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.

1 files changed, 2 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index e60af602..b7ba45dc 100644
--- a/configure.ac
+++ b/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.