[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:

Good point. There are only a handful of cases that this applies to, but they should also break loudly. In that case, I'd prefer it if getting that diagnostic logic into Clang preceded submitting this PR.

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