[clang] [C] Do not diagnose flexible array members with -Wdefault-const-init-field-unsafe (PR #140578)

[Bill Wendling via cfe-commits]

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@@ -6513,6 +6513,14 @@ static bool canPerformArrayCopy(const InitializedEntity &Entity) {
 static const FieldDecl *getConstField(const RecordDecl *RD) {
   assert(!isa<CXXRecordDecl>(RD) && "Only expect to call this in C mode");
   for (const FieldDecl *FD : RD->fields()) {
+    // If the field is a flexible array member, we don't want to consider it
+    // as a const field because there's no way to initialize the FAM anyway.
+    if (Decl::isFlexibleArrayMemberLike(
+            FD->getASTContext(), FD, FD->getType(),
+            LangOptions::StrictFlexArraysLevelKind::ZeroOrIncomplete,
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bwendling wrote:

Would it be better to use `getLangOpts().getStrictFlexArraysLevel()` instead of hard-coding `ZeroOrIncomplete`? If `-fstrict-flex-arrays=<n>` is set, then we may not expect there to be storage available in the flex array.

https://github.com/llvm/llvm-project/pull/140578