Type
+	{
+		return $this->unionTypes(static fn (Type $type): Type => $type->getKeysArrayFiltered($filterValueType, $strict));
+	}
+
 	public function getKeysArray(): Type
 	{
 		return $this->unionTypes(static fn (Type $type): Type => $type->getKeysArray());

tests/PHPStan/Analyser/nsrt/bug-11928.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace Bug11928;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo()
+{
+	$a = [2 => 1, 3 => 2, 4 => 1];
+
+	$keys = array_keys($a, 1); // returns [2, 4]
+	assertType('list<2|3|4>', $keys);
+
+	$keys = array_keys($a); // returns [2, 3, 4]
+	assertType('array{2, 3, 4}', $keys);
+}
+
+/**
+ * @param array<1|2|3, 4|5|6> $unionKeyedArray
+ * @param 4|5 $fourOrFive
+ * @return void
+ */
+function doFooStrings($unionKeyedArray, $fourOrFive) {
+	$a = [2 => 'hi', 3 => '123', 'xy' => 5];
+	$keys = array_keys($a, 1);
+	assertType("list<2|3|'xy'>", $keys);
+
+	$keys = array_keys($a);
+	assertType("array{2, 3, 'xy'}", $keys);
+
+	$keys = array_keys($unionKeyedArray, 1);
+	assertType("list<1|2|3>", $keys); // could be array{}
+
+	$keys = array_keys($unionKeyedArray, 4);
+	assertType("list<1|2|3>", $keys);
+
+	$keys = array_keys($unionKeyedArray, $fourOrFive);
+	assertType("list<1|2|3>", $keys);
+
+	$keys = array_keys($unionKeyedArray);
+	assertType("list<1|2|3>", $keys);
+}
+
+/**
+ * @param array<int, int> $array
+ * @param list<int> $list
+ * @param array<string, string> $strings
+ * @return void
+ */
+function doFooBar(array $array, array $list, array $strings) {
+	$keys = array_keys($strings, "a", true);
+	assertType('list<string>', $keys);
+
+	$keys = array_keys($strings, "a", false);
+	assertType('list<string>', $keys);
+
+	$keys = array_keys($array, 1, true);
+	assertType('list<int>', $keys);
+
+	$keys = array_keys($array, 1, false);
+	assertType('list<int>', $keys);
+
+	$keys = array_keys($list, 1, true);
+	assertType('list<int<0, max>>', $keys);
+
+	$keys = array_keys($list, 1, true);
+	assertType('list<int<0, max>>', $keys);
+}