C# compiler error when generated lambda refers to out parameter

[lukemaurer]

If a lambda in the source code mentions an out parameter, the out parameter's value is captured in a temporary variable at the moment the closure is created, since C# doesn't allow out parameters to occur inside lambdas. However, this isn't done for source-level constructs that are compiled into lambdas, such as set comprehensions:

method Test(a : int) returns (b : int, c : int) requires a != 0 {
  var xs := set x | x in { 1, 2, 3, 4, 5 } :: x % a == b;
  b := |xs|;
}

method Main() {
  var x, _ := Test(2);
  print x, "\n";
}

Errors compiling program
/var/folders/4z/3lglrccd2ts1ns79fkybj4cwlwfdyg/T/fd0rhgw2.0.cs(1649,71) : error CS1628: Parameter `b' cannot be used inside `lambda expression' when using `ref' or `out' modifier

(Note that Test() must have two out parameters, as otherwise the generated C# code has a return type rather than an out parameter.)