[clang] [clang][Sema] Fix wrong assumption in `tryDiagnoseOverloadedCast` (PR #108021)

[Alejandro Álvarez Ayllón via cfe-commits]

https://github.com/alejandro-alvarez-sonarsource created https://github.com/llvm/llvm-project/pull/108021

A constructor may have failed to be used due to a broken templated dependent parameter. The `InitializationSequence` itself can succeed.

Due to the assumption that it is in a failed state, it can either cause an assertion failure, or undefined behavior in release mode, since `sequence.Failure` is not initialized.

>From b414b6e184fc566efd6a2dbe631ee68a25b461a1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alejandro=20=C3=81lvarez=20Ayll=C3=B3n?=
 <alejandro.alvarez at sonarsource.com>
Date: Fri, 16 Jun 2023 18:01:33 +0200
Subject: [PATCH] [clang][Sema] Fix wrong assumption in
 `tryDiagnoseOverloadedCast`

A constructor may have failed to be used due to a broken templated
dependent parameter. The `InitializationSequence` itself can
succeed.

Due to the assumption that it is in a failed state, it can
either cause an assertion failure, or undefined behavior in release
mode, since `sequence.Failure` is not initialized.
---
 clang/lib/Sema/SemaCast.cpp                   |  7 ++++-
 .../cxx-bad-cast-diagnose-broken-template.cpp | 31 +++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/Parser/cxx-bad-cast-diagnose-broken-template.cpp

diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index f01b22a72915c8..6ac6201843476b 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/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;
 
diff --git a/clang/test/Parser/cxx-bad-cast-diagnose-broken-template.cpp b/clang/test/Parser/cxx-bad-cast-diagnose-broken-template.cpp
new file mode 100644
index 00000000000000..f9fc3e6082da10
--- /dev/null
+++ b/clang/test/Parser/cxx-bad-cast-diagnose-broken-template.cpp
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify %s
+
+template< typename>
+struct IsConstCharArray
+{
+  static const bool value = false;
+};
+
+template< int N >
+struct IsConstCharArray< const char[ N ] >
+{
+  typedef char CharType;
+  static const bool value = true;
+  static const missing_int_t length = N - 1; // expected-error {{unknown type name 'missing_int_t'}}
+};
+
+class String {
+public:
+  template <typename T>
+  String(T& str, typename IsConstCharArray<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}}
+}