[clang][Sema] Fix assertion in `tryDiagnoseOverloadedCast` (#108021)

Fixed an assertion failure in debug mode, and potential crashes in
release mode, when
diagnosing a failed cast caused indirectly by a failed implicit
conversion to the type of the constructor parameter.

For instance

```
template<typename>
struct StringTrait {};

template< int N >
struct StringTrait< const char[ N ] > {
  typedef char CharType;
  static const MissingIntT length = N - 1;
};

class String {
public:
  template <typename T>
  String(T& str, typename StringTrait<T>::CharType = 0);
};


class Exception {
public:
  Exception(String const&);
};

void foo() {
  throw Exception("some error");
}
```

`Exception(String const&)` is a matching constructor for `Exception`
from a `const char*`, via an implicit conversion to `String`. However,
the instantiation of the `String` constructor will fail because of the
missing type `MissingIntT` inside the specialization of `StringTrait`.

When trying to emit a diagnosis, `tryDiagnoseOverloadedCast` expects not
to have a matching constructor, but there is; it just could not be
instantiated.

0 files changed, 0 insertions, 0 deletions