Fix false positive unresolvable template type on optional variadic params

- When a template type is only used on a variadic parameter and no arguments are passed, it was incorrectly reported as unresolvable
- Fixed by inferring NeverType for variadic parameters with no arguments in GenericParametersAcceptorResolver, since an empty variadic means the element type is never
- Added regression tests in both rule test and NSRT format

Closes phpstan/phpstan#2572

Author: github-actions[bot]

src/Reflection/GenericParametersAcceptorResolver.php

@@ -10,6 +10,7 @@
 use PHPStan\Type\Generic\TemplateTypeMap;
 use PHPStan\Type\Generic\TemplateTypeVarianceMap;
 use PHPStan\Type\MixedType;
+use PHPStan\Type\NeverType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
 use function array_key_exists;
@@ -61,6 +62,8 @@ public static function resolve(array $argTypes, ParametersAcceptor $parametersAc
 				$argType = $namedArgTypes[$param->getName()];
 			} elseif ($param->getDefaultValue() !== null) {
 				$argType = $param->getDefaultValue();
+			} elseif ($param->isVariadic()) {
+				$argType = new NeverType();
 			} else {
 				continue;
 			}

tests/PHPStan/Analyser/nsrt/bug-2572.php

@@ -0,0 +1,27 @@
+<?php declare(strict_types = 1);
+
+namespace Bug2572Nsrt;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @template TE
+ * @template TR
+ *
+ * @param TE $elt
+ * @param TR ...$elts
+ *
+ * @return TE|TR
+ */
+function collect($elt, ...$elts) {
+	$ret = $elt;
+	foreach ($elts as $item) {
+		if (rand(0, 1)) {
+			$ret = $item;
+		}
+	}
+	return $ret;
+}
+
+assertType("'a'", collect("a"));
+assertType("'a'|'b'|'c'", collect("a", "b", "c"));

tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php

@@ -2691,6 +2691,11 @@ public function testBug8936(): void
 		$this->analyse([__DIR__ . '/data/bug-8936.php'], []);
 	}
 
+	public function testBug2572(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-2572.php'], []);
+	}
+
 	public function testDumpFunctions(): void
 	{
 		$this->analyse([__DIR__ . '/data/dump-functions.php'], [

tests/PHPStan/Rules/Functions/data/bug-2572.php

@@ -0,0 +1,25 @@
+<?php declare(strict_types = 1);
+
+namespace Bug2572;
+
+/**
+ * @template TE
+ * @template TR
+ *
+ * @param TE $elt
+ * @param TR ...$elts
+ *
+ * @return TE|TR
+ */
+function collect($elt, ...$elts) {
+	$ret = $elt;
+	foreach ($elts as $item) {
+		if (rand(0, 1)) {
+			$ret = $item;
+		}
+	}
+	return $ret;
+}
+
+echo collect("a");
+echo collect("a", "b", "c");