Support narrowing down array_filter return type via string callable

src/Type/Php/ArrayFilterFunctionReturnTypeReturnTypeExtension.php

@@ -2,13 +2,16 @@
 
 namespace PHPStan\Type\Php;
 
+use PhpParser\Node\Arg;
 use PhpParser\Node\Expr;
 use PhpParser\Node\Expr\ArrowFunction;
 use PhpParser\Node\Expr\Closure;
 use PhpParser\Node\Expr\ConstFetch;
 use PhpParser\Node\Expr\Error;
 use PhpParser\Node\Expr\FuncCall;
 use PhpParser\Node\Expr\Variable;
+use PhpParser\Node\Name;
+use PhpParser\Node\Scalar\String_;
 use PhpParser\Node\Stmt\Return_;
 use PHPStan\Analyser\MutatingScope;
 use PHPStan\Analyser\Scope;
@@ -74,6 +77,10 @@ public function getTypeFromFunctionCall(FunctionReflection $functionReflection,
 				}
 			} elseif ($callbackArg instanceof ArrowFunction && count($callbackArg->params) > 0) {
 				[$itemType, $keyType] = $this->filterByTruthyValue($scope, $callbackArg->params[0]->var, $itemType, null, $keyType, $callbackArg->expr);
+			} elseif ($callbackArg instanceof String_) {
+				$itemVar = new Variable('item');
+				$expr = new FuncCall(new Name($callbackArg->value), [new Arg($itemVar)]);
+				[$itemType, $keyType] = $this->filterByTruthyValue($scope, $itemVar, $itemType, null, $keyType, $expr);
 			}
 		}
 
@@ -85,6 +92,10 @@ public function getTypeFromFunctionCall(FunctionReflection $functionReflection,
 				}
 			} elseif ($callbackArg instanceof ArrowFunction && count($callbackArg->params) > 0) {
 				[$itemType, $keyType] = $this->filterByTruthyValue($scope, null, $itemType, $callbackArg->params[0]->var, $keyType, $callbackArg->expr);
+			} elseif ($callbackArg instanceof String_) {
+				$keyVar = new Variable('key');
+				$expr = new FuncCall(new Name($callbackArg->value), [new Arg($keyVar)]);
+				[$itemType, $keyType] = $this->filterByTruthyValue($scope, null, $itemType, $keyVar, $keyType, $expr);
 			}
 		}
 
@@ -96,6 +107,11 @@ public function getTypeFromFunctionCall(FunctionReflection $functionReflection,
 				}
 			} elseif ($callbackArg instanceof ArrowFunction && count($callbackArg->params) > 0) {
 				[$itemType, $keyType] = $this->filterByTruthyValue($scope, $callbackArg->params[0]->var, $itemType, $callbackArg->params[1]->var ?? null, $keyType, $callbackArg->expr);
+			} elseif ($callbackArg instanceof String_) {
+				$itemVar = new Variable('item');
+				$keyVar = new Variable('key');
+				$expr = new FuncCall(new Name($callbackArg->value), [new Arg($itemVar), new Arg($keyVar)]);
+				[$itemType, $keyType] = $this->filterByTruthyValue($scope, $itemVar, $itemType, $keyVar, $keyType, $expr);
 			}
 		}
 

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -281,6 +281,7 @@ public function dataFileAsserts(): iterable
 
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/array-filter.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/array-filter-callables.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/array-filter-string-callables.php');
 		if (PHP_VERSION_ID >= 70400) {
 			yield from $this->gatherAssertTypes(__DIR__ . '/data/array-filter-arrow-functions.php');
 		}

tests/PHPStan/Analyser/data/array-filter-string-callables.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace ArrayFilter;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param array<int, int> $list1
+ * @param array<int, int> $list2
+ * @param array<int, int> $list3
+ */
+function alwaysEvaluatesToFalse(array $list1, array $list2, array $list3): void
+{
+	$filtered1 = array_filter($list1, 'is_string');
+	assertType('array{}', $filtered1);
+
+	$filtered2 = array_filter($list2, 'is_string', ARRAY_FILTER_USE_KEY);
+	assertType('array{}', $filtered2);
+
+	$filtered3 = array_filter($list3, 'is_string', ARRAY_FILTER_USE_BOTH);
+	assertType('array{}', $filtered3);
+}
+
+/**
+ * @param array<int|string, int|string> $map1
+ * @param array<int|string, int|string> $map2
+ * @param array<int|string, int|string> $map3
+ */
+function filtersString(array $map1, array $map2, array $map3): void
+{
+	$filtered1 = array_filter($map1, 'is_string');
+	assertType('array<int|string, string>', $filtered1);
+
+	$filtered2 = array_filter($map2, 'is_string', ARRAY_FILTER_USE_KEY);
+	assertType('array<string, int|string>', $filtered2);
+
+	$filtered3 = array_filter($map3, 'is_string', ARRAY_FILTER_USE_BOTH);
+	assertType('array<int|string, string>', $filtered3);
+}
+
+/**
+ * @param array<int, int> $list1
+ */
+function nonCallableStringIsIgnored(array $list1): void
+{
+	$filtered1 = array_filter($list1, 'foo');
+	assertType('array<int, int>', $filtered1);
+}
+
+/**
+ * @param array<int|string, int|string> $map1
+ * @param array<int|string, int|string> $map2
+ */
+function nonBuiltInFunctionsAreNotSupportedYetAndThereforeIgnored(array $map1, array $map2): void
+{
+	$filtered1 = array_filter($map1, '\ArrayFilter\isString');
+	assertType('array<int|string, int|string>', $filtered1);
+
+	$filtered2 = array_filter($map2, '\ArrayFilter\Filters::isString');
+	assertType('array<int|string, int|string>', $filtered2);
+}
+
+/**
+ * @param mixed $value
+ */
+function isString($value): bool
+{
+	return is_string($value);
+}
+
+class Filters {
+	/**
+	 * @param mixed $value
+	 */
+	public static function isString($value): bool
+	{
+		return is_string($value);
+	}
+}