Update grammar rule for enums to allow specifying an underlying type

Author: danpoe

src/ansi-c/c_typecheck_type.cpp

@@ -1160,6 +1160,12 @@ void c_typecheck_baset::typecheck_c_enum_type(typet &type)
     throw 0;
   }
 
+  if(as_expr.find(ID_enum_underlying_type).is_not_nil())
+  {
+    warning().source_location = source_location;
+    warning() << "ignoring specification of underlying type for enum" << eom;
+  }
+
   // enums start at zero;
   // we also track min and max to find a nice base type
   mp_integer value=0, min_value=0, max_value=0;
@@ -1352,6 +1358,12 @@ void c_typecheck_baset::typecheck_c_enum_tag_type(c_enum_tag_typet &type)
     throw 0;
   }
 
+  if(type.find(ID_enum_underlying_type).is_not_nil())
+  {
+    warning().source_location = type.source_location();
+    warning() << "ignoring specification of underlying type for enum" << eom;
+  }
+
   source_locationt source_location=type.source_location();
 
   irept &tag=type.add(ID_tag);

src/ansi-c/parser.y

@@ -267,10 +267,14 @@ extern char *yyansi_ctext;
 
 %start grammar
 
-%expect 1 /* the famous "dangling `else'" ambiguity */
+%expect 2 /* the famous "dangling `else'" ambiguity */
           /* results in one shift/reduce conflict   */
           /* that we don't want to be reported      */
 
+          /* a second shift/reduce conflict arises due to enum underlying */
+          /* type specifications and bitfield specifications, which are both */
+          /* introduced by a ':' and follow a type */
+
 %{
 /************************************************************************/
 /*** rules **************************************************************/
@@ -1790,38 +1794,70 @@ bit_field_size:
 enum_name:
           enum_key
           gcc_type_attribute_opt
+          enum_underlying_type_opt
         {
           // an anon enum
+          if(parser_stack($3).is_not_nil())
+          {
+            parser_stack($1).set(ID_enum_underlying_type, parser_stack($3));
+          }
         }
          '{' enumerator_list_opt '}'
           gcc_type_attribute_opt
         {
-          parser_stack($1).operands().swap(parser_stack($5).operands());
-          $$=merge($1, merge($2, $7)); // throw in the gcc attributes
+          parser_stack($1).operands().swap(parser_stack($6).operands());
+          $$=merge($1, merge($2, $8)); // throw in the gcc attributes
         }
         | enum_key
           gcc_type_attribute_opt
           identifier_or_typedef_name
+          enum_underlying_type_opt
         {
           // an enum with tag
           parser_stack($1).set(ID_tag, parser_stack($3));
+
+          if(parser_stack($4).is_not_nil())
+          {
+            parser_stack($1).set(ID_enum_underlying_type, parser_stack($4));
+          }
         }
-          '{' enumerator_list_opt '}'
+          braced_enumerator_list_opt
           gcc_type_attribute_opt
         {
-          parser_stack($1).operands().swap(parser_stack($6).operands());
-          $$=merge($1, merge($2, $8)); // throw in the gcc attributes
+          if(parser_stack($6).is_not_nil())
+          {
+            parser_stack($1).operands().swap(parser_stack($6).operands());
+          }
+          else
+          {
+            parser_stack($1).id(ID_c_enum_tag);
+          }
+
+          $$=merge($1, merge($2, $7)); // throw in the gcc attributes
         }
-        | enum_key
-          gcc_type_attribute_opt
-          identifier_or_typedef_name
-          gcc_type_attribute_opt
+        ;
+
+enum_underlying_type_opt:
+        /* empty */
         {
-          parser_stack($1).id(ID_c_enum_tag); // tag only
-          parser_stack($1).set(ID_tag, parser_stack($3));
-          $$=merge($1, merge($2, $4)); // throw in the gcc attributes
+          init($$);
+          parser_stack($$).make_nil();
+        }
+        | ':' basic_type_name
+        {
+          $$=$2;
+        }
+
+braced_enumerator_list_opt:
+        /* empty */
+        {
+          init($$);
+          parser_stack($$).make_nil();
+        }
+        | '{' enumerator_list_opt '}'
+        {
+          $$=$2;
         }
-        ;
 
 enum_key: TOK_ENUM
         {

src/util/irep_ids.def

@@ -212,6 +212,7 @@ IREP_ID_ONE(NULL)
 IREP_ID_ONE(null)
 IREP_ID_ONE(nullptr)
 IREP_ID_ONE(c_enum)
+IREP_ID_ONE(enum_underlying_type)
 IREP_ID_ONE(enumeration)
 IREP_ID_ONE(elements)
 IREP_ID_ONE(unknown)