Bug 23791 – Rvalue default argument for ref parameter gives misleading error

As of DMD 2.102.2, attempting to compile the following program: --- void f1(ref int n = 123) {} struct S {} void f2(ref S s = S.init) {} --- ...produces the following error messages: --- bug.d(1): Error: cannot modify constant `123` bug.d(4): Error: `S()` is not an lvalue and cannot be modified --- These messages are misleading, because the program is not attempting to modify either `123` or `S()`. The actual cause of the errors is that rvalues cannot be used as default arguments for `ref` parameters. The messages should reflect this.

@ntrel created dlang/dmd pull request #15596 "Fix isLvalue error message" fixing this issue: - Fix isLvalue error message Requiring an lvalue is not the same as requiring modification. Fix Issue 23791 - Rvalue default argument for ref parameter gives misleading error. https://github.com/dlang/dmd/pull/15596

Fixed by https://github.com/dlang/dmd/pull/15793: /home/nick/os/rval.d(1): Error: cannot create default argument for `ref` / `out` parameter from constant `123` /home/nick/os/rval.d(4): Error: cannot create default argument for `ref` / `out` parameter from expression `S()` because it is not an lvalue