Delay analysis of assignability of default values to invocation

[CeylonMigrationBot]

[@quintesse] I'd asked @gavinking if this was possible:

Integer|Absent parseInt<Absent>(String? intstr, Integer|Absent default = null)
       given Absent satisfies Null {
   if (exists intstr) {
       return parseInteger(intstr) else default;
   } else {
       return default;
   }
}

and it is except for the default value of null, which results in null not assignable to Integer|Absent. Postponing that check to the invocation site would make this work.

Low priority.

[Migrated from ceylon/ceylon-spec#594]

[CeylonMigrationBot]

[@lucaswerkmeister] I’m a bit afraid that the error message for

Integer i = parseInt<Nothing>("10");

might be confusing, because the argument that’s not assignable to its type (null) isn’t present here. Would the error be attached to the parseInt node?

[CeylonMigrationBot]

[@gavinking]

Would the error be attached to the parseInt node?

Um, that, or to the argument list, perhaps. Not sure.

[CeylonMigrationBot]

[@lucaswerkmeister] Would this also apply to default type arguments? (Of course, that question doesn’t make much sense until #4067 is fixed.)

That would make my type system Turing machine a lot easier to use. At the moment, I have the following function:

// humongous return type omitted
t
<out State, out LeftStack, out LeftRest, out RightStack, out RightRest, Left, Right>
(B<State, LeftStack, RightStack> state, Left left, LeftRest leftRest, Right right, RightRest rightRest)
given State satisfies Q
given Left satisfies S
given Right satisfies S
given LeftRest satisfies Stack
given RightRest satisfies Stack
given LeftStack of StackEnd|StackHead<Left, LeftRest>
             satisfies Stack
given RightStack of StackEnd|StackHead<Right, RightRest>
             satisfies Stack

The arguments for left, leftRest, right and rightRest are always state.second.first, state.second.rest, state.third.first, state.third.rest, but in the t declaration that’s not necessarily well-typed, so I can’t use these as default arguments and instead need to always specify them like this.

[CeylonMigrationBot]

[@lucaswerkmeister] Maybe this should only happen for especially annotated parameters (e. g. with 'late')?