[clang] [Clang] prevent assertion failure by avoiding casts on type declarations that require complete types (PR #101426)

[Mital Ashok via cfe-commits]

================
@@ -9267,14 +9267,14 @@ bool Sema::RequireLiteralType(SourceLocation Loc, QualType T,
   if (!RT)
     return true;
 
-  const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
-
   // A partially-defined class type can't be a literal type, because a literal
   // class type must have a trivial destructor (which can't be checked until
   // the class definition is complete).
   if (RequireCompleteType(Loc, ElemType, diag::note_non_literal_incomplete, T))
     return true;
 
+  const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
----------------
MitalAshok wrote:

You don't require a literal type for C23 `constexpr`, so part of the condition should be `if (getLangOpts().CPlusPlus && NewVD->isConstexpr() && ... && RequireLiteralType`, just like how `CheckC23ConstexprVarType` is gated behind `getLangOpts().C23 &&`. And add an assertion for `getLangOpts().CPlusPlus` in RequireLiteralType for good measure.

The declaration will be marked invalid later by the same code that marks `/*!constexpr*/ struct Incomplete s = {};` invalid.

https://github.com/llvm/llvm-project/pull/101426