Libadalang appears to treat any single string wrapped in square brackets as invalid, reporting "invalid bracket encoding". See the following valid code as an example:
pragma Ada_2022;
with Ada.Containers.Vectors;
procedure Example is
type String_1 is new String (1 .. 1);
package String_Vec is new Ada.Containers.Vectors (Positive, String_1);
package Integer_Vec is new Ada.Containers.Vectors (Positive, Integer);
V1 : String_Vec.Vector := ["a", "b"];
V2 : Integer_Vec.Vector := [1];
V3 : String_Vec.Vector := ["a"]; -- Error here
begin
null;
end Example;
This has only been tested against the version of libadalang included in ALS 2026.0.202510141.