[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.

Author: alejandro-alvarez-sonarsource

clang/docs/ReleaseNotes.rst

@@ -397,6 +397,8 @@ Bug Fixes to C++ Support
 - Fixed a crash when clang tries to subtitute parameter pack while retaining the parameter
   pack. #GH63819, #GH107560
 - Fix a crash when a static assert declaration has an invalid close location. (#GH108687)
+- 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.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaCast.cpp

@@ -446,7 +446,12 @@ static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
     : InitializationKind::CreateCast(/*type range?*/ range);
   InitializationSequence sequence(S, entity, initKind, src);
 
-  assert(sequence.Failed() && "initialization succeeded on second try?");
+  // It could happen that a constructor failed to be used because
+  // it requires a temporary of a broken type. Still, it will be found when
+  // looking for a match.
+  if (!sequence.Failed())
+    return false;
+
   switch (sequence.getFailureKind()) {
   default: return false;
 

clang/test/Parser/cxx-bad-cast-diagnose-broken-template.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s
+
+template<typename>
+struct StringTrait {};
+
+template< int N >
+struct StringTrait< const char[ N ] > {
+  typedef char CharType;
+  static const MissingIntT length = N - 1; // expected-error {{unknown type name 'MissingIntT'}}
+};
+
+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"); // expected-error {{functional-style cast from 'const char[11]' to 'Exception' is not allowed}}
+}