I seem to have stumbled into something similar to what was mentioned in this thread: Function overriding causes warning
however, I can’t spot the trouble.

I have a “base” module that provides some default implementations of functions (i.e. not functions that are optional in any way that could be notated via @optional_callbacks). 90% of the time, the default implementation is what’s used. But when I do need to override the one of these functions, I keep seeing warnings, .e.g

warning: this clause for foo/1 cannot match because a previous clause at line 8 always matches
  lib/my_app/foo.ex:130

My tests are passing – the proper code is getting executed. Can anyone shed light on the source of the warning? I have the proper use statement in my modules, so I’m wondering if there’s something else I’ve missed.