https://issues.dlang.org/show_bug.cgi?id=19118

          Issue ID: 19118
           Summary: Types defined in the template specification 'if' are
                    not usable in the template
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: yshuiv7@gmail.com

template Test(T) if (is(T == Nullable!S, S)) {
    alias tmp = S; // S is undefined
}

This is quite inconvenient.

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