Explanation about decltype used in typechecker_spec.lua

[stefanos82]

In the following lines

nelua-lang/spec/typechecker_spec.lua

Lines 1814 to 1816 in e6727dd

	expect.ast_type_equals([[
	local x: auto = 1
	local y: decltype(x) = x

we see

local x: auto = 1 

and then x is used inside decltype:

local y: decltype(x) = x

Does this mean y's decltype behavior is like C++14's decltype(auto) in this case?

[edubart]

The decltype in Nelua is a generic that returns the type of an identifier or expression.

Does this mean y's decltype behavior is like C++14's decltype(auto) in this case?

Not exactly, Nelua's decltype is similar to C++'s decltype because you can use both to get a type from an identifier or expression, but decltype(auto) in C++ is used only to automatic detect the type for function returns while in Nelua the same is done just with auto or leaving the function returns without type notations.

Here is a small example of decltype usage:

local function bounded_increment(x: auto): auto
  local y: decltype(x) = x + 1
  if y <= x then -- value wrapped around due to integer overflow
    y = x
  end
  return y
end

print(bounded_increment(0.0)) -- outputs: 1.0
print(bounded_increment(4294967295_u32)) -- outputs: 4294967295

So decltype can be used enforce a type early, but usually this is not needed as the compiler can infer types by itself, so it's very little used, it's rare to need it.

[stefanos82]

Thank you for the explanation @edubart.

Coming from C and C++, using __typeof__, auto, and decltype(auto) can make you see such behavior in Nelua's auto and decltype, thus the mild confusion.