Mangling enclosing template arguments used in the base of an unresolved-name

[John McCall]

How should we mangle the reference to 'T' in the following example?
  template <class T> struct A {
    template <class U> auto foo(U &u) -> decltype(T::bar(u));
  };

The ABI says:
  Class template parameter references are mangled using the standard
  mangling for the actual parameter type, typically a substitution. Note that
  a template parameter reference is a substitution candidate, distinct from
  the type (or other substitutible entity) that is the actual parameter.

We can't mangle T as a <template-param> because it would look exactly
like U.  We can't mangle it as a <source-name> because we're required
to be alpha-invariant over template parameter names.  As far as I can tell,
the mangling grammar doesn't allow us to mangle it as the substituted
type.  So we have a problem, and there's a parallel problem if the base
of the <unresolved-name> is a template-id where the template name is a
template template parameter from an enclosing template.

In both cases the underlying template argument should be available
as a substitution.  I suggest adding a new production:
  <existing-substitution> ::= <substitution>
and then extending unresolved-type:
  <unresolved-type> ::= <existing-substitution> [ <template-args> ]
with the proviso that this production should only be used for enclosing
template template parameters.

John.