diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/FunctionCallAnalyzer.php b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/FunctionCallAnalyzer.php
index f852b5a2a..d093dedbb 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/FunctionCallAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/FunctionCallAnalyzer.php
@@ -897,9 +897,15 @@ class FunctionCallAnalyzer extends CallAnalyzer
                 if ($function_storage && $function_storage->template_types) {
                     foreach ($function_storage->template_types as $template_name => $_) {
                         if (!isset($template_result->upper_bounds[$template_name])) {
-                            $template_result->upper_bounds[$template_name] = [
-                                'fn-' . $function_id => [Type::getEmpty(), 0]
-                            ];
+                            if ($template_name === 'TFunctionArgCount') {
+                                $template_result->upper_bounds[$template_name] = [
+                                    'fn-' . $function_id => [Type::getInt(false, count($stmt->args)), 0]
+                                ];
+                            } else {
+                                $template_result->upper_bounds[$template_name] = [
+                                    'fn-' . $function_id => [Type::getEmpty(), 0]
+                                ];
+                            }
                         }
                     }
                 }
diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/MethodCallReturnTypeFetcher.php b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/MethodCallReturnTypeFetcher.php
index c28b86397..c20e07b69 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/MethodCallReturnTypeFetcher.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/MethodCallReturnTypeFetcher.php
@@ -140,9 +140,15 @@ class MethodCallReturnTypeFetcher
                                     [$template_type->defining_class]
                             )
                         ) {
-                            $template_result->upper_bounds[$template_type->param_name] = [
-                                ($template_type->defining_class) => [Type::getEmpty(), 0]
-                            ];
+                            if ($template_type->param_name === 'TFunctionArgCount') {
+                                $template_result->upper_bounds[$template_type->param_name] = [
+                                    'fn-' . $method_id => [Type::getInt(false, \count($stmt->args)), 0]
+                                ];
+                            } else {
+                                $template_result->upper_bounds[$template_type->param_name] = [
+                                    ($template_type->defining_class) => [Type::getEmpty(), 0]
+                                ];
+                            }
                         }
                     }
                 }
diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/StaticCallAnalyzer.php b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/StaticCallAnalyzer.php
index 70d4ae72b..0608ed533 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/Call/StaticCallAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/Call/StaticCallAnalyzer.php
@@ -916,9 +916,15 @@ class StaticCallAnalyzer extends \Psalm\Internal\Analyzer\Statements\Expression\
                                         [$template_type->param_name]
                                         [$template_type->defining_class]
                                 )) {
-                                    $template_result->upper_bounds[$template_type->param_name] = [
-                                        ($template_type->defining_class) => [Type::getEmpty(), 0]
-                                    ];
+                                    if ($template_type->param_name === 'TFunctionArgCount') {
+                                        $template_result->upper_bounds[$template_type->param_name] = [
+                                            'fn-' . $method_id => [Type::getInt(false, count($stmt->args)), 0]
+                                        ];
+                                    } else {
+                                        $template_result->upper_bounds[$template_type->param_name] = [
+                                            ($template_type->defining_class) => [Type::getEmpty(), 0]
+                                        ];
+                                    }
                                 }
                             }
                         }
diff --git a/src/Psalm/Internal/PhpVisitor/ReflectorVisitor.php b/src/Psalm/Internal/PhpVisitor/ReflectorVisitor.php
index e246fe021..34a90db90 100644
--- a/src/Psalm/Internal/PhpVisitor/ReflectorVisitor.php
+++ b/src/Psalm/Internal/PhpVisitor/ReflectorVisitor.php
@@ -2513,16 +2513,15 @@ class ReflectorVisitor extends PhpParser\NodeVisitorAbstract implements PhpParse
                 // This checks for param references in the return type tokens
                 // If found, the param is replaced with a generated template param
                 foreach ($fixed_type_tokens as $i => $type_token) {
-                    if ($type_token[0][0] === '$') {
-                        $token_body = $type_token[0];
+                    $token_body = $type_token[0];
+                    $template_function_id = 'fn-' . strtolower($cased_function_id);
 
+                    if ($token_body[0] === '$') {
                         foreach ($storage->params as $j => $param_storage) {
                             if ('$' . $param_storage->name === $token_body) {
                                 if (!isset($param_type_mapping[$token_body])) {
                                     $template_name = 'TGeneratedFromParam' . $j;
 
-                                    $template_function_id = 'fn-' . strtolower($cased_function_id);
-
                                     $template_as_type = $param_storage->type
                                         ? clone $param_storage->type
                                         : Type::getMixed();
@@ -2556,9 +2555,26 @@ class ReflectorVisitor extends PhpParser\NodeVisitorAbstract implements PhpParse
                                 }
 
                                 $fixed_type_tokens[$i][0] = $param_type_mapping[$token_body];
+
+                                continue 2;
                             }
                         }
                     }
+
+                    if ($token_body === 'func_num_args()') {
+                        $template_name = 'TFunctionArgCount';
+
+                        $storage->template_types[$template_name] = [
+                            $template_function_id => [
+                                Type::getInt()
+                            ],
+                        ];
+
+                        $this->function_template_types[$template_name]
+                            = $storage->template_types[$template_name];
+
+                        $fixed_type_tokens[$i][0] = $template_name;
+                    }
                 }
 
                 $storage->return_type = Type::parseTokens(
diff --git a/src/Psalm/Type.php b/src/Psalm/Type.php
index e9972d1f9..867f2c195 100644
--- a/src/Psalm/Type.php
+++ b/src/Psalm/Type.php
@@ -1147,6 +1147,17 @@ abstract class Type
                 || $char === '&'
                 || $char === '='
             ) {
+                if ($char === '('
+                    && $type_tokens[$rtc][0] === 'func_num_args'
+                    && isset($chars[$i + 1])
+                    && $chars[$i + 1] === ')'
+                ) {
+                    $type_tokens[$rtc][0] = 'func_num_args()';
+                    ++$i;
+
+                    continue;
+                }
+
                 if ($type_tokens[$rtc][0] === '') {
                     $type_tokens[$rtc] = [$char, $i];
                 } else {
diff --git a/tests/Template/ConditionalReturnTypeTest.php b/tests/Template/ConditionalReturnTypeTest.php
index 062d9bf44..b2a7b7864 100644
--- a/tests/Template/ConditionalReturnTypeTest.php
+++ b/tests/Template/ConditionalReturnTypeTest.php
@@ -439,6 +439,28 @@ class ConditionalReturnTypeTest extends TestCase
                         }
                     }'
             ],
+            'conditionalOnArgCount' => [
+                '<?php
+                    /**
+                     * @return (func_num_args() is 0 ? false : string)
+                     */
+                    function zeroArgsFalseOneArgString(string $s = "") {
+                        if (func_num_args() === 0) {
+                            return false;
+                        }
+
+                        return $s;
+                    }
+
+                    $a = zeroArgsFalseOneArgString();
+                    $b = zeroArgsFalseOneArgString("");
+                    $c = zeroArgsFalseOneArgString("hello");',
+                [
+                    '$a' => 'false',
+                    '$b' => 'string',
+                    '$c' => 'string',
+                ]
+            ],
         ];
     }
 }