Prevent false-positive for Exception::__toString overriding

src/Psalm/Internal/Stubs/CoreImmutableClasses.phpstub

@@ -60,9 +60,10 @@ interface Throwable
     public function getTraceAsString() : string;
 
     /**
+     * @return string
      * @psalm-taint-source input
      */
-    public function __toString() : string;
+    public function __toString();
 }
 
 /**
@@ -137,9 +138,10 @@ class Exception implements Throwable
     public final function getTraceAsString() : string {}
 
     /**
+     * @return string
      * @psalm-taint-source input
      */
-    public function __toString() : string {}
+    public function __toString() {}
 }
 
 /**
@@ -212,7 +214,8 @@ class Error implements Throwable
     public final function getTraceAsString() : string {}
 
     /**
+     * @return string
      * @psalm-taint-source input
      */
-    public function __toString() : string {}
+    public function __toString() {}
 }

tests/MethodSignatureTest.php

@@ -791,6 +791,25 @@ class MethodSignatureTest extends TestCase
                         public function getPrevious(): ?\Throwable;
                         public function getTraceAsString(): string;
                     }'
+            ],
+            'allowExecptionToStringWithNoType' => [
+                '<?php
+                    class E extends Exception {
+                        public function __toString() {
+                            return "hello";
+                        }
+                    }'
+            ],
+            'allowExecptionToStringIn71' => [
+                '<?php
+                    class E extends Exception {
+                        public function __toString() : string {
+                            return "hello";
+                        }
+                    }',
+                [],
+                [],
+                '7.1'
             ]
         ];
     }