c, c++: avoid implicit fall-throw

Reported by Derek Clegg.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00004.html

* configure.ac (warn_common): Add -Wimplicit-fallthrough.
This does trigger failures in the test suite.
* data/skeletons/glr.c, data/skeletons/lalr1.cc,
* data/skeletons/yacc.c, tests/c++.at:
Make fall-throws explicit.

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
+    -Wformat -Wimplicit-fallthrough -Wnull-dereference
+    -Wpointer-arith -Wshadow
     -Wundefined-func-template -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
   warn_cxx='-Wextra-semi -Wnoexcept'

data/skeletons/glr.c

@@ -590,7 +590,10 @@ yytnamerr (char *yyres, const char *yystr)
           case '\\':
             if (*++yyp != '\\')
               goto do_not_strip_quotes;
-            /* Fall through.  */
+            else
+              goto append;
+
+          append:
           default:
             if (yyres)
               yyres[yyn] = *yyp;

data/skeletons/lalr1.cc

@@ -532,7 +532,10 @@ m4_if(b4_prefix, [yy], [],
             case '\\':
               if (*++yyp != '\\')
                 goto do_not_strip_quotes;
-              // Fall through.
+              else
+                goto append;
+
+            append:
             default:
               yyr += *yyp;
               break;

data/skeletons/yacc.c

@@ -1066,7 +1066,10 @@ yytnamerr (char *yyres, const char *yystr)
           case '\\':
             if (*++yyp != '\\')
               goto do_not_strip_quotes;
-            /* Fall through.  */
+            else
+              goto append;
+
+          append:
           default:
             if (yyres)
               yyres[yyn] = *yyp;

doc/bison.texi

@@ -6650,6 +6650,28 @@ For example, if you use @samp{%define api.prefix @{c@}}, the names become
 @code{cparse}, @code{clex}, @dots{}, @code{CSTYPE}, @code{CLTYPE}, and so
 on.
 
+Users of Flex must update the signature of the generated @code{yylex}
+function.  Since the Flex scanner usually includes the generated header of
+the parser (to get the definitions of the tokens, etc.), the most convenient
+way is to insert the declaration of @code{yylex} in the @code{provides}
+section:
+
+@example
+%define api.prefix @{c@}
+// Emitted in the header file, after the definition of YYSTYPE.
+%code provides
+@{
+  // Tell Flex the expected prototype of yylex.
+  #define YY_DECL                             \
+    int clex (CSTYPE*y ylval, CLTYPE *yylloc)
+
+  // Declare the scanner.
+  YY_DECL;
+@}
+@end example
+
+@sp 1
+
 The @code{%define} variable @code{api.prefix} works in two different ways.
 In the implementation file, it works by adding macro definitions to the
 beginning of the parser implementation file, defining @code{yyparse} as

tests/c++.at

@@ -1242,7 +1242,8 @@ yylex (yy::parser::semantic_type *lvalp)
   default:
     lvalp->]AT_VARIANT_IF([build<Object> (res)],
                           [obj = new Object (res)])[;
-    // Fall through.
+    goto zero;
+  zero:
   case 0:
     return res;
   }