How to raise a warning when using the default constructor directly?

Hellow! I have a code :

module Test

export custom_constructor_1, custom_constructor_2

struct Foo
	a::Int
	b::Int
	c::Int
	function Foo(a::Int, b::Int)
		# @warn "You should use a custom constructor!"  # this must work only in the default constructor
		return new(a, b, a + b)
	end
end

function custom_constructor_1(;
		a::Int = 5,
		b::Int = 8,
	)
	@info "Good, you use custom constructor β„– 1"
	println("a=$a, b=$b")
	return Foo(a, b)
end

function custom_constructor_2(;
		a::Int = 3,
		b::Int = 4,
	)
	@info "Good, you use custom constructor β„– 2"
	println("a=$a, b=$b")
	return Foo(a, b)
end

end # module Test

How to raise a warning when using the default constructor directly? like this:

julia> using .Test

julia> foo = Test.Foo(1,2)
β”Œ Warning: You should use a custom constructor!
β”” @ Main.Test REPL[2]:10
Main.Test.Foo(1, 2, 3)

julia> foo = custom_constructor_1() # this is OK
[ Info: Good, you use custom constructor β„– 1
a=5, b=8
Main.Test.Foo(5, 8, 13)

julia> foo = Test.custom_constructor_2(a=3, b=4) # and this is OK
[ Info: Good, you use custom constructor β„– 2
a=3, b=4
Main.Test.Foo(3, 4, 7)

The first idea which comes to my mind is to add a keyword argument to your inner constructor which mutes the warning (punintented).

struct Foo
	a::Int
	b::Int
	c::Int
	function Foo(a::Int, b::Int; warnifused=true)
		warnifused && @warn "You should use a custom constructor!"
		return new(a, b, a + b)
	end
end

and then

function custom_constructor_1(;
		a::Int = 5,
		b::Int = 8,
	)
	@info "Good, you use custom constructor β„– 1"
	println("a=$a, b=$b")
	return Foo(a, b; warnifused=false)
end
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