[Issue 21585] New: add __traits(totype, string) to convert mangled type string to an existing type

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21585

          Issue ID: 21585
           Summary: add __traits(totype, string) to convert mangled type
                    string to an existing type
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Add totype as a __trait:

---
__trait ( totype, AssignExpression )
---

AssignExpression is evaluated at compile time, and the result must be
a string. The string must be a sequence of characters representing the
mangling of an existing type. The __trait then represents that type.

For example:

---
pragma(msg, 1.mangleof); // prints `i`
__traits(totype, "i") x;         // declares `x` as having type `int`
__traits(totype, "Pi") p;        // declares `p` as having type `int*`
__traits(totype, 3) y;           // error: `3` is not a string
__traits(totype, "#Hello") z;    // error: `#Hello` is not a recognized
mangling of a type
---

Earlier work: https://github.com/dlang/dmd/pull/11797

--