[Bug #18973] Promote US-ASCII to ASCII-8BIT when adding 8-bit char

1 files changed, 9 insertions, 4 deletions

diff --git a/sprintf.c b/sprintf.c
index b2bdd4a072..bfe25e1d3c 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -454,13 +454,18 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
                     str = tmp;
                     goto format_s1;
                 }
-                else {
-                    n = NUM2INT(val);
-                    if (n >= 0) n = rb_enc_codelen((c = n), enc);
-                }
+                n = NUM2INT(val);
+                if (n >= 0) n = rb_enc_codelen((c = n), enc);
                 if (n <= 0) {
                     rb_raise(rb_eArgError, "invalid character");
                 }
+                int encidx = rb_ascii8bit_appendable_encoding_index(enc, c);
+                if (encidx >= 0 && encidx != rb_enc_to_index(enc)) {
+                    /* special case */
+                    rb_enc_associate_index(result, encidx);
+                    enc = rb_enc_from_index(encidx);
+                    coderange = ENC_CODERANGE_VALID;
+                }
                 if (!(flags & FWIDTH)) {
                     CHECK(n);
                     rb_enc_mbcput(c, &buf[blen], enc);