Potentially misleading error message for incomplete generic return types

[jeremiah-corrado]

Summary

The following proc has an array return type with an unspecified domain. Throwing causes an "illegal use of function that does not return a value" error, originating from the last line of this program:

proc throwsError(): [] int throws {
  throw new Error("asdf");
}

var x = throwsError();

Specifying a domain value in the return type allows the program to compile:

proc throwsError(d: domain(?)): [d] int throws {
  throw new Error("asdf");
}

var x = throwsError({1..10});

EDIT: an improved error message like: "unable to determine concrete return type for the routine: 'throwsError'" or "unable to determine type of 'x' because procedure 'throwsError' doesn't have a concrete return type" would make it easier for users to understand why the first program fails to compile.

Example

The following is a more realistic example of how this issue could come up:

proc transpose(a: [?d] ?t): [] t throws
    where d.rank >= 2
{
    var dims = d.dims();
    dims[d.rank - 2] <=> dims[d.rank - 1];
    var ret: [{(...dims)}] t;

    forall idx in d {
        var rIdx = idx;
        rIdx[d.rank-1] <=> rIdx[d.rank-2];
        ret[rIdx] = a[idx];
    }

    return ret;
}

proc transpose(a: [?d] ?t): [] t throws
    where d.rank < 2
{
    throw new Error("rank must be at least 2 for transpose");
}

var x: [1..10, 1..5] int;
var y: [1..10] int;

// should work
var xx = transpose(x);

// should throw
try {
    var yy = transpose(y);
} catch e {
    writeln(e);
}

Here, the compiler emits an "illegal use of function that does not return a value" error from the var yy = transpose(y); line.

Discussion

The root cause of this issue is likely related to the compiler being unable to create a fully instantiated return type with a runtime-valued domain. Because the return type is missing, the compiler proceeds as if the function does not return a value when resolving expressions like var yy = transpose(y);.

Potentially related: https://github.com/chapel-lang/chapel/issues/15691, https://github.com/chapel-lang/chapel/issues/14191.