stdlib: Allow lists with binaries in the Format argument

As a consequence of some refactoring (OTP-14983, option
'chars_limit'), the Format argument of io_lib:format() no longer
accepts binaries in lists in Erlang/OTP 21.0.

Note that if Format is not of type io:format(), control sequences in
Format are ignored. This can result in unpredictable behaviour if, for
example, the output of io_lib:format() is used as Format: if the
output happens to be a string(), then control sequences are
interpreted, otherwise not.

A check that Format is of type io:format() will likely be introduced
in Erlang/OTP 22.0. That will probably mean work for some users, but
considering how unpredictable io_lib:format() is, we should try to fix
that.

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/stdlib/src/io_lib_format.erl b/lib/stdlib/src/io_lib_format.erl
index e247b00a04..ab9031573b 100644
--- a/lib/stdlib/src/io_lib_format.erl
+++ b/lib/stdlib/src/io_lib_format.erl
@@ -246,7 +246,8 @@ count_small([#{control_char := $W}|Cs], #{w := W} = Cnts) ->
     count_small(Cs, Cnts#{w := W + 1});
 count_small([#{control_char := $s}|Cs], #{w := W} = Cnts) ->
     count_small(Cs, Cnts#{w := W + 1});
-count_small([S|Cs], #{other := Other} = Cnts) when is_list(S) ->
+count_small([S|Cs], #{other := Other} = Cnts) when is_list(S);
+                                                   is_binary(S) ->
     count_small(Cs, Cnts#{other := Other + string:length(S)});
 count_small([C|Cs], #{other := Other} = Cnts) when is_integer(C) ->
     count_small(Cs, Cnts#{other := Other + 1});