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

[ayzhao]

Summary

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

[shafik]

clang/lib/Sema/SemaCast.cpp

458

Can you quote the sections in the standard that say this?

[ayzhao]

quote C++20 standard

[aaron.ballman]

LGTM, thank you for the fix!

clang/docs/ReleaseNotes.rst

- Fix the contains-errors bit not being set for DeclRefExpr that refers to a
   (`#50236 <https://github.com/llvm/llvm-project/issues/50236>`_),
   (`#50243 <https://github.com/llvm/llvm-project/issues/50243>`_),
   (`#48636 <https://github.com/llvm/llvm-project/issues/48636>`_),
   (`#50320 <https://github.com/llvm/llvm-project/issues/50320>`_).
 - Fix an assertion when using ``\u0024`` (``$``) as an identifier, by disallowing
   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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a bug where attribute annotations on type specifiers (enums, classes,
   structs, unions, and scoped enums) were not properly ignored, resulting in

clang/lib/Sema/SemaCast.cpp

: (CT == CT_Functional)? InitializationKind::CreateFunctionalCast(range,
                                                              listInitialization)
     : InitializationKind::CreateCast(/*type range?*/ range);
   InitializationSequence sequence(S, entity, initKind, src);
 
   assert(sequence.Failed() && "initialization succeeded on second try?");
   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;
   }
 
   OverloadCandidateSet &candidates = sequence.getFailedCandidateSet();
 
   unsigned msg = 0;
   OverloadCandidateDisplayKind howManyCandidates = OCD_AllCandidates;
 

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

int test() {
   S c(0, 0, 1); // beforecxx20-warning {{aggregate initialization of type 'S' from a parenthesized list of values is a C++20 extension}}
 
   S d {0, 1};
   S e {0};
   S f {0, 0, 1};
 }
 
 }
+
+namespace gh63758 {
+  struct S {} s;
+  auto words = (char[])s; // expected-error {{C-style cast from 'struct S' to 'char[]' is not allowed}}
+};