http://d.puremagic.com/issues/show_bug.cgi?id=5020

           Summary: Forward implicit bool conversions to alias this
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: rsinfu@gmail.com


--- Comment #0 from Shin Fujishiro <rsinfu@gmail.com> 2010-10-08 12:14:52 PDT ---
Alias this isn't implemented for implicit conversions to bool in conditional expressions.  The following presumably valid code doesn't compile:
--------------------
void main()
{
    S s;
    if (!s) {}  // (4)
}
struct S
{
    bool cond;
    alias cond this;
}
--------------------
% dmd -o- -c test.d
test.d(4): Error: expression s of type S does not have a boolean value

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Shin Fujishiro <rsinfu@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch


--- Comment #1 from Shin Fujishiro <rsinfu@gmail.com> 2010-10-08 13:14:45 PDT ---
It's simply Expression::checkToBoolean() not looking for alias this.  Here's a
proposed patch against dmd r707:
====================
--- src/expression.c
+++ src/expression.c
@@ -1269,6 +1269,15 @@ Expression *Expression::checkToBoolean(Scope *sc)
             e = e->semantic(sc);
             return e;
         }
+
+        // Forward to aliasthis.
+        if (ad->aliasthis)
+        {
+            Expression *e = new DotIdExp(loc, this, ad->aliasthis->ident);
+            e = e->semantic(sc);
+            e = e->checkToBoolean(sc);
+            return e;
+        }
     }

     if (!type->checkBoolean())
====================

Note: Since CastExp takes care of aliasthis, adding a test for ad->aliasthis in a preceding if-block also makes the repro code work.  But the if-block doesn't check for implicit convertible-ness (i.e. checkToBoolean), and it will eventually allow the following wrong code to be accepted:

void main()
{
    S s;
    if (s) {}   // wrong
}
struct S
{
    struct R {} // not implicitly convertible to bool
    R r;
    alias r this;
}

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bearophile_hugs@eml.cc
         Resolution|                            |FIXED


--- Comment #2 from bearophile_hugs@eml.cc 2010-12-21 01:50:31 PST ---
Fixed in DMD 2.051

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