[PATCH 1/4] glr2.cc: disable incorrect warnings from GCC6 to 9

For instance with GCC8:

    616. regression.at:1560: testing Lex and parse params: glr2.cc ...
    tests/regression.at:1560: COLUMNS=1000; export COLUMNS; NO_TERM_HYPERLINKS=1; export NO_TERM_HYPERLINKS;  bison --color=no -fno-caret  -o input.cc input.y
    tests/regression.at:1560: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o input input.cc $LIBS
    stderr:
    input.cc: In member function 'YYRESULTTAG glr_stack::yyresolveValue(glr_state*)':
    input.cc:1796:10: error: potential null pointer dereference [-Werror=null-dereference]
       return yypred ? &(asItem(this) - yypred)->getState() : YY_NULLPTR;
              ^~~~~~
    input.cc:1796:10: error: potential null pointer dereference [-Werror=null-dereference]
       return yypred ? &(asItem(this) - yypred)->getState() : YY_NULLPTR;
              ^~~~~~
    cc1plus: all warnings being treated as errors

It complains that the implicit this in yypred might be null.  It fears
it because of loops such as

    for (glr_state* yys = firstTopState();
         yys != yystateStack.yysplitPoint;
         yys = yys->pred())
      yyn += 1;

that could possibly set yys to null, since yys->pred might return
null.  However, the warning is incorrect, since in C++ `this` cannot
be null.  GCC 10 no longer emits this warning.

GCC 7 also complains many times about glr_stack::yyresolveLocations
when NDEBUG is enabled (when it is not, YYASSERT (yyoption !=
YY_NULLPTR) is probably enough to pacify GCC):

    616. regression.at:1560: testing Lex and parse params: glr2.cc ...
    tests/regression.at:1560: COLUMNS=1000; export COLUMNS; NO_TERM_HYPERLINKS=1; export NO_TERM_HYPERLINKS;  bison --color=no -fno-caret  -o input.cc input.y
    tests/regression.at:1560: $CXX $CXXFLAGS $CPPFLAGS  $LDFLAGS -o input input.cc $LIBS
    stderr:
    input.cc: In member function 'void glr_stack::yyresolveLocations(glr_state*, int)':
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc: In member function 'YYRESULTTAG glr_stack::yyresolveValue(glr_state*)':
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    input.cc:3061:46: error: potential null pointer dereference [-Werror=null-dereference]
                     yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

* data/skeletons/glr2.cc (YY_IGNORE_NULL_DEREFERENCE_BEGIN)
(YY_IGNORE_NULL_DEREFERENCE_BEGIN): New.
(glr_state::pred, glr_stack::yyresolveLocations): Use them.
---
 TODO                   | 13 ++++++++++---
 data/skeletons/glr2.cc | 27 +++++++++++++++++++++++----
 2 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/TODO b/TODO
index 3f4d1978..c692b9f9 100644
--- a/TODO
+++ b/TODO
@@ -158,9 +158,12 @@ Les catégories d'avertissements incluent :
 Line -1 and -3 should mention CATEGORIE, not CATEGORY.
 
 * Bison 3.8
-** Rewrite glr.cc
-*** glr.c
-Get rid of scaffolding in glr.c.
+** Rewrite glr.cc (currently glr2.cc)
+*** Remove jumps
+We can probably replace setjmp/longjmp with exceptions.  That would help
+tremendously other languages such as D and Java that probably have no
+similar feature.  If we remove jumps, we probably no longer need _Noreturn,
+so simplify `b4_attribute_define([noreturn])` into `b4_attribute_define`.
 
 *** Coding style
 Move to our coding conventions.  In particular names such as yy_glr_stack,
@@ -178,6 +181,10 @@ There are many places where pointers should be replaced with references.
 Don't use YYSTYPE and YYLTYPE but parser::semantic_type and
 parser::location_type.  Undefine YYSTYPE and YYLTYPE.
 
+*** glr.c
+When glr2.cc fully replaces glr.cc, get rid of the glr.cc scaffolding in
+glr.c.
+
 * Chains
 ** Unit rules / Injection rules (Akim Demaille)
 Maybe we could expand unit rules (or "injections", see
diff --git a/data/skeletons/glr2.cc b/data/skeletons/glr2.cc
index a7f2d129..e463ffb3 100644
--- a/data/skeletons/glr2.cc
+++ b/data/skeletons/glr2.cc
@@ -126,6 +126,17 @@ b4_percent_code_get([[requires]])[
 ]b4_cast_define[
 ]b4_null_define[
 
+#if defined __GNUC__ && ! defined __ICC && 6 <= __GNUC__ && __GNUC__ <= 9
+# define YY_IGNORE_NULL_DEREFERENCE_BEGIN                               \
+  _Pragma ("GCC diagnostic push")                                       \
+  _Pragma ("GCC diagnostic ignored \"-Wnull-dereference\"")
+# define YY_IGNORE_NULL_DEREFERENCE_END         \
+  _Pragma ("GCC diagnostic pop")
+#else
+# define YY_IGNORE_NULL_DEREFERENCE_BEGIN
+# define YY_IGNORE_NULL_DEREFERENCE_END
+#endif
+
 // Whether we are compiled with exception support.
 #ifndef YY_EXCEPTIONS
 # if defined __GNUC__ && !defined __EXCEPTIONS
@@ -1271,12 +1282,18 @@ public:
   bool is_state_;
 };
 
-glr_state* glr_state::pred() {
-  return yypred ? &(asItem(this) - yypred)->getState() : YY_NULLPTR;
+glr_state* glr_state::pred ()
+{
+  YY_IGNORE_NULL_DEREFERENCE_BEGIN
+  return yypred ? &(asItem (this) - yypred)->getState () : YY_NULLPTR;
+  YY_IGNORE_NULL_DEREFERENCE_END
 }
 
-const glr_state* glr_state::pred() const {
-  return yypred ? &(asItem(this) - yypred)->getState() : YY_NULLPTR;
+const glr_state* glr_state::pred () const
+{
+  YY_IGNORE_NULL_DEREFERENCE_BEGIN
+  return yypred ? &(asItem (this) - yypred)->getState () : YY_NULLPTR;
+  YY_IGNORE_NULL_DEREFERENCE_END
 }
 
 void glr_state::setPred(const glr_state* state) {
@@ -2606,7 +2623,9 @@ class glr_stack {
                    detected.  Thus the location of the previous state (but not
                    necessarily the previous state itself) is guaranteed to be
                    resolved already.  */
+                YY_IGNORE_NULL_DEREFERENCE_BEGIN
                 yyrhsloc[0].getState().yyloc = yyoption->state()->yyloc;
+                YY_IGNORE_NULL_DEREFERENCE_END
               }
             YYLLOC_DEFAULT ((yys1->yyloc), yyrhsloc, yynrhs);
           }
-- 
2.28.0