When importing C++ class template instantiations, Swift generates a type name for each instantiation. The generated names must be unique, since they are used for mangling.
If multiple different C++ types declare nested types with the same name, which are then used as template arguments, Swift was generating the same name for those template instantiations (e.g. `shared_ptr<Impl>` for different `Impl` types).
This change makes sure we use fully-qualified type names of template parameters when generating Swift type names for class template instantiations (e.g. `shared_ptr<MyNamespace.MyClass.Impl>`).
This fixes an assertion failure coming out of IRGen:
```
Assertion failed: (Buffer.empty() && "didn't claim all values out of buffer"), function ~ConstantInitBuilderBase, file ConstantInitBuilder.h, line 75.
```
rdar://141962480
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>
