PR24076, PR33655, C++ CWG 1558: Consider the instantiation-dependence of

the nested-name-specifier when determining whether a qualified type is
instantiation-dependent.

Previously reverted in 25a02c3 due to
causing us to reject some code. It turns out that the rejected code was
ill-formed (no diagnostic required).

Author: zygoloid

clang/include/clang/AST/Type.h

@@ -5412,7 +5412,9 @@ class ElaboratedType final
   ElaboratedType(ElaboratedTypeKeyword Keyword, NestedNameSpecifier *NNS,
                  QualType NamedType, QualType CanonType, TagDecl *OwnedTagDecl)
       : TypeWithKeyword(Keyword, Elaborated, CanonType,
-                        NamedType->getDependence()),
+                        NamedType->getDependence() |
+                            (NNS ? toTypeDependence(NNS->getDependence())
+                                 : TypeDependence::None)),
         NNS(NNS), NamedType(NamedType) {
     ElaboratedTypeBits.HasOwnedTagDecl = false;
     if (OwnedTagDecl) {

clang/lib/AST/ItaniumMangle.cpp

@@ -2583,6 +2583,10 @@ void CXXNameMangler::mangleType(QualType T) {
       if (isa<DecltypeType, TypeOfType>(T))
         break;
 
+      // FIXME: We presumably shouldn't strip off ElaboratedTypes with
+      // instantation-dependent qualifiers. See
+      // https://github.com/itanium-cxx-abi/cxx-abi/issues/114.
+
       QualType Desugared
         = T.getSingleStepDesugaredType(Context.getASTContext());
       if (Desugared == T)

clang/test/CXX/drs/dr15xx.cpp

@@ -239,6 +239,20 @@ namespace dr1550 { // dr1550: yes
   }
 }
 
+namespace dr1558 { // dr1558: 12
+#if __cplusplus >= 201103L
+  template<class T, class...> using first_of = T;
+  template<class T> first_of<void, typename T::type> f(int); // expected-note {{'int' cannot be used prior to '::'}}
+  template<class T> void f(...) = delete; // expected-note {{deleted}}
+
+  struct X { typedef void type; };
+  void test() {
+    f<X>(0);
+    f<int>(0); // expected-error {{deleted}}
+  }
+#endif
+}
+
 namespace dr1560 { // dr1560: 3.5
   void f(bool b, int n) {
     (b ? throw 0 : n) = (b ? n : throw 0) = 0;

clang/test/CodeGenCXX/mangle-template.cpp

@@ -342,3 +342,23 @@ namespace fixed_size_parameter_pack {
   template<int ...Ns> void f(A<unsigned, char, long long>::B<0, Ns...>);
   void g() { f<1, 2>({}); }
 }
+
+namespace type_qualifier {
+  template<typename T> using int_t = int;
+  template<typename T> void f(decltype(int_t<T*>() + 1)) {}
+  // FIXME: This mangling doesn't work: we need to mangle the
+  // instantiation-dependent 'int_t' operand.
+  // CHECK: @_ZN14type_qualifier1fIPiEEvDTplcvi_ELi1EE
+  template void f<int*>(int);
+
+  // Note that this template has different constraints but would mangle the
+  // same:
+  //template<typename T> void f(decltype(int_t<typename T::type>() + 1)) {}
+
+  struct impl { using type = void; };
+  template<typename T> using alias = impl;
+  template<typename T> void g(decltype(alias<T*>::type(), 1)) {}
+  // FIXME: Similarly we need to mangle the `T*` in here.
+  // CHECK: @_ZN14type_qualifier1gIPiEEvDTcmcvv_ELi1EE
+  template void g<int*>(int);
+}

