https://issues.dlang.org/show_bug.cgi?id=21356

          Issue ID: 21356
           Summary: mixin of mixin template in indexed tuple not allowed
           Product: D
           Version: D2
          Hardware: x86
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: destructionator@gmail.com

Easier to describe in code:

---
struct A {
        mixin template Foo() {}
}

alias AliasSeq(A...) = A;

alias thing = AliasSeq!(A);

mixin thing[0].Foo; // error
---

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