method overwrite warning

[daviehh]

Re: #15602 and #36037 (possibly others)

Currently, expressions like

function myfun(n::Int)
     if n < 3
        println("branch 1")
        doit() = 3
     else
        println("branch 1")
        doit() = 5
     end 
end

and

function do_test()
    function innerfunc()
        println(stderr, "innerfunc")
    end
    println(stderr, "pre-innerfunc")
    innerfunc()
    println(stderr, "post-innerfunc")

    # Overwrite innerfunc
    innerfunc() = nothing
    println("innerfunc is now overwritten")
end

do_test()

would "silently" overwrite the function, such that e.g. with the 1st example, a = myfun(1) would print branch 1 but a() returns 5 since method is silently overwritten, making such scenarios a bit difficult to debug.

A long-term solution could be to error in such cases, meanwhile, this PR set the default setting to warn about such overwrites, and add a function to flip the warning setting (and a convenience macro) to temporarily disable the warning when such overwrite is intentional.

Example:

julia> f(x) = 1
 f (generic function with 1 method)
julia> f(x) = x
WARNING: Method definition f(Any) in module Main at REPL[1]:1 overwritten at REPL[2]:1.
 f (generic function with 1 method)
julia> Base.@overwrite f(x) = x * 3
 f (generic function with 1 method)
julia> f(1)
 3

[KristofferC]

The whole reason this setting was introduced was to not show them by default (#23002).

Call site overriding was thumbed down #23002 (comment).

[mbauman]

I would much rather this be done in the lowering for local functions only.

[daviehh]

got it, thanks! closing for now