Providing a string() instead of [string()] in 𝚎𝚛𝚕_𝚏𝚒𝚛𝚜𝚝_𝚏𝚒𝚕𝚎𝚜 crashes rebar3

[kvakvs]

Current behaviour

I have specified {erl_first_files, "nnn.erl"} as a string instead of list of strings.

Running rebar3 compile crashes with internal error trying to join a string with a single integer (a character).

===> Uncaught error in rebar_core. Run with DEBUG=1 to see stacktrace or consult rebar3.crashdump
===> Uncaught error: function_clause
===> Stack trace to the error location:
[{filename,join,
           ["/home/kv/M/_build/default/lib/lib_directory",115],
           [{file,"filename.erl"},{line,410}]},
 {rebar_erlc_compiler,'-erl_first_files/4-lc$^0/1-0-',3,
                      [{file,"/home/tristan/Devel/rebar3/_build/default/lib/rebar/src/rebar_erlc_compiler.erl"},
                       {line,292}]},

Expected behaviour

There should be a check that the value provided is a list of strings and error should be printed.

Environment

[kv@UbuntuVbox M (BC-173-rebar3)]$ rebar3 report
Rebar3 report
 version 3.4.4
 generated at 2017-10-11T13:29:08+00:00
=================
Please submit this along with your issue at https://github.com/erlang/rebar3/issues (and feel free to edit out private information, if any)
-----------------
Task: 
Entered as:
  
-----------------
Operating System: x86_64-unknown-linux-gnu
ERTS: Erlang/OTP 19 [erts-8.3] [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]
Root Directory: /home/kv/r19
Library directory: /home/kv/r19/lib
-----------------
Loaded Applications:
bbmustache: 1.3.0
certifi: 2.0.0
cf: 0.2.2
common_test: 1.14
compiler: 7.0.4
crypto: 3.7.3
cth_readable: 1.3.0
dialyzer: 3.1
edoc: 0.8.1
erlware_commons: 1.0.0
eunit: 2.3.2
eunit_formatters: 0.3.1
getopt: 0.8.2
inets: 6.3.6
kernel: 5.2
providers: 1.6.0
public_key: 1.4
relx: 3.23.1
sasl: 3.0.3
snmp: 5.2.5
ssl_verify_fun: 1.1.2
stdlib: 3.3
syntax_tools: 2.1.1
tools: 2.9.1

-----------------
Escript path: /home/kv/bin/rebar3
Providers:
  app_discovery as clean compile compile cover ct deps dialyzer do edoc escriptize eunit get-deps help install install_deps list lock new path pkgs release relup report shell state tar tree unlock update upgrade upgrade upgrade version xref 

[ferd]

Might make sense. A bit tricky from the point of view that "" will be valid, but "abc" will not, but that's what we get for string representations. There's also multiple places where the line can be used (you may get failures in plugins if the compiler does its own validation for example), but a best effort in the main code base would be reasonable.