[cxx-interop] Disambiguate instantiations with template template parameters

[egorzhdan]

• Explanation:
  template<template <typename> typename> was tripping up the template class name printer – different instantiations of a class template were getting assigned the same name.
• Scope:
  This adjusts the instantiation name printer to emit an actual name of the template parameter instead of a placeholder (_).
• Issues:
  rdar://169199221
• Risk:
  Low, this is covered by tests.
• Testing:
  Added compiler tests.

[egorzhdan]

@swift-ci please smoke test

[Xazax-hun]

This looks good, but I wonder if we should also cover other (non-integral) template arguments like:

struct S {
    constexpr S(int) {}
};

template <S s>
void f();

void g() {
    constexpr S s{5};
    f<s>();
}

Could be a separate PR though.

[egorzhdan]

@Xazax-hun that particular example wouldn't trigger this code path in the compiler, since the template name printer is exclusive to types, it isn't used for functions. Supporting more kinds of template scaffolding would definitely be an improvement!

[egorzhdan]

@swift-ci please test Windows

[egorzhdan]

@swift-ci please test Windows

[j-hui]

Just for posteriority: do you have any record of what the crash looked like prior to your fix?

[j-hui]

@Xazax-hun that particular example wouldn't trigger this code path in the compiler, since the template name printer is exclusive to types, it isn't used for functions. Supporting more kinds of template scaffolding would definitely be an improvement!

Perhaps something like this?

struct S { constexpr S(int) {} };
template <S s> struct T {};
using T5 = T<{5}>;

[patrykstefanski]

Do we care about qualified names, for example:

namespace foo {
template <typename> struct Traits {};
} // namespace foo

namespace bar {
template <typename> struct Traits {};
} // namespace bar

using HasTraits3 = HasTemplateTemplateParam<foo::Traits>;
using HasTraits4 = HasTemplateTemplateParam<bar::Traits>;

prints:

typealias HasTraits3 = HasTemplateTemplateParam<Traits>
typealias HasTraits4 = HasTemplateTemplateParam<Traits>