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

[Nikolas Klauser via cfe-commits]

================
@@ -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;
+
+  // 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) -> std::optional<QualType> {
+        EnterExpressionEvaluationContext UnevaluatedContext(
+            S, Sema::ExpressionEvaluationContext::Unevaluated);
+        Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
+        Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
+
+        // false
+        OpaqueValueExpr CondExpr({}, S.Context.BoolTy,
+                                 ExprValueKind::VK_PRValue);
+        ExprResult Cond = &CondExpr;
+
+        auto EVK =
+            ConstRefQual ? ExprValueKind::VK_LValue : ExprValueKind::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);
+
+        if (Result.isNull() || SFINAE.hasErrorOccurred())
+          return std::nullopt;
+
+        // decay_t<decltype(false ? declval<D1>() : declval<D2>())>
+        return S.BuiltinDecay(Result, TemplateLoc);
+      };
+
+      if (auto Res = CheckConditionalOperands(false))
+        return Res;
+
+      // Let:
+      // CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,
+      // COND-RES(X, Y) be
+      //   decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()()).
+
+      // C++20 only
+      // Otherwise, if COND-RES(CREF(D1), CREF(D2)) denotes a type, let C denote
+      // the type decay_t<COND-RES(CREF(D1), CREF(D2))>.
+      if (!S.Context.getLangOpts().CPlusPlus20)
+        return std::nullopt;
+      return CheckConditionalOperands(true);
+    }
+  }
+
+  // If sizeof...(T) is greater than two, let T1, T2, and R, respectively,
+  // denote the first, second, and (pack of) remaining types constituting T. Let
+  // C denote the same type, if any, as common_type_t<T1, T2>. If there is such
+  // a type C, the member typedef-name type shall denote the same type, if any,
+  // as common_type_t<C, R...>. Otherwise, there shall be no member type.
+  default: {
+    std::optional<QualType> Result = Ts[Ts.size() - 1].getAsType();
+    for (size_t i = Ts.size() - 1; i != 0; --i) {
+      Result = lookUpCommonType(Ts[i - 1].getAsType(), *Result);
----------------
philnik777 wrote:

I'm not sure I understand your question here. `lookUpCommonType` will either call `commonTypeImpl` directly if it's known to be safe, or it will instantiate `common_type<T1, T2>`, which will either be a user specialization or the base template, which then goes into this function again. So in the end we are calling this recursively if there isn't a user specialization of `common_type`.

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