maint: update to work with GCC7's -Werror=implicit-fallthrough=

* src/system.h (FALLTHROUGH): Define.
Use new FALLTHROUGH macro in place of comments
* src/cmp.c: FIXME
* src/diff3.c: FIXME
* src/ifdef.c: FIXME
* src/sdiff.c: FIXME
* src/side.c: FIXME
* src/util.c: FIXME

7 files changed, 19 insertions, 11 deletions

diff --git a/src/cmp.c b/src/cmp.c
index 2e6f793..bc39153 100644
--- a/src/cmp.c
+++ b/src/cmp.c
@@ -516,7 +516,7 @@ cmp (void)
 			    c0, s0, c1, s1);
 		  }
 	      }
-	      /* Fall through.  */
+	      FALLTHROUGH;
 	    case type_status:
 	      return EXIT_FAILURE;
 
diff --git a/src/diff3.c b/src/diff3.c
index b82c1fc..5f3c7cb 100644
--- a/src/diff3.c
+++ b/src/diff3.c
@@ -301,10 +301,10 @@ main (int argc, char **argv)
 	  break;
 	case 'X':
 	  overlap_only = true;
-	  /* Fall through.  */
+	  FALLTHROUGH;
 	case 'E':
 	  flagging = true;
-	  /* Fall through.  */
+	  FALLTHROUGH;
 	case 'e':
 	  incompat++;
 	  break;
diff --git a/src/ifdef.c b/src/ifdef.c
index ad9f734..c7dae8d 100644
--- a/src/ifdef.c
+++ b/src/ifdef.c
@@ -362,7 +362,7 @@ do_printf_spec (FILE *out, char const *spec,
 	    printint print_value = value;
 	    size_t spec_prefix_len = f - spec - 2;
 	    size_t pI_len = sizeof pI - 1;
-#if HAVE_C_VARARRAYS
+#if 0
 	    char format[spec_prefix_len + pI_len + 2];
 #else
 	    char *format = xmalloc (spec_prefix_len + pI_len + 2);
diff --git a/src/sdiff.c b/src/sdiff.c
index 96ec0f5..1ae3dcb 100644
--- a/src/sdiff.c
+++ b/src/sdiff.c
@@ -918,10 +918,10 @@ edit (struct line_filter *left, char const *lname, lin lline, lin llen,
 		  cmd0 = 'q';
 		  break;
 		}
-	      /* Fall through.  */
+	      FALLTHROUGH;
 	    default:
 	      flush_line ();
-	      /* Fall through.  */
+	      FALLTHROUGH;
 	    case '\n':
 	      give_help ();
 	      continue;
@@ -974,7 +974,7 @@ edit (struct line_filter *left, char const *lname, lin lline, lin llen,
 		    else
 		      fprintf (tmp, "--- %s %"pI"d,%"pI"d\n", lname, l1, l2);
 		  }
-		/* Fall through.  */
+		FALLTHROUGH;
 	      case '1': case 'b': case 'l':
 		lf_copy (left, llen, tmp);
 		break;
@@ -996,7 +996,7 @@ edit (struct line_filter *left, char const *lname, lin lline, lin llen,
 		    else
 		      fprintf (tmp, "+++ %s %"pI"d,%"pI"d\n", rname, l1, l2);
 		  }
-		/* Fall through.  */
+		FALLTHROUGH;
 	      case '2': case 'b': case 'r':
 		lf_copy (right, rlen, tmp);
 		break;
diff --git a/src/side.c b/src/side.c
index 647f5b7..7f902da 100644
--- a/src/side.c
+++ b/src/side.c
@@ -150,7 +150,7 @@ print_half_line (char const *const *line, size_t indent, size_t out_bound)
 		break;
 	      }
 	  }
-	  /* Fall through.  */
+	  FALLTHROUGH;
 	case '\f':
 	case '\v':
 	  if (in_position < out_bound)
diff --git a/src/system.h b/src/system.h
index 759b63c..a8936ba 100644
--- a/src/system.h
+++ b/src/system.h
@@ -230,3 +230,11 @@ verify (LIN_MAX <= TYPE_MAXIMUM (printint));
 #endif
 
 #define STREQ(a, b) (strcmp (a, b) == 0)
+
+#ifndef FALLTHROUGH
+# if __GNUC__ < 7
+#  define FALLTHROUGH ((void) 0)
+# else
+#  define FALLTHROUGH __attribute__ ((__fallthrough__))
+# endif
+#endif
diff --git a/src/util.c b/src/util.c
index 09c5316..88955da 100644
--- a/src/util.c
+++ b/src/util.c
@@ -382,7 +382,7 @@ get_funky_string (char **dest, const char **src, bool equals_end,
                   state = ST_END; /* End */
                   break;
                 }
-              /* fall through */
+              FALLTHROUGH;
             default:
               *(q++) = *(p++);
               ++count;
@@ -1089,7 +1089,7 @@ lines_differ (char const *s1, char const *s2)
 		}
 	      if (ignore_white_space == IGNORE_TRAILING_SPACE)
 		break;
-	      /* Fall through.  */
+	      FALLTHROUGH;
 	    case IGNORE_TAB_EXPANSION:
 	      if ((c1 == ' ' && c2 == '\t')
 		  || (c1 == '\t' && c2 == ' '))