Optimize default(T) deconstruction

[alrz]

Version Used: master

Steps to Reproduce:

void M(out int i, out string j) {
  (i, j) = default((int, string));
}

Expected Behavior: (decompiled)

void M(out int i, out string j) {
  i = 0;    // default(int)
  j = null; // default(string)
}

Actual Behavior:

void M(out int i, out string j) {
  i = default((int, string)).Item1;
  j = default((int, string)).Item2;
}

PS: Ideally, this should work for default literal as well (dotnet/csharplang#583).

[HaloFour]

Interesting case. I bet the confusion is the compiler dealing with a default of the tuple literal. If you used (default(string), default(int)) instead the compiler does optimize out the tuple. What's particularly strange is that the compiler is emitting and initializing two tuples instead of one.

At least the JIT is correctly optimizing away the use of the tuples, but I agree that the compiler could be handling this better.

[alrz]

What's particularly strange is that the compiler is emitting and initializing two tuples instead of one.

I think that's because default(..) is known to be side-effect free between reads so it's not stored in a local.

PS: Looking at IL, actually there is a local to store the result, but I suppose that's how initobj works.

[jcouv]

Fixed in the feature branch for "deconstruction on default". Actual release not yet finalized.