As you have observed, for:
var list = new List<(int,int)>();
One would at least expect to be able to do the following:
list.Select((x,y) => x*2 + y/2);
But the C# 7 compiler doesn’t (yet) support this. It is also reasonable to desire sugar that would allow the following:
void Foo(int x, int y) => ...
Foo(list[0]);
with the compiler converting Foo(list[0]); to Foo(list[0].Item1, list[0].Item2); automatically.
Neither of these is currently possible. However, the issue, Proposal: Tuple deconstruction in lambda argument list, exists on the dotnet/csharplang repo on GitHub, requesting that the language team consider these features for a future version of C#. Please do add your voices to that thread if you too would like to see support for this.