using X loads a file called X

[shashi]

file X (note: not X.jl):

println("foo")

Inside a repl in the same directory

julia> using X
foo
Warning: requiring "X" did not define a corresponding module.

This means that we cannot have an executable named foo for a module named Foo which does using Foo inside it. Worse on case insensitive file systems.

[SimonDanisch]

Also: #9007 #9079 #7256