Allow object reference in C++ trailing return type

Fixes #231

CHANGES.current

@@ -7,6 +7,10 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-02-01: olly
+	    #231 Handle returning an object by reference in a C++ trailing
+	    return type.
+
 2022-02-01: davidcl
             [Scilab] #745 use SWIG_<module>_Init() as a C module init function.
 

Examples/test-suite/cpp11_alternate_function_syntax.i

@@ -3,6 +3,8 @@
 %module cpp11_alternate_function_syntax
 
 %inline %{
+struct Hello {};
+
 struct SomeStruct {
   int addNormal(int x, int y);
   auto addAlternate(int x, int y) -> int;
@@ -12,6 +14,9 @@ struct SomeStruct {
   auto addAlternateMemberPtrParm(int x, int (SomeStruct::*mp)(int, int)) -> int;
   auto addAlternateMemberPtrConstParm(int x, int (SomeStruct::*mp)(int, int) const) const -> int;
 
+  // Returning a reference didn't parse in SWIG < 4.1.0 (#231)
+  auto output() -> Hello&;
+
   virtual auto addFinal(int x, int y) const noexcept -> int final { return x + y; }
   virtual ~SomeStruct() = default;
 };
@@ -27,5 +32,6 @@ auto SomeStruct::addAlternateMemberPtrParm(int x, int (SomeStruct::*mp)(int, int
 auto SomeStruct::addAlternateMemberPtrConstParm(int x, int (SomeStruct::*mp)(int, int) const) const -> int {
   return 1000*x + (this->*mp)(x, x);
 }
+auto SomeStruct::output() -> Hello& { static Hello h; return h; }
 
 %}

Source/CParse/parser.y

@@ -3383,6 +3383,10 @@ cpp_alternate_rettype : primitive_type { $$ = $1; }
 */
               | TYPE_RAW { $$ = $1; }
               | idcolon { $$ = $1; }
+              | idcolon AND {
+                $$ = $1;
+                SwigType_add_reference($$);
+              }
               | decltype { $$ = $1; }
               ;