https://issues.dlang.org/show_bug.cgi?id=21668

          Issue ID: 21668
           Summary: Cannot declare ref parameter of opaque type
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: moonlightsentinel@disroot.org

Functions cannot accept opaque types by ref (pointer and slices work already):

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struct Opaque;

void byPtr(Opaque*) {}
void byRef(ref Opaque) {} // Fails
void bySlice(Opaque[]) {}
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