warning: avoid warnings about unreachable code

Reported by Uxio Prego.
https://lists.gnu.org/archive/html/help-bison/2018-11/msg00031.html

We also need to move the unreachable 'goto' to a reachable place,
otherwise clang complains about the code being unreachable anyway.
See also https://bugs.llvm.org/show_bug.cgi?id=39736.

Interestingly, we don't have to apply that trick to
`#define YYCDEBUG if (false) std::cerr`, clang does not warn when the
code comes from macro expansion.

* configure.ac: Use -Wunreachable-code when supported.
* data/lalr1.cc, data/yacc.c: Pacify clang's warning about `if (0)`
by using a macro.
Another possibility was to move this statement to a reachable place.
* tests/actions.at, tests/c++.at: Avoid generating unreachable code.

configure.ac

@@ -113,6 +113,7 @@ if test "$enable_gcc_warnings" = yes; then
     -fno-color-diagnostics
     -Wno-keyword-macro'
 
+
   AC_LANG_PUSH([C])
   # Clang supports many of GCC's -W options, but only issues warnings
   # on the ones it does not recognize.  In that case, gl_WARN_ADD
@@ -126,6 +127,11 @@ if test "$enable_gcc_warnings" = yes; then
   # CFLAGS, and restore CFLAGS after the tests.
   save_CFLAGS=$CFLAGS
   gl_WARN_ADD([-Werror=unknown-warning-option], [CFLAGS])
+  # Accept this warning only if it is not too touchy (e.g., clang 3.3
+  # and 3.4).
+  gl_WARN_ADD([-Wunreachable-code], [WARN_CFLAGS],
+              [AC_LANG_PROGRAM([],
+              [[if (sizeof (long) < sizeof (int)) return 1;]])])
   for i in $warn_common $warn_c;
   do
     gl_WARN_ADD([$i], [WARN_CFLAGS])
@@ -147,6 +153,7 @@ if test "$enable_gcc_warnings" = yes; then
   CFLAGS=$save_CFLAGS
   AC_LANG_POP([C])
 
+
   AC_LANG_PUSH([C++])
   save_CXXFLAGS=$CXXFLAGS
   gl_WARN_ADD([-Werror=unknown-warning-option], [CXXFLAGS])
@@ -154,6 +161,11 @@ if test "$enable_gcc_warnings" = yes; then
   do
     gl_WARN_ADD([$i], [WARN_CXXFLAGS])
   done
+  # Accept this warning only if it is not too touchy (e.g., clang 3.3
+  # and 3.4).
+  gl_WARN_ADD([-Wunreachable-code], [WARN_CXXFLAGS],
+              [AC_LANG_PROGRAM([],
+              [[if (sizeof (long) < sizeof (int)) return 1;]])])
   gl_WARN_ADD([-Wzero-as-null-pointer-constant], [WARN_CXXFLAGS],
               [AC_LANG_PROGRAM([], [nullptr])])
   gl_WARN_ADD([-Werror], [WERROR_CXXFLAGS])

data/lalr1.cc

@@ -971,11 +971,11 @@ b4_dollar_popdef])[]dnl
   | yyerrorlab -- error raised explicitly by YYERROR.  |
   `---------------------------------------------------*/
   yyerrorlab:
-    /* Pacify compilers like GCC when the user code never invokes
+    /* Pacify compilers when the user code never invokes YYERROR and
-       YYERROR and the label yyerrorlab therefore never appears in user
+       the label yyerrorlab therefore never appears in user code.  */
-       code.  */
     if (false)
-      goto yyerrorlab;
+      YYERROR;
+
     /* Do not reclaim the symbols of the rule whose action triggered
        this YYERROR.  */
     yypop_ (yylen);

data/yacc.c

@@ -1777,11 +1777,10 @@ yyerrlab:
 | yyerrorlab -- error raised explicitly by YYERROR.  |
 `---------------------------------------------------*/
 yyerrorlab:
-  /* Pacify compilers like GCC when the user code never invokes
+  /* Pacify compilers when the user code never invokes YYERROR and the
-     YYERROR and the label yyerrorlab therefore never appears in user
+     label yyerrorlab therefore never appears in user code.  */
-     code.  */
   if (0)
-    goto yyerrorlab;
+    YYERROR;
 
   /* Do not reclaim the symbols of the rule whose action triggered
      this YYERROR.  */

examples/calc++/local.mk

@@ -20,7 +20,7 @@
 
 # Don't depend on $(BISON) otherwise we would rebuild these files
 # in srcdir, including during distcheck, which is forbidden.
-%D%/parser.stamp: $(BISON_IN)
+%D%/parser.stamp: $(BISON_IN) $(dist_pkgdata_DATA)
 SUFFIXES += .yy .stamp
 .yy.stamp:
 	$(AM_V_YACC)rm -f $@

tests/actions.at

@@ -519,6 +519,8 @@ AT_DATA_GRAMMAR([[input.y]],
 #define RANGE(Location) ]AT_LALR1_CC_IF([(Location).begin.line, (Location).end.line],
       [(Location).first_line, (Location).last_line])[
 
+#define USE(SYM)
+
 /* Display the symbol type Symbol.  */
 #define V(Symbol, Value, Location, Sep) \
    fprintf (stderr, #Symbol " (%d@%d-%d)%s", Value, RANGE(Location), Sep)
@@ -610,8 +612,8 @@ input:
 ;
 
 check-spontaneous-errors:
-  raise         { abort(); $$ = $1; }
+  raise         { abort(); USE(($$, $1)); }
-| '(' raise ')' { abort(); $$ = $2; }
+| '(' raise ')' { abort(); USE(($$, $2)); }
 | error
   {
     $$ = 5;

tests/c++.at

@@ -435,8 +435,6 @@ namespace yy
            return parser::token::TEXT;]])[
          }
     }
-
-    abort ();
   }
 }