Calling macro from within generated function in Julia

General Usage
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1

I have been messing around with generated functions in Julia, and have come to a weird problem I do not understand fully: My final goal would involve calling a macro (more specifically @tullio) from within a generated function (to perform some tensor contractions that depend on the input tensors). But I have been having problems, which I narrowed down to calling the macro from within the generated function.

To illustrate the problem, let’s consider a very simple example that also fails:

macro my_add(a,b) 
    return :($a + $b)
end

function add_one_expr(x::T) where T
    y = one(T)
    return :( @my_add($x,$y) )
end

@generated function add_one_gen(x::T) where T
    y = one(T)
    return :( @my_add($x,$y) )
end

With these declarations, I find that eval(add_one_expr(2.0)) works just as expected and returns and expression

:(@my_add 2.0 1.0)

which correctly evaluates to 3.0.

However evaluating add_one_gen(2.0) returns the following error:

MethodError: no method matching +(::Type{Float64}, ::Float64)

Doing some research, I have found that @generated actually produces two codes, and in one only the types of the variables can be used. I think this is what is happening here, but I do not understand what is happening at all. It must be some weird interaction between macros and generated functions.

Can someone explain and/or propose a solution? Thank you!