allow replacing const binding with new binding?

[StefanKarpinski]

People are often quite confused and frustrated by this sort of thing: JuliaAcademy/JuliaTutorials#27. The core issue is this:

julia> sum(rand(10)) # resolves `Main.sum` to be `Base.sum`
4.042512623756739

julia> sum = 3 + 4
ERROR: cannot assign variable Base.sum from module Main
Stacktrace:
 [1] top-level scope at none:0

This also crops up when some bit of machinery accidentally and mistakenly resolves bindings that you didn't even use, e.g. #30234, #23677, #29962, etc. But the end result is the same: frustration that one cannot use a particular name without restarting Julia. And this is not limited to newbies: if I'm debugging code that uses e.g. sum any other function name as a local variable—which is fine in local scope—then I have to either start a new REPL and be very careful not to use that function before then or I have to change the name throughout the code.

To that end, I would propose this relaxation of the current behavior: if someone assigns to an already-bound constant, we print a warning along these lines:

WARNING: cannot assign variable Base.sum from module Main, creating a new binding instead

or in the case of a plain const with an incompatible assignment:

WARNING: invalid redefinition of constant c, creating a new binding instead

Any function definition that used a previous binding would continue to use that binding and be unaffected by the new one, so no recompilation would be required. I think this would make this behavior feel like less of a mine field to navigate.

[lukeburns]

This can also be frustrating if using packages that pollute the global namespace