clang/test/SemaTemplate/instantiation-dependence.cpp

@@ -0,0 +1,82 @@
+// RUN: %clang_cc1 -std=c++2b -verify %s
+
+// Ensure we substitute into instantiation-dependent but non-dependent
+// constructs. The poster-child for this is...
+template<class ...> using void_t = void;
+
+namespace PR24076 {
+  template<class T> T declval();
+  struct s {};
+
+  template<class T,
+           class = void_t<decltype(declval<T>() + 1)>>
+    void foo(T) {} // expected-note {{invalid operands to binary expression}}
+
+  void f() {
+    foo(s{}); // expected-error {{no matching function}}
+  }
+
+  template<class T,
+           class = void_t<decltype(declval<T>() + 1)>> // expected-error {{invalid operands to binary expression}}
+  struct bar {};
+
+  bar<s> bar; // expected-note {{in instantiation of}}
+}
+
+namespace PR33655 {
+  struct One { using x = int; };
+  struct Two { using y = int; };
+
+  template<typename T, void_t<typename T::x> * = nullptr> int &func() {}
+  template<typename T, void_t<typename T::y> * = nullptr> float &func() {}
+
+  int &test1 = func<One>();
+  float &test2 = func<Two>();
+
+  template<class ...Args> struct indirect_void_t_imp { using type = void; };
+  template<class ...Args> using indirect_void_t = typename indirect_void_t_imp<Args...>::type;
+
+  template<class T> void foo() {
+    static_assert(!__is_void(indirect_void_t<T>)); // "ok", dependent
+    static_assert(!__is_void(void_t<T>)); // expected-error {{failed}}
+  }
+}
+
+namespace PR46791 { // also PR45782
+  template<typename T, typename = void>
+  struct trait {
+    static constexpr int specialization = 0;
+  };
+
+  // FIXME: Per a strict interpretation of the C++ rules, the two void_t<...>
+  // types below are equivalent -- we only (effectively) do token-by-token
+  // comparison for *expressions* appearing within types. But all other
+  // implementations accept this, using rules that are unclear.
+  template<typename T>
+  struct trait<T, void_t<typename T::value_type>> { // expected-note {{previous}} FIXME-note {{matches}}
+    static constexpr int specialization = 1;
+  };
+
+  template<typename T>
+  struct trait<T, void_t<typename T::element_type>> { // expected-error {{redefinition}} FIXME-note {{matches}}
+    static constexpr int specialization = 2;
+  };
+
+  struct A {};
+  struct B { typedef int value_type; };
+  struct C { typedef int element_type; };
+  struct D : B, C {};
+
+  static_assert(trait<A>::specialization == 0);
+  static_assert(trait<B>::specialization == 1); // FIXME expected-error {{failed}}
+  static_assert(trait<C>::specialization == 2); // FIXME expected-error {{failed}}
+  static_assert(trait<D>::specialization == 0); // FIXME-error {{ambiguous partial specialization}}
+}
+
+namespace TypeQualifier {
+  // Ensure that we substitute into an instantiation-dependent but
+  // non-dependent qualifier.
+  template<int> struct A { using type = int; };
+  template<typename T> A<sizeof(sizeof(T::error))>::type f() {} // expected-note {{'int' cannot be used prior to '::'}}
+  int k = f<int>(); // expected-error {{no matching}}
+}

clang/test/SemaTemplate/partial-spec-instantiate.cpp

@@ -51,8 +51,6 @@ namespace rdar9169404 {
   X<bool, -1>::type value;
 #if __cplusplus >= 201103L
   // expected-error@-2 {{non-type template argument evaluates to -1, which cannot be narrowed to type 'bool'}}
-#else
-  // expected-no-diagnostics
 #endif
 }
 
@@ -98,3 +96,19 @@ namespace rdar39524996 {
     takesWrapperInContainer(c);
   }
 }
+
+namespace InstantiationDependent {
+  template<typename> using ignore = void; // expected-warning 0-1{{extension}}
+  template<typename T, typename = void> struct A {
+    static const bool specialized = false;
+  };
+  template<typename T> struct Hide { typedef void type; };
+  template<typename T> struct A<T, Hide<ignore<typename T::type> >::type> {
+    static const bool specialized = true;
+  };
+
+  struct X {};
+  struct Y { typedef int type; };
+  _Static_assert(!A<X>::specialized, "");
+  _Static_assert(A<Y>::specialized, "");
+}

clang/www/cxx_dr_status.html

@@ -9162,7 +9162,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://wg21.link/cwg1558">1558</a></td>
     <td>CD4</td>
     <td>Unused arguments in alias template specializations</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 12</td>
   </tr>
   <tr id="1559">
     <td><a href="https://wg21.link/cwg1559">1559</a></td>