Weird error: nonoverridable aspect "String_Literal"

General
1

Consider this code:
(from AARM 4.2.1 User-Defined Literals)

13.    Max_Roman_Number : constant := 3_999;  -- MMMCMXCIX
14. 
15.    type Roman_Number is range 1 .. Max_Roman_Number
16.      with String_Literal => To_Roman_Number;
17. 
18.    function To_Roman_Number (S : Wide_Wide_String) return Roman_Number;

I got:
error: nonoverridable aspect "String_Literal" of type "Roman_Number" requires "To_Roman_Number" declared at line 18 to be a primitive operation

What could its meaning possibly be?

2

Are you compiling with Ada 2022 enabled? -gnat2022

IIRC String_Literal is Ada 2022 only. The error may come from an issue in the compiler not detecting correctly the function.

3

The error is correct but not useful if you’re doing what I think you’re doing. Put your declarations inside of a package and it should work. I don’t remember off the top of my head why a package is required, however I do remember that that RM is very clear about the fact that it is.

4

It comes from the definition of primitives in 3.2.3, specifically the fact that there’s no equivalent of this for declarative regions:

For a specific type declared immediately within a package_specification, any subprograms (in addition to the enumeration literals) that are explicitly declared immediately within the same package_specification and that operate on the type

5

Thanks for quoting that!

It’s not very clear to me why operations are not primitive in a procedure declaration part.

Note: declaring a package in the procedure declaration part solve the issue!

procedure Test_SL_decl is
   package Roman is
   Max_Roman_Number : constant := 3_999;  -- MMMCMXCIX
   type Roman_Number is range 1 .. Max_Roman_Number
     with String_Literal => To_Roman_Number;
   function To_Roman_Number (S : Wide_Wide_String) return Roman_Number;
   end;
...
 X : Roman.Roman_Number;
...

Both RM and GNAT compiler messages should be clearer on this topic.

@Irvise Yes with -gnat2022, I hadn’t mentioned it.

6

I encountered something similar when playing with the Put_Image aspect, but I cannot reproduce it with your code.
It would be better if you provided a reproducible example.

The following compiles with $ gnatmake test_sl_decl.adb and GCC 14.3.1 and with 16.1.0 (no -gnat2022 needed).

with Ada.Text_IO;
procedure Test_SL_decl is

   Max_Roman_Number : constant := 3_999;  -- MMMCMXCIX
   type Roman_Number is range 1 .. Max_Roman_Number
     with String_Literal => To_Roman_Number;
   function To_Roman_Number (S : Wide_Wide_String) return Roman_Number is (1);

   X : constant Roman_Number := "I";
begin
   Ada.Text_IO.Put_Line (X'Image);
end Test_SL_decl;