Fix Unexpected ctor storing error value

[PetarKirov]

Summary

The Unexpected exception constructor at source/expected.d:1312 had this.error = error; — assigning the field to itself rather than the value parameter. As a result, the error value was never actually stored on the exception.

Newer compilers now emit:

source/expected.d(1312,13): Deprecation: cannot initialize field `error` with itself
             this.error = error;
             ^

This will eventually become a hard error.

Fix

-            static if (isAssignable!(string, T)) super(forward!value, file, line);
+            static if (isAssignable!(string, T)) super(value, file, line);
             else super("Unexpected error", file, line);

-            this.error = error;
+            this.error = forward!value;

I also dropped the forward!value from the super(...) call. Otherwise we'd be forwarding value to super and to this.error, which would use value after a potential move. The static if branch only fires when T is assignable to string (slice types — no meaningful move semantics), so passing value by ref to super is safe.

Test plan

• dub test — 43/43 pass
• dub build — clean, no deprecation

[tchaloupka]

thanks

[PetarKirov]

thanks as well for the quick merge @tchaloupka! Can you tag v0.4.1?