Support error recovery for missing return type

The return type is set to "null" in this case.

Fixes #544.

CHANGELOG.md

@@ -1,7 +1,9 @@
 Version 4.1.1-dev
 -----------------
 
-Nothing yet.
+### Added
+
+* Support error recovery for missing return type (`function foo() : {}`).
 
 Version 4.1.0 (2018-10-10)
 --------------------------

grammar/php7.y

@@ -477,6 +477,7 @@ optional_param_type:
 optional_return_type:
       /* empty */                                           { $$ = null; }
     | ':' type_expr                                         { $$ = $2; }
+    | ':' error                                             { $$ = null; }
 ;
 
 argument_list:

test/code/parser/errorHandling/recovery.test

@@ -1374,3 +1374,30 @@ array(
         )
     )
 )
+-----
+<?php
+function foo() :
+{
+    return $a;
+}
+-----
+!!php7
+Syntax error, unexpected '{' from 3:1 to 3:1
+array(
+    0: Stmt_Function(
+        byRef: false
+        name: Identifier(
+            name: foo
+        )
+        params: array(
+        )
+        returnType: null
+        stmts: array(
+            0: Stmt_Return(
+                expr: Expr_Variable(
+                    name: a
+                )
+            )
+        )
+    )
+)