Cannot match warnings with location: :keep show the wrong location

[gabrielpra1]

Environment

• Erlang/OTP 21 [erts-10.2], Elixir 1.7.2 (compiled with Erlang/OTP 21)
• Operating system: Ubuntu 16.04

Current behavior

When using location: :keep, "cannot match" warnings are generated with the file name where the macro is defined but line number where the macro is called.

# bug_test.ex
defmodule BugTest do
  defmacro generate_function() do
    quote location: :keep do
      def unquote(:test)() do # Line 4
        :ok
      end
    end
  end
end

# test.exs
test "bug" do
  defmodule Test do
    require BugTest
    BugTest.generate_function # Line 113
    BugTest.generate_function # Line 114
  end
end

Generates the warning

warning: this clause cannot match because a previous clause at line 113 always matches
  lib/bug_test.ex:114

If the macro is defined without location: :keep, the warning is

warning: this clause cannot match because a previous clause at line 113 always matches
  test/test.exs:114

Expected behavior

I'm not sure if this is a bug or my misunderstanding, but the first warning looks weird, since the lines 113 and 114 don't even exist in bug_test.ex.

I understand that it's tricky to show a proper warning message here because the issue comes from calling the macro twice, so we need to reference this. However, I think a clearer warning message would be something like:

warning: this clause cannot match because a previous clause at line 113 in test/text.exs always matches
  lib/bug_test.ex:4

or simply

warning: this clause cannot match because a previous clause at line 113 always matches
  lib/bug_test.ex:4

That way, if we had multiple functions being generated by the generate_function macro we could see which function cannot be matched.

[josevalim]

Unfortunately there is not much we can do in this case. This error happens from a pass in the compiler that is very far removed from the original code, so any extra information is lost. The location: :keep affects the runtime behaviour but the definition code uses the caller information, otherwise we would actually report:

warning: this clause cannot match because a previous clause at line 4 always matches
lib/bug_test.ex:4

Which is possibly more confusing.