ERL-633: io_lib:format/2 incompatibility between 21-rc1 and 21-rc2

[OTP-Maintainer]

Original reporter: dane
Affected version: OTP-20.1
Component: stdlib
Migrated from: https://bugs.erlang.org/browse/ERL-633

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G'Day,
This commit:
https://github.com/erlang/otp/commit/29a347ffd408c68861a914db4efc75d8ea20a762
seems to change behaviour of:
io_lib:format("~.16B", [1521415153])
now it returns "5AAEF3F1"
while with this commit reverted it's
["5AAEF3F1"]

Problem seems to be somewhere in io_lib_format:build() function.

[OTP-Maintainer]

hasse said:

Hi,

The change is compatible with respect to the contract, so it's not a bug.

You should not rely on the exact return value from the formatting
functions. Use lists:flatten/1 to get a string(), which you can
compare and traverse.

BR