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

[via libcxx-commits]

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@@ -3058,6 +3058,141 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
   }
 }
 
+static std::optional<QualType> commonTypeImpl(Sema &S,
+                                              TemplateName BaseTemplate,
+                                              SourceLocation TemplateLoc,
+                                              ArrayRef<TemplateArgument> Ts) {
+  auto lookUpCommonType = [&](TemplateArgument T1,
+                              TemplateArgument T2) -> std::optional<QualType> {
+    // Don't bother looking for other specializations if both types are
+    // builtins - users aren't allowed to specialize for them
+    if (T1.getAsType()->isBuiltinType() && T2.getAsType()->isBuiltinType())
+      return commonTypeImpl(S, BaseTemplate, TemplateLoc, {T1, T2});
+
+    TemplateArgumentListInfo Args;
+    Args.addArgument(TemplateArgumentLoc(
+        T1, S.Context.getTrivialTypeSourceInfo(T1.getAsType())));
+    Args.addArgument(TemplateArgumentLoc(
+        T2, S.Context.getTrivialTypeSourceInfo(T2.getAsType())));
+    QualType BaseTemplateInst =
+        S.CheckTemplateIdType(BaseTemplate, TemplateLoc, Args);
+    if (S.RequireCompleteType(TemplateLoc, BaseTemplateInst,
+                              diag::err_incomplete_type))
+      return std::nullopt;
+    if (QualType Type = S.getTypeMember("type", BaseTemplateInst);
+        !Type.isNull()) {
+      return Type;
+    }
+    return std::nullopt;
+  };
+
+  // Note A: For the common_type trait applied to a template parameter pack T of
+  // types, the member type shall be either defined or not present as follows:
+  switch (Ts.size()) {
+
+  // If sizeof...(T) is zero, there shall be no member type.
+  case 0:
+    return std::nullopt;
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Sirraide wrote:

`QualType` already has a ‘null’ state, so this could just be `return QualType();` (in which case the function should just return a `QualType`). Is there a reason why we’re using `std::optional` here? I don’t think we really use `std::optional<QualType>` all that often...

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