[Issue 2571] New: const static arrays: const(T)[N] and const(T[N]) are the same, but DMD treats them as different

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=2571

           Summary: const static arrays: const(T)[N] and const(T[N]) are the
                    same, but DMD treats them as different
           Product: D
           Version: 2.023
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid, wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: smjg at iname.com


You can apply const to the whole of a static array type or to the element type
thereof.  Since static arrays have value semantics, the result is the same: you
can neither change an element in the array nor reassign to it a whole new
array.  Nonetheless, DMD treats them as different types:

----- const_static_array_1.d -----
const(char)[26] abc1 = "abcdefghijklmnopqrstuvwxyz";
const(char[26]) abc2 = "abcdefghijklmnopqrstuvwxyz";

pragma(msg, typeof(abc1).stringof);
pragma(msg, typeof(abc2).stringof);
pragma(msg, (const(char)[26]).stringof);
pragma(msg, (const(char[26])).stringof);

pragma(msg, is(typeof(abc1) : typeof(abc2)).stringof);
pragma(msg, is(typeof(abc2) : typeof(abc1)).stringof);
pragma(msg, is(typeof(abc1) == typeof(abc2)).stringof);
----------
C:\Users\Stewart\Documents\Programming\D\d2\tests>dmd -c const_static_array_1.d
const(char)[26u]
const(char[26u])
const(char)[26u]
const(char[26u])
true
true
false
----------

Moreover, while they are implicitly convertible, that they are treated as
different types creates a silly template incompatibility:
----- const_static_array_2.d -----
const(char)[26] abc1 = "abcdefghijklmnopqrstuvwxyz";
const(char[26]) abc2 = "abcdefghijklmnopqrstuvwxyz";

class Temp(T) {}

void main() {
    Temp!(const(char)[26]) t1;
    Temp!(const(char[26])) t2;

    t1 = t2;
    t2 = t1;
}
----------
C:\Users\Stewart\Documents\Programming\D\d2\tests>dmd const_static_array_2.d
const_static_array_2.d(10): Error: cannot implicitly convert expression (t2) of
type const_static_array_2.Temp!(const(char[26u])).Temp to
const_static_array_2.Temp!(const(char)[26u]).Temp
const_static_array_2.d(11): Error: cannot implicitly convert expression (t1) of
type const_static_array_2.Temp!(const(char)[26u]).Temp to
const_static_array_2.Temp!(const(char[26u])).Temp
----------


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