A patch to allow "nested union".

patch.diff

diff --git a/src/struct.c b/src/struct.c
index f6fd857..3a6fe5f 100644
--- a/src/struct.c
+++ b/src/struct.c
@@ -684,9 +684,19 @@ void StructDeclaration::finalizeSize(Scope *sc)
     // Set the offsets of the fields and determine the size of the struct
     unsigned offset = 0;
     bool isunion = isUnionDeclaration() != NULL;
+    unsigned maxofs = 0;
     for (size_t i = 0; i < members->dim; i++)
     {   Dsymbol *s = (*members)[i];
-        s->setFieldOffset(this, &offset, isunion);
+        //printf("%s [%d] offset = %d, u = %d\n", toChars(), i, offset, maxofs);
+        if (isunion)
+        {
+            if (i == members->dim - 1 && s->isThisDeclaration())
+                offset = maxofs;
+            else
+                offset = 0;
+        }
+        s->setFieldOffset(this, &offset, false);
+        if (maxofs < offset) maxofs = offset;
     }
     if (sizeok == SIZEOKfwd)
         return;

testcase.d

void main()
{
    union U {
        int x;
        float y;
        void foo() {}
    }
    pragma(msg, U.sizeof);  // prints 12 in 32bit system
    pragma(msg, U.tupleof[0].offsetof); // prints 0
    pragma(msg, U.tupleof[1].offsetof); // prints 0
    pragma(msg, U.tupleof[2].offsetof); // prints 8
}