--- a/lib/intprops.h
+++ b/lib/intprops.h
@@ -53,12 +53,25 @@
 	? (t) -1 \
 	: ~ (~ (t) 0 << (sizeof (t) * CHAR_BIT - 1))))
 
+/* Return zero if T can be determined to be an unsigned type.
+   Otherwise, return 1.
+   When compiling with GCC, INT_STRLEN_BOUND uses this macro to obtain a
+   tighter bound.  Otherwise, it overestimates the true bound by one byte
+   when applied to unsigned types of size 2, 4, 16, ... bytes.
+   The symbol signed_type_or_expr__ is private to this header file.  */
+#if __GNUC__ >= 2
+# define signed_type_or_expr__(t) TYPE_SIGNED (__typeof__ (t))
+#else
+# define signed_type_or_expr__(t) 1
+#endif
+
 /* Bound on length of the string representing an integer type or expression T.
-   Subtract 1 for the sign bit if t is signed; log10 (2.0) < 146/485;
+   Subtract 1 for the sign bit if T is signed; log10 (2.0) < 146/485;
    add 1 for integer division truncation; add 1 more for a minus sign
    if needed.  */
 #define INT_STRLEN_BOUND(t) \
-  ((sizeof (t) * CHAR_BIT - 1) * 146 / 485 + 2)
+  ((sizeof (t) * CHAR_BIT - signed_type_or_expr__ (t)) * 146 / 485 \
+   + signed_type_or_expr__ (t) + 1)
 
 /* Bound on buffer size needed to represent an integer type or expression T,
    including the terminating null.  */