Fix testsuite build for GCC 7 -Wformat-truncation.

This patch fixes the glibc testsuite build for GCC 7
-Wformat-truncation, newly moved out of -Wformat-length and with some
further warnings that didn't previously appear.  Two tests that
previously disabled -Wformat-length are changed to disable
-Wformat-truncation instead; two others are made to disable that
option as well.

Tested (compilation only) with build-many-glibcs.py for aarch64 with
GCC mainline.

	* stdio-common/tst-printf.c [__GNUC_PREREQ (7, 0)]: Ignore
	-Wformat-truncation instead of -Wformat-length.
	* time/tst-strptime2.c (mkbuf) [__GNUC_PREREQ (7, 0)]: Likewise.
	* stdio-common/tstdiomisc.c (F): Ignore -Wformat-truncation for
	GCC 7.
	* wcsmbs/tst-wcstof.c: Include <libc-internal.h>.
	(do_test): Ignore -Wformat-truncation for GCC 7.

(cherry picked from commit 3c9378265a8633e2c85a393b54a16abcf64fe616)

4 files changed, 22 insertions, 3 deletions

diff --git a/stdio-common/tst-printf.c b/stdio-common/tst-printf.c
index ffe7ac7753..8870d4ab6d 100644
--- a/stdio-common/tst-printf.c
+++ b/stdio-common/tst-printf.c
@@ -33,8 +33,9 @@
 DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
 
 #if __GNUC_PREREQ (7, 0)
-/* Compiler warnings about format lengths should also be ignored.  */
-DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-length");
+/* Compiler warnings about snprintf output truncation should also be
+   ignored.  */
+DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-truncation");
 #endif
 
 static void rfg1 (void);
diff --git a/stdio-common/tstdiomisc.c b/stdio-common/tstdiomisc.c
index 5548a71e52..9c7342d55f 100644
--- a/stdio-common/tstdiomisc.c
+++ b/stdio-common/tstdiomisc.c
@@ -94,6 +94,14 @@ F (void)
 
   qnanval = NAN;
 
+  /* The %f and %F arguments are in fact constants, but GCC is
+     prevented from seeing this (volatile is used) so it cannot tell
+     that the output is not truncated.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (7, 0)
+  DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-truncation");
+#endif
+
   snprintf (buf, sizeof buf, "%a %A %e %E %f %F %g %G",
 	    qnanval, qnanval, qnanval, qnanval,
 	    qnanval, qnanval, qnanval, qnanval);
@@ -260,6 +268,8 @@ F (void)
   printf ("expected L\"-inf -INF -inf -INF -inf -INF -inf -INF\", got L\"%S\"\n",
 	  wbuf);
 
+  DIAG_POP_NEEDS_COMMENT;
+
   return result;
 }
 
diff --git a/time/tst-strptime2.c b/time/tst-strptime2.c
index 04f7cff437..9273568b6f 100644
--- a/time/tst-strptime2.c
+++ b/time/tst-strptime2.c
@@ -73,7 +73,7 @@ mkbuf (char *buf, bool neg, bool colon, unsigned int hhmm, size_t ndigits)
      This test is explicitly using short buffers to force snprintf to truncate
      the output so we ignore the warnings.  */
   DIAG_PUSH_NEEDS_COMMENT;
-  DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-length");
+  DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-truncation");
 #endif
   if (colon)
     snprintf (buf + i, ndigits + 2, "%02u:%02u", hh, mm);
diff --git a/wcsmbs/tst-wcstof.c b/wcsmbs/tst-wcstof.c
index 576a58c023..861f65905a 100644
--- a/wcsmbs/tst-wcstof.c
+++ b/wcsmbs/tst-wcstof.c
@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <wctype.h>
+#include <libc-internal.h>
 
 static int
 do_test (void)
@@ -14,7 +15,14 @@ do_test (void)
   tmp[1] = '1';
   tmp[2] = 0;
 
+  /* GCC does not know the result of wcstof so cannot see that the
+     snprintf output is not truncated.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+#if __GNUC_PREREQ (7, 0)
+  DIAG_IGNORE_NEEDS_COMMENT (7.0, "-Wformat-truncation");
+#endif
   snprintf (buf, 100, "%S = %f", tmp, wcstof (tmp, NULL));
+  DIAG_POP_NEEDS_COMMENT;
   printf ("\"%s\" -> %s\n", buf,
 	  strcmp (buf, "81 = 81.000000") == 0 ? "okay" : "buggy");
   result |= strcmp (buf, "81 = 81.000000") != 0;