[Sema] Ensure that we retain __restrict qualifiers when substituting a reference type.

[erik.pilkington]

Summary

Fixes rdar://43760099. Thanks for taking a look!

[rjmccall]

clang/lib/Sema/TreeTransform.h

4268 ↗

(On Diff #166150)

restrict is unique in being able to be applied to reference types. You should just rebuild Quals from scratch, preserving only restrict.

[erik.pilkington]

Sure, the new patch just preserves __restrict qualifiers. Thanks!

[rjmccall]

Thanks, LGTM.

lib/Sema/TreeTransform.h

 
 template<typename Derived>
 QualType TreeTransform<Derived>::RebuildQualifiedType(QualType T,
                                                       SourceLocation Loc,
                                                       Qualifiers Quals) {
   // C++ [dcl.fct]p7:
   //   [When] adding cv-qualifications on top of the function type [...] the
   //   cv-qualifiers are ignored.
+  if (T->isFunctionType())
+    return T;
+
   // C++ [dcl.ref]p1:
   //   when the cv-qualifiers are introduced through the use of a typedef-name
   //   or decltype-specifier [...] the cv-qualifiers are ignored.
   // Note that [dcl.ref]p1 lists all cases in which cv-qualifiers can be
   // applied to a reference type.
-  // FIXME: This removes all qualifiers, not just cv-qualifiers!
-  if (T->isFunctionType() || T->isReferenceType())
+  if (T->isReferenceType()) {
+    // The only qualifier that applies to a reference type is restrict.
+    if (!Quals.hasRestrict())
       return T;
+    Quals = Qualifiers::fromCVRMask(Qualifiers::Restrict);
+  }
 
   // Suppress Objective-C lifetime qualifiers if they don't make sense for the
   // resulting type.
   if (Quals.hasObjCLifetime()) {
     if (!T->isObjCLifetimeType() && !T->isDependentType())
       Quals.removeObjCLifetime();
     else if (T.getObjCLifetime()) {
       // Objective-C ARC:

test/SemaCXX/subst-restrict.cpp

+// RUN: %clang_cc1 -std=c++17 -verify %s
+
+// expected-no-diagnostics
+
+template <class T> struct add_restrict {
+  typedef T __restrict type;
+};
+
+template <class T, class V> struct is_same {
+  static constexpr bool value = false;
+};
+
+template <class T> struct is_same<T, T> {
+  static constexpr bool value = true;
+};
+
+static_assert(is_same<int & __restrict, add_restrict<int &>::type>::value, "");
+static_assert(is_same<int(), add_restrict<int()>::type>::value, "");