Make function_exists() similar to is_callable()

src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php

@@ -12,6 +12,7 @@
 use PHPStan\Analyser\TypeSpecifierAwareExtension;
 use PHPStan\Analyser\TypeSpecifierContext;
 use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Type\CallableType;
 use PHPStan\Type\Constant\ConstantBooleanType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\FunctionTypeSpecifyingExtension;
@@ -45,7 +46,13 @@ public function specifyTypes(FunctionReflection $functionReflection, FuncCall $n
 			);
 		}
 
-		return new SpecifiedTypes();
+		return $this->typeSpecifier->create(
+			$node->getArgs()[0]->value,
+			new CallableType(),
+			$context,
+			false,
+			$scope
+		);
 	}
 
 	public function setTypeSpecifier(TypeSpecifier $typeSpecifier): void

tests/PHPStan/Rules/Functions/CallCallablesRuleTest.php

@@ -213,4 +213,9 @@ public function testRuleWithNullsafeVariant(): void
 		]);
 	}
 
+	public function testBug1849(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-1849.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-1849.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace Bug1849;
+
+function (string $type): void {
+	$func = 'is_' . $type;
+
+	if (function_exists($func)) {
+		$func([]);
+	}
+};