[clang] [Clang][RFC] Intrododuce a builtin to determine the structure binding size (PR #131515)

[Erich Keane via cfe-commits]

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@@ -5813,6 +5817,34 @@ static ExprResult CheckConvertibilityForTypeTraits(
   return Result;
 }
 
+static APValue EvaluateSizeTTypeTrait(Sema &S, TypeTrait Kind,
+                                      SourceLocation KWLoc,
+                                      ArrayRef<TypeSourceInfo *> Args,
+                                      SourceLocation RParenLoc,
+                                      bool IsDependent) {
+  if (IsDependent)
+    return APValue();
+
+  switch (Kind) {
+  case TypeTrait::UTT_StructuredBindingSize: {
+    QualType T = Args[0]->getType();
+    SourceRange ArgRange = Args[0]->getTypeLoc().getSourceRange();
+    std::optional<unsigned> Size =
+        S.GetDecompositionElementCount(T, ArgRange.getBegin());
+    if (!Size) {
+      S.Diag(KWLoc, diag::err_arg_is_not_destructurable) << T << ArgRange;
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erichkeane wrote:

Ah, hmm... This would result in a complete-type diagnostic, followed by 'not destructurable'.  I guess that is OK.

https://github.com/llvm/llvm-project/pull/131515