[clang] Fix a crash when casting to an array type

In C++20, if Clang fails to perform constructor overload on a
RecordType, then Clang will try to perform parentesized aggregate
initialization. If that fails and the initialization was attempted as
part of a cast, then we should get the diagnostics from the failed
constructor overload attempt. However, we don't attempt constructor
overloading for arrays, so previously, if we try to diagnose an
overloaded cast for a parenthesized aggregate initialization of an
array, we crash. To fix this, we now exit tryDiagnoseOverloadedCast(...)
for failed parentesized list initialization if the destination type is
an array.

Fixes #63758

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D155523

clang/docs/ReleaseNotes.rst

@@ -634,6 +634,8 @@ Bug Fixes in This Version
   that construct (`#62133 <https://github.com/llvm/llvm-project/issues/38717>_`).
 - Fix crash caused by PseudoObjectExprBitfields: NumSubExprs overflow.
   (`#63169 <https://github.com/llvm/llvm-project/issues/63169>_`)
+- Fix crash when casting an object to an array type.
+  (`#63758 <https://github.com/llvm/llvm-project/issues/63758>_`)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaCast.cpp

@@ -454,9 +454,27 @@ static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
   switch (sequence.getFailureKind()) {
   default: return false;
 
+  case InitializationSequence::FK_ParenthesizedListInitFailed:
+    // In C++20, if the underlying destination type is a RecordType, Clang
+    // attempts to perform parentesized aggregate initialization if constructor
+    // overload fails:
+    //
+    // C++20 [expr.static.cast]p4:
+    //   An expression E can be explicitly converted to a type T...if overload
+    //   resolution for a direct-initialization...would find at least one viable
+    //   function ([over.match.viable]), or if T is an aggregate type having a
+    //   first element X and there is an implicit conversion sequence from E to
+    //   the type of X.
+    //
+    // If that fails, then we'll generate the diagnostics from the failed
+    // previous constructor overload attempt. Array initialization, however, is
+    // not done after attempting constructor overloading, so we exit as there
+    // won't be a failed overload result.
+    if (destType->isArrayType())
+      return false;
+    break;
   case InitializationSequence::FK_ConstructorOverloadFailed:
   case InitializationSequence::FK_UserConversionOverloadFailed:
-  case InitializationSequence::FK_ParenthesizedListInitFailed:
     break;
   }
 

clang/test/SemaCXX/paren-list-agg-init.cpp

@@ -294,3 +294,8 @@ int test() {
 }
 
 }
+
+namespace gh63758 {
+  struct S {} s;
+  auto words = (char[])s; // expected-error {{C-style cast from 'struct S' to 'char[]' is not allowed}}
+};