diff -Naurd mpfr-3.1.5-a/PATCHES mpfr-3.1.5-b/PATCHES
--- mpfr-3.1.5-a/PATCHES	2016-12-19 22:11:17.022676737 +0000
+++ mpfr-3.1.5-b/PATCHES	2016-12-19 22:11:17.094676820 +0000
@@ -0,0 +1 @@
+strtofr
diff -Naurd mpfr-3.1.5-a/VERSION mpfr-3.1.5-b/VERSION
--- mpfr-3.1.5-a/VERSION	2016-12-15 08:35:46.544430346 +0000
+++ mpfr-3.1.5-b/VERSION	2016-12-19 22:11:17.094676820 +0000
@@ -1 +1 @@
-3.1.5-p1
+3.1.5-p2
diff -Naurd mpfr-3.1.5-a/src/mpfr.h mpfr-3.1.5-b/src/mpfr.h
--- mpfr-3.1.5-a/src/mpfr.h	2016-12-15 08:35:46.540430340 +0000
+++ mpfr-3.1.5-b/src/mpfr.h	2016-12-19 22:11:17.090676815 +0000
@@ -27,7 +27,7 @@
 #define MPFR_VERSION_MAJOR 3
 #define MPFR_VERSION_MINOR 1
 #define MPFR_VERSION_PATCHLEVEL 5
-#define MPFR_VERSION_STRING "3.1.5-p1"
+#define MPFR_VERSION_STRING "3.1.5-p2"
 
 /* Macros dealing with MPFR VERSION */
 #define MPFR_VERSION_NUM(a,b,c) (((a) << 16L) | ((b) << 8) | (c))
diff -Naurd mpfr-3.1.5-a/src/strtofr.c mpfr-3.1.5-b/src/strtofr.c
--- mpfr-3.1.5-a/src/strtofr.c	2016-09-27 07:58:15.000000000 +0000
+++ mpfr-3.1.5-b/src/strtofr.c	2016-12-19 22:11:17.066676788 +0000
@@ -743,11 +743,14 @@
          of the pstr_size most significant digits of pstr->mant, with
          equality in case exact is non-zero. */
 
-      /* test if rounding is possible, and if so exit the loop */
-      if (exact || mpfr_can_round_raw (result, ysize,
-                                       (pstr->negative) ? -1 : 1,
-                                       ysize_bits - err - 1,
-                                       MPFR_RNDN, rnd, MPFR_PREC(x)))
+      /* test if rounding is possible, and if so exit the loop.
+         Note: we also need to be able to determine the correct ternary value,
+         thus we use the MPFR_PREC(x) + (rnd == MPFR_RNDN) trick.
+         For example if result = xxx...xxx111...111 and rnd = RNDN,
+         then we know the correct rounding is xxx...xx(x+1), but we cannot know
+         the correct ternary value. */
+      if (exact || mpfr_round_p (result, ysize, ysize_bits - err - 1,
+                                 MPFR_PREC(x) + (rnd == MPFR_RNDN)))
         break;
 
     next_loop:
diff -Naurd mpfr-3.1.5-a/src/version.c mpfr-3.1.5-b/src/version.c
--- mpfr-3.1.5-a/src/version.c	2016-12-15 08:35:46.544430346 +0000
+++ mpfr-3.1.5-b/src/version.c	2016-12-19 22:11:17.094676820 +0000
@@ -25,5 +25,5 @@
 const char *
 mpfr_get_version (void)
 {
-  return "3.1.5-p1";
+  return "3.1.5-p2";
 }
diff -Naurd mpfr-3.1.5-a/tests/tstrtofr.c mpfr-3.1.5-b/tests/tstrtofr.c
--- mpfr-3.1.5-a/tests/tstrtofr.c	2016-09-27 07:58:14.000000000 +0000
+++ mpfr-3.1.5-b/tests/tstrtofr.c	2016-12-19 22:11:17.066676788 +0000
@@ -1191,6 +1191,24 @@
   mpfr_clears (e, x1, x2, (mpfr_ptr) 0);
 }
 
+/* Note: the number is 5^47/2^9. */
+static void
+bug20161217 (void)
+{
+  mpfr_t fp, z;
+  static const char * num = "0.1387778780781445675529539585113525390625e31";
+  int inex;
+
+  mpfr_init2 (fp, 110);
+  mpfr_init2 (z, 110);
+  inex = mpfr_strtofr (fp, num, NULL, 10, MPFR_RNDN);
+  MPFR_ASSERTN(inex == 0);
+  mpfr_set_str_binary (z, "10001100001000010011110110011101101001010000001011011110010001010100010100100110111101000010001011001100001101E-9");
+  MPFR_ASSERTN(mpfr_equal_p (fp, z));
+  mpfr_clear (fp);
+  mpfr_clear (z);
+}
+
 int
 main (int argc, char *argv[])
 {
@@ -1205,6 +1223,7 @@
   test20100310 ();
   bug20120814 ();
   bug20120829 ();
+  bug20161217 ();
 
   tests_end_mpfr ();
   return 0;