Param folding of if expression can result in compiler error

[dlongnecke-cray]

Summary of Problem

Param folding of if expressions can result in a compiler error depending on the branch expressions. In my specific case, futures filed for tuples in #15394 failed because they were unable to resolve a call to chpl__initCopy. Relying on a different mechanism such as a where clause sidesteps the issue.

While our specific failures occurred when a branch expression tried to copy initialize an owned class or a borrowed class, I think that the failure might occur any time a copy initializer requires a ref argument.

Steps to Reproduce

Source Code:

class C { var x: int = 0; }

proc test(type T) {
  // Trivially evaluate to second branch.
  var a = if isTuple(T) then 128 else (128, new T(256));
  writeln(a);
}

type T1 = owned C;

proc main() {
  test(T1);
}

Output:

test.chpl:3: In function 'test':
test.chpl:5: error: non-lvalue actual is passed to a 'ref' formal of chpl__initCopy()

Compile command:
chpl test.chpl

Execution command:
./test

Associated Future Test(s):

test/param/ifExprCopyInitBug.chpl

[e-kayrakli]

Possibly related: #14317, #14309, #14301, #14279

[mppf]

Here are two issues about how param folding interacts with return (which is not the topic of this issue but I tend to find this one with my searches so am leaving a breadcrumb here): #14683 #18597