lalr1.cc: support move semantics

Modern C++ (i.e., C++11 and later) introduced "move only" types: types such
as std::unique_ptr<T> that can never be duplicated.  They must never be
copied (by assignments and constructors), they must be "moved".  The
implementation of lalr1.cc used to copy symbols (including their semantic
values).  This commit ensures that values are only moved in modern C++, yet
remain compatible with C++98/C++03.

Suggested by Frank Heckenbach, who provided a full implementation on
top of C++17's std::variant.
See http://lists.gnu.org/archive/html/bug-bison/2018-03/msg00002.html,
and https://lists.gnu.org/archive/html/bison-patches/2018-04/msg00002.html.

Symbols (terminal/non terminal) are handled by several functions that used
to take const-refs, which resulted eventually in a copy pushed on the stack.
With modern C++ (C++11 and later) the callers must use std::move, and the
callees must take their arguments as rvalue refs (foo&&).  In order to avoid
duplicating these functions to support both legacy C++ and modern C++, let's
introduce macros (YY_MOVE, YY_RVREF, etc.)  that rely on copy-semantics for
C++98/03, and move-semantics for modern C++.

That's easy for inner types, when the parser's functions pass arguments to
each other.  Functions facing the user (make_NUMBER, make_STRING, etc.)
should support both rvalue-refs (for instance to support move-only types:
make_INT (std::make_unique<int> (1))), and lvalue-refs (so that we can pass
a variable: make_INT (my_int)).  To avoid the multiplication of the
signatures (there is also the location), let's take the argument by value.

See:
https://lists.gnu.org/archive/html/bison-patches/2018-09/msg00024.html.

* data/c++.m4 (b4_cxx_portability): New.
(basic_symbol): In C++11, replace copy-ctors with move-ctors.
In C++11, replace copies with moves.
* data/lalr1.cc (stack_symbol_type, yypush_): Likewise.
Use YY_MOVE to avoid useless copies.
* data/variant.hh (variant): Support move-semantics.
(make_SYMBOL): In C++11, in order to support both read-only lvalues,
and rvalues, take the argument as a copy.
* data/stack.hh (yypush_): Use rvalue-refs in C++11.
* tests/c++.at: Use move semantics.

* tests/headers.at: Adjust to the new macros (YY_MOVE, etc.).

* configure.ac (CXX98_CXXFLAGS, CXX11_CXXFLAGS, CXX14_CXXFLAGS)
(CXX17_CXXFLAGS, ENABLE_CXX11): New.
* tests/atlocal.in: Receive them.

* examples/variant.yy: Don't define things in std.
* examples/variant-11.test, examples/variant-11.yy: New.
Check the support of move-only types.
* examples/README, examples/local.mk: Adjust.

data/variant.hh

@@ -101,11 +101,11 @@ m4_define([b4_variant_define],
 
     /// Construct and fill.
     template <typename T>
-    variant (const T& t)]b4_parse_assert_if([
+    variant (YY_RVREF (T) t)]b4_parse_assert_if([
       : yytypeid_ (&typeid (T))])[
     {
       YYASSERT (sizeof (T) <= S);
-      new (yyas_<T> ()) T (t);
+      new (yyas_<T> ()) T (YY_MOVE (t));
     }
 
     /// Destruction, allowed only if empty.
@@ -183,10 +183,26 @@ m4_define([b4_variant_define],
     move (self_type& other)
     {
       build<T> ();
+# if defined __cplusplus && 201103L <= __cplusplus
+      as<T> () = YY_MOVE (other.as<T> ());
+# else
       swap<T> (other);
+# endif
       other.destroy<T> ();
     }
 
+# if defined __cplusplus && 201103L <= __cplusplus
+    /// Move the content of \a other to this.
+    template <typename T>
+    void
+    move (self_type&& other)
+    {
+      build<T> ();
+      as<T> () = YY_MOVE (other.as<T> ());
+      other.destroy<T> ();
+    }
+#endif
+
     /// Copy the content of \a other to this.
     template <typename T>
     void
@@ -293,8 +309,8 @@ m4_define([b4_symbol_constructor_declare_],
     symbol_type
     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])));
+                     [YY_COPY (b4_symbol([$1], [type])) v]),
+        b4_locations_if([YY_COPY (location_type) l])));
 
 ])])])
 
@@ -318,12 +334,12 @@ m4_define([b4_symbol_constructor_define_],
   b4_parser_class_name::symbol_type
   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])))
+                     [YY_COPY (b4_symbol([$1], [type])) v]),
+        b4_locations_if([YY_COPY (location_type) l])))
   {
     return symbol_type (b4_join([token::b4_symbol([$1], [id])],
-                                b4_symbol_if([$1], [has_type], [v]),
-                                b4_locations_if([l])));
+                                b4_symbol_if([$1], [has_type], [YY_MOVE (v)]),
+                                b4_locations_if([YY_MOVE (l)])));
   }
 
 ])])])
@@ -335,8 +351,8 @@ b4_join(b4_symbol_if([$1], [has_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_symbol_if([$1], [has_type], [YY_RVREF (b4_symbol([$1], [type])) v]),
+          b4_locations_if([YY_RVREF (location_type) l]))[);
 ]])
 
 # b4_basic_symbol_constructor_define
@@ -346,11 +362,11 @@ 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]))[)
+          b4_symbol_if([$1], [has_type], [YY_RVREF (b4_symbol([$1], [type])) v]),
+          b4_locations_if([YY_RVREF (location_type) l]))[)
     : Base (t)]b4_symbol_if([$1], [has_type], [
-    , value (v)])[]b4_locations_if([
-    , location (l)])[
+    , value (YY_MOVE (v))])[]b4_locations_if([
+    , location (YY_MOVE (l))])[
   {}
 
 ]])