[Bug]: cannot explicitly specify a generic range return type

[jabraham17]

Summary of Problem

Description:

Attempting to specify a generic return type for a range with a non-default type results in an error. However, fully specifying the return type does work. And just removing the explicit return type also works.

I believe this is a quirk of range being generic with defaults, such that range(?) gives you the defaults. I would have expected that range says "the return type is a range with the type defaults" and range(?) says "the return type is a range with unknown type"

Steps to Reproduce

Source Code:

proc foo(): range(?) { // error: initializing a range with strideKind.one from a range with strideKind.positive
  return 1..10 by 2;
}
writeln(foo());

proc foo() { // this is fine and is a workaround
  return 1..10 by 2;
}

proc foo(): range(int, boundKind.both, strideKind.positive) { // this is also fine, being full explicit
  return 1..10 by 2;
}

Compile command: chpl foo.chpl

Configuration Information

• Output of chpl --version: 2.2
• Output of $CHPL_HOME/util/printchplenv --anonymize:

  CHPL_TARGET_PLATFORM: darwin
  CHPL_TARGET_COMPILER: llvm
  CHPL_TARGET_ARCH: arm64
  CHPL_TARGET_CPU: native
  CHPL_LOCALE_MODEL: flat
  CHPL_COMM: none
  CHPL_TASKS: qthreads
  CHPL_LAUNCHER: none
  CHPL_TIMERS: generic
  CHPL_UNWIND: none
  CHPL_MEM: jemalloc +
  CHPL_ATOMICS: cstdlib
  CHPL_GMP: system +
  CHPL_HWLOC: system +
  CHPL_RE2: bundled +
  CHPL_LLVM: system
  CHPL_AUX_FILESYS: none

• Back-end compiler and version, e.g. gcc --version or clang --version: LLVM 18

[bradcray]

I believe this is a quirk of range being generic with defaults, such that range(?) gives you the defaults.

I'd also expect that range(?) should not get you the defaults. In other cases, I believe that's the case, such as:

var r: range(?) = 1..10 by -2;
writeln(r);

proc foo(r: range(?)) {
  writeln(r);
}

foo(r);

which works as expected.

Your issue also seems to be consistent for other fully-defaulted generics:

record R {
  param n = 1;
}

proc foo(): R(?) {
  return new R(n=2);
}
writeln(foo());  // similar error

But interestingly, not for partially-defaulted generics:

record R {
  param n = 1;
  param k;
}

proc foo(): R(?) {
  return new R(n=2, k=33);
}
writeln(foo());  // works

[edited to update comments, which I'd copied without updating]