[clang] [libcxx] [Clang] Add __common_type builtin (PR #99473)

[Christopher Di Bella via cfe-commits]

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@@ -3058,6 +3058,133 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
   }
 }
 
+static QualType commonTypeImpl(Sema &S, TemplateName BaseTemplate,
+                               SourceLocation TemplateLoc,
+                               ArrayRef<TemplateArgument> Ts) {
+  auto lookUpCommonType = [&](TemplateArgument T1,
+                              TemplateArgument T2) -> QualType {
+    // Don't bother looking for other specializations if both types are
+    // builtins - users aren't allowed to specialize for them
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cjdb wrote:

Can we diagnose the case where users do this, please? Similarly for specialising `common_type<>`, `common_type<T>`, and `common_type<T1, T2, T3, Ts...>`.

https://github.com/llvm/llvm-project/pull/99473