c++: avoid -Wundefined-func-template warnings from clang

Reported by Derek Clegg.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00006.html

Clang does not like this:

    template <typename D>
    struct basic_symbol : D
    {
      basic_symbol();
    };

    struct by_type {};

    struct symbol_type : basic_symbol<by_type>
    {
      symbol_type(){}
    };

It gives:

    $ clang++-mp-7.0 -Wundefined-func-template foo.cc -c
    foo.cc:11:3: warning: instantiation of function 'basic_symbol<by_type>::basic_symbol'
                 required here, but no definition is available [-Wundefined-func-template]
      symbol_type(){}
      ^
    foo.cc:4:3: note: forward declaration of template entity is here
      basic_symbol();
      ^
    foo.cc:11:3: note: add an explicit instantiation declaration to suppress this warning
                 if 'basic_symbol<by_type>::basic_symbol' is explicitly instantiated in
                 another translation unit
      symbol_type(){}
      ^
    1 warning generated.

The same applies for the basic_symbol's destructor and `clear()`.

* configure.ac (warn_cxx): Add -Wundefined-func-template.
This triggered one failure in the test suite:
* tests/headers.at (Sane headers): here, where we check that we can
compile the generated headers in other compilation units than the
parser's.
Add a variant type to make sure that basic_symbol and symbol_type are
properly generated in this case.
* data/skeletons/c++.m4 (basic_symbol): Inline the definitions of the
destructor and of `clear` in the class definition.

configure.ac

@@ -96,7 +96,8 @@ AM_CONDITIONAL([ENABLE_GCC_WARNINGS], [test "$enable_gcc_warnings" = yes])
 if test "$enable_gcc_warnings" = yes; then
   warn_common='-Wall -Wextra -Wno-sign-compare -Wcast-align
     -fparse-all-comments -Wdocumentation
-    -Wformat -Wnull-dereference -Wpointer-arith -Wshadow -Wwrite-strings'
+    -Wformat -Wnull-dereference -Wpointer-arith -Wshadow
+    -Wundefined-func-template -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
   warn_cxx='-Wextra-semi -Wnoexcept'
   # Warnings for the test suite only.

data/skeletons/c++.m4

@@ -260,7 +260,10 @@ m4_define([b4_symbol_type_define],
       typedef Base super_type;
 
       /// Default constructor.
-      basic_symbol ();
+      basic_symbol ()
+        : value ()]b4_locations_if([
+        , location ()])[
+      {}
 
 #if 201103L <= YY_CPLUSPLUS
       /// Move constructor.
@@ -282,10 +285,29 @@ m4_define([b4_symbol_type_define],
                     YY_RVREF (location_type) l])[);
 ]])[
       /// Destroy the symbol.
-      ~basic_symbol ();
+      ~basic_symbol ()
+      {
+        clear ();
+      }
 
       /// Destroy contents, and record that is empty.
-      void clear ();
+      void clear ()
+      {]b4_variant_if([[
+        // User destructor.
+        symbol_number_type yytype = this->type_get ();
+        basic_symbol<Base>& yysym = *this;
+        (void) yysym;
+        switch (yytype)
+        {
+]b4_symbol_foreach([b4_symbol_destructor])dnl
+[       default:
+          break;
+        }
+
+        // Type destructor.
+]b4_symbol_variant([[yytype]], [[value]], [[template destroy]])])[
+        Base::clear ();
+      }
 
       /// Whether empty.
       bool empty () const YY_NOEXCEPT;
@@ -365,12 +387,6 @@ m4_define([b4_symbol_type_define],
 # Provide the implementation needed by the public types.
 m4_define([b4_public_types_define],
 [[  // basic_symbol.
-  template <typename Base>
-  ]b4_parser_class[::basic_symbol<Base>::basic_symbol ()
-    : value ()]b4_locations_if([
-    , location ()])[
-  {}
-
 #if 201103L <= YY_CPLUSPLUS
   template <typename Base>
   ]b4_parser_class[::basic_symbol<Base>::basic_symbol (basic_symbol&& that)
@@ -416,32 +432,6 @@ m4_define([b4_public_types_define],
     (void) v;
     ]b4_symbol_variant([this->type_get ()], [value], [YY_MOVE_OR_COPY], [YY_MOVE (v)])])[}]])[
 
-  template <typename Base>
-  ]b4_parser_class[::basic_symbol<Base>::~basic_symbol ()
-  {
-    clear ();
-  }
-
-  template <typename Base>
-  void
-  ]b4_parser_class[::basic_symbol<Base>::clear ()
-  {]b4_variant_if([[
-    // User destructor.
-    symbol_number_type yytype = this->type_get ();
-    basic_symbol<Base>& yysym = *this;
-    (void) yysym;
-    switch (yytype)
-    {
-]b4_symbol_foreach([b4_symbol_destructor])dnl
-[   default:
-      break;
-    }
-
-    // Type destructor.
-  ]b4_symbol_variant([[yytype]], [[value]], [[template destroy]])])[
-    Base::clear ();
-  }
-
   template <typename Base>
   bool
   ]b4_parser_class[::basic_symbol<Base>::empty () const YY_NOEXCEPT

tests/headers.at

@@ -125,7 +125,7 @@ AT_BISON_OPTION_PUSHDEFS([$1])
 AT_DATA_GRAMMAR([input.y],
 [[$1
 %define parse.error verbose
-]AT_VARIANT_IF([], [%union {int integer;}])[
+]AT_VARIANT_IF([%token <int> 'x'], [%union {int integer;}])[
 %code {
 ]AT_PUSH_IF([[
 #if defined __GNUC__ && 7 == __GNUC__