Swift was previously crashing while emitting an error ("could not generate C++ types from the generic Swift types provided") for C++ decls that do not have a name, such as constructors: `func->getName()` triggered an assertion.
Lazily instantiate class template members. This means we no longer
reject some programs that clang accepts, such as the following:
```
template<class T> struct Foo { void fail(T value) { value.fail(); } };
using Bar = Foo<int>;
```
The above program will not error so long as `Bar::fail` isn't called.
(Previously, we'd fail to import `Bar`.)
This PR makes it possible to instantiate C++ class templates from Swift. Given a C++ header:
```c++
// C++ module `ClassTemplates`
template<class T>
struct MagicWrapper {
T t;
};
struct MagicNumber {};
```
it is now possible to write in Swift:
```swift
import ClassTemplates
func x() -> MagicWrapper<MagicNumber> {
return MagicWrapper<MagicNumber>()
}
```
This is achieved by importing C++ class templates as generic structs, and then when Swift type checker calls `applyGenericArguments` we detect when the generic struct is backed by the C++ class template and call Clang to instantiate the template. In order to make it possible to put class instantiations such as `MagicWrapper<MagicNumber>` into Swift signatures, we have created a new field in `StructDecl` named `TemplateInstantiationType` where the typechecker stores the `BoundGenericType` which we serialize. Deserializer then notices that the `BoundGenericType` is actually a C++ class template and performs the instantiation logic.
Depends on https://github.com/apple/swift/pull/33420.
Progress towards https://bugs.swift.org/browse/SR-13261.
Fixes https://bugs.swift.org/browse/SR-13775.
Co-authored-by: Dmitri Gribenko <gribozavr@gmail.com>
Co-authored-by: Rosica Dejanovska <rosica@google.com>
