date function can sometimes return numeric string

conf/config.neon

@@ -790,6 +790,11 @@ services:
 		tags:
 			- phpstan.broker.dynamicFunctionReturnTypeExtension
 
+	-
+		class: PHPStan\Type\Php\DateFunctionReturnTypeExtension
+		tags:
+			- phpstan.broker.dynamicFunctionReturnTypeExtension
+
 	-
 		class: PHPStan\Type\Php\DsMapDynamicReturnTypeExtension
 		tags:

src/Type/Php/DateFunctionReturnTypeExtension.php

@@ -0,0 +1,51 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use PhpParser\Node\Expr\FuncCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Type\Accessory\AccessoryNumericStringType;
+use PHPStan\Type\DynamicFunctionReturnTypeExtension;
+use PHPStan\Type\IntersectionType;
+use PHPStan\Type\StringType;
+use PHPStan\Type\Type;
+use PHPStan\Type\TypeUtils;
+
+class DateFunctionReturnTypeExtension implements DynamicFunctionReturnTypeExtension
+{
+
+	public function isFunctionSupported(FunctionReflection $functionReflection): bool
+	{
+		return $functionReflection->getName() === 'date';
+	}
+
+	public function getTypeFromFunctionCall(
+		FunctionReflection $functionReflection,
+		FuncCall $functionCall,
+		Scope $scope
+	): Type
+	{
+		if (count($functionCall->args) === 0) {
+			return new StringType();
+		}
+		$argType = $scope->getType($functionCall->args[0]->value);
+		$constantStrings = TypeUtils::getConstantStrings($argType);
+		if (count($constantStrings) === 0) {
+			return new StringType();
+		}
+
+		foreach ($constantStrings as $constantString) {
+			$formattedDate = date($constantString->getValue());
+			if (!is_numeric($formattedDate)) {
+				return new StringType();
+			}
+		}
+
+		return new IntersectionType([
+			new StringType(),
+			new AccessoryNumericStringType(),
+		]);
+	}
+
+}

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10043,6 +10043,11 @@ public function dataBug2550(): array
 		return $this->gatherAssertTypes(__DIR__ . '/../Rules/Comparison/data/bug-2550.php');
 	}
 
+	public function dataBug2899(): array
+	{
+		return $this->gatherAssertTypes(__DIR__ . '/data/bug-2899.php');
+	}
+
 	/**
 	 * @dataProvider dataBug2574
 	 * @dataProvider dataBug2577
@@ -10110,6 +10115,7 @@ public function dataBug2550(): array
 	 * @dataProvider dataNonEmptyArrayKeyType
 	 * @dataProvider dataBug3133
 	 * @dataProvider dataBug2550
+	 * @dataProvider dataBug2899
 	 * @param string $assertType
 	 * @param string $file
 	 * @param mixed ...$args

tests/PHPStan/Analyser/data/bug-2899.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Bug2899;
+
+use function PHPStan\Analyser\assertType;
+
+class Foo
+{
+
+	public function doFoo(string $s, $mixed)
+	{
+		assertType('string&numeric', date('Y'));
+		assertType('string', date('Y.m.d'));
+		assertType('string', date($s));
+		assertType('string', date($mixed));
+	}
+
+}