Code coverage over reported? Difference between Julia 0.6, 0.7, 1.0

[fingolfin]

While working on getting Coverage.jl up to speed for use with Julia 0.7 and 1.0 (see JuliaCI/Coverage.jl#179) I noticed the following; I am unsure whether this is a bug, a misfeature, or (most likely) just something I don't understand regarding the code coverage reported by Julia.

Consider this simple input file, stored as tc.jl:

f(x) = 2x

module TestModule
end

Now I run Julia like this:

julia --code-coverage -L tc.jl -e "using Compat, Compat.Test; @test f(2) == 4"

Then I get the following results ...

... with Julia 0.6.4:

        1 f(x) = 2x
        - 
        - module TestModule
        - end
        - 

... with Julia 0.7.0.

        2 f(x) = 2x
        - 
        3 module TestModule
        - end
        - 

... with Julia 1.0.1 and master:

        2 f(x) = 2x
        - 
        - module TestModule
        - end
        - 

So the "3" for the module seems to be a bug that's been fixed in Julia 1.x, fine. But what puzzles me is that 2 (instead of 1 for Julia 0.6) for the line f(x) = 2x. Is that a bug? Or a quirk of the code coverage implementation? Or something else?

[ararslan]

Coverage on Julia 0.7 and higher is kind of messed up, I believe due to changes in Julia's optimizer. See also #28192.