m4: generate a basic_symbol constructor for each symbol type

Recently, there was a slightly vicious bug hidden in the make_ functions:

  parser::symbol_type
  parser::make_TEXT (const ::std::string& v)
  {
    return symbol_type (token::TOK_TEXT, v);
  }

The constructor for symbol_type doesn't take an ::std::string& as
argument, but a constant variant.  However, because there is a variant
constructor which takes an ::std::string&, this caused the implicit
construction of a built variant.  Considering that the variant argument
for the symbol_type constructor was cv-qualified, this temporary variant
was never destroyed.

As a temporary solution, the symbol was built in two stages:

  symbol_type res (token::TOK_TEXT);
  res.value.build< ::std::string&> (v);
  return res;

However, the solution introduced in this patch contributes to letting
the symbols handle themselves, by supplying them with constructors that
take a non-variant value and build the symbol's own variant with that
value.

* data/variant.hh (b4_symbol_constructor_define_): Use the new
constructors rather than building in a temporary symbol.
(b4_basic_symbol_constructor_declare,
b4_basic_symbol_constructor_define): New macros generating the
constructors.
* data/c++.m4 (basic_symbol): Invoke the macros here.

data/c++.m4

@@ -180,10 +180,12 @@ m4_define([b4_public_types_declare],
 
       /// Copy constructor.
       basic_symbol (const basic_symbol& other);
-
+]b4_variant_if([[
+      /// Constructor for valueless symbols, and symbols from each type.
+]b4_type_foreach([b4_basic_symbol_constructor_declare])], [[
       /// Constructor for valueless symbols.
       basic_symbol (typename Base::kind_type t]b4_locations_if([,
-                    const location_type& l])[);
+                    const location_type& l])[);]])[
 
       /// Constructor for symbols with semantic value.
       basic_symbol (typename Base::kind_type t,
@@ -287,6 +289,10 @@ m4_define([b4_public_types_define],
     (void) v;
     ]b4_symbol_variant([this->type_get ()], [value], [copy], [v])])[}
 
+]b4_variant_if([[
+  // Implementation of basic_symbol constructor for each type.
+]b4_type_foreach([b4_basic_symbol_constructor_define])], [[
+  /// Constructor for valueless symbols.
   template <typename Base>
   inline
   ]b4_parser_class_name[::basic_symbol<Base>::basic_symbol (]b4_join(
@@ -295,7 +301,7 @@ m4_define([b4_public_types_define],
     : Base (t)
     , value ()]b4_locations_if([
     , location (l)])[
-  {}
+  {}]])[
 
   template <typename Base>
   inline

data/variant.hh

@@ -311,16 +311,44 @@ m4_define([b4_symbol_constructor_define_],
   b4_parser_class_name::make_[]b4_symbol_([$1], [id]) (dnl
 b4_join(b4_symbol_if([$1], [has_type],
                      [const b4_symbol([$1], [type])& v]),
-        b4_locations_if([const location_type& l])))[
+        b4_locations_if([const location_type& l])))
   {
-    symbol_type res (token::]b4_symbol([$1], [id])[]b4_locations_if([, l])[);
+    return symbol_type (b4_join([token::b4_symbol([$1], [id])],
-    ]b4_symbol_if([$1], [has_type], [res.value.build (v);
+                                b4_symbol_if([$1], [has_type], [v]),
-    ])[return res;
+                                b4_locations_if([l])));
+
   }
 
-]])])])
+])])])
 
 
+# b4_basic_symbol_constructor_declare
+# ----------------------------------
+# Generate a constructor declaration for basic_symbol from given type.
+m4_define([b4_basic_symbol_constructor_declare],
+[[
+  basic_symbol (]b4_join(
+          [typename Base::kind_type t],
+          b4_symbol_if([$1], [has_type], const b4_symbol([$1], [type])[ v]),
+          b4_locations_if([const location_type& l]))[);
+]])
+
+# b4_basic_symbol_constructor_define
+# ----------------------------------
+# Generate a constructor implementation for basic_symbol from given type.
+m4_define([b4_basic_symbol_constructor_define],
+[[
+  template <typename Base>
+  ]b4_parser_class_name[::basic_symbol<Base>::basic_symbol (]b4_join(
+          [typename Base::kind_type t],
+          b4_symbol_if([$1], [has_type], const b4_symbol([$1], [type])[ v]),
+          b4_locations_if([const location_type& l]))[)
+    : Base (t)
+    , value (]b4_symbol_if([$1], [has_type], [v])[)]b4_locations_if([
+    , location (l)])[
+  {}
+]])
+
 # b4_symbol_constructor_define
 # ----------------------------
 # Define the overloaded versions of make_symbol for all the value types.