https://issues.dlang.org/show_bug.cgi?id=21144

          Issue ID: 21144
           Summary: A __trait should give access to interface
                    implementation vtbl and their offsets
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: andrei@erdani.com

Currently TypeInfo_Class implements a primitive interfaces() that returns an array of struct Interface. The vtbl and offset of the interface are magic. Should be available as __traits.

This blocks https://github.com/dlang/druntime/pull/3174.

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