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

[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
+    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 QualType();
+    return S.getTypeMember(BaseTemplateInst, "type");
+  };
+
+  // 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 QualType();
+
+  // If sizeof...(T) is one, let T0 denote the sole type constituting the
+  // pack T. The member typedef-name type shall denote the same type, if any, as
+  // common_type_t<T0, T0>; otherwise there shall be no member type.
+  case 1:
+    return lookUpCommonType(Ts[0], Ts[0]);
+
+  // If sizeof...(T) is two, let the first and second types constituting T be
+  // denoted by T1 and T2, respectively, and let D1 and D2 denote the same types
+  // as decay_t<T1> and decay_t<T2>, respectively.
+  case 2: {
+    QualType T1 = Ts[0].getAsType();
+    QualType T2 = Ts[1].getAsType();
+    QualType D1 = S.BuiltinDecay(T1, {});
+    QualType D2 = S.BuiltinDecay(T2, {});
+
+    // If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false, let C denote
+    // the same type, if any, as common_type_t<D1, D2>.
+    if (!S.Context.hasSameType(T1, D1) || !S.Context.hasSameType(T2, D2))
+      return lookUpCommonType(D1, D2);
+
+    // Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())>
+    // denotes a valid type, let C denote that type.
+    {
+      auto CheckConditionalOperands = [&](bool ConstRefQual) -> QualType {
+        EnterExpressionEvaluationContext UnevaluatedContext(
+            S, Sema::ExpressionEvaluationContext::Unevaluated);
+        Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+        Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
+
+        // false
+        OpaqueValueExpr CondExpr(SourceLocation(), S.Context.BoolTy,
+                                 VK_PRValue);
+        ExprResult Cond = &CondExpr;
+
+        auto EVK = ConstRefQual ? VK_LValue : VK_PRValue;
+        if (ConstRefQual) {
+          D1.addConst();
+          D2.addConst();
+        }
+
+        // declval<D1>()
+        OpaqueValueExpr LHSExpr(TemplateLoc, D1, EVK);
+        ExprResult LHS = &LHSExpr;
+
+        // declval<D2>()
+        OpaqueValueExpr RHSExpr(TemplateLoc, D2, EVK);
+        ExprResult RHS = &RHSExpr;
+
+        ExprValueKind VK = VK_PRValue;
+        ExprObjectKind OK = OK_Ordinary;
+
+        // decltype(false ? declval<D1>() : declval<D2>())
+        QualType Result =
+            S.CheckConditionalOperands(Cond, LHS, RHS, VK, OK, TemplateLoc);
----------------
cor3ntin wrote:

So, I think we can call `CXXCheckConditionalOperands` here.

But the question I have is, should we factor out the part of `CXXCheckConditionalOperands` that deals with types directly and use that?

One one hand, doing surgery to `CXXCheckConditionalOperands` requires care and might be a tad difficult, on the other hand the whole danse we are doing here is a bit convoluted 





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