[cxx2a] P0641R2: (Some) type mismatches on defaulted functions only

render the function deleted instead of rendering the program ill-formed.

This change also adds an enabled-by-default warning for the case where
an explicitly-defaulted special member function of a non-template class
is implicitly deleted by the type checking rules. (This fires either due
to this language change or due to pre-C++20 reasons for the member being
implicitly deleted). I've tested this on a large codebase and found only
bugs (where the program means something that's clearly different from
what the programmer intended), so this is enabled by default, but we
should revisit this if there are problems with this being enabled by
default.

llvm-svn: 343285

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7760,9 +7760,19 @@ def err_incorrect_defaulted_exception_spec : Error<
 def err_incorrect_defaulted_constexpr : Error<
   "defaulted definition of %sub{select_special_member_kind}0 "
   "is not constexpr">;
+def warn_defaulted_method_deleted : Warning<
+  "explicitly defaulted %sub{select_special_member_kind}0 is implicitly "
+  "deleted">, InGroup<DiagGroup<"defaulted-function-deleted">>;
 def err_out_of_line_default_deletes : Error<
   "defaulting this %sub{select_special_member_kind}0 "
   "would delete it after its first declaration">;
+def note_deleted_type_mismatch : Note<
+  "function is implicitly deleted because its declared type does not match "
+  "the type of an implicit %sub{select_special_member_kind}0">;
+def warn_cxx17_compat_defaulted_method_type_mismatch : Warning<
+  "explicitly defaulting this %sub{select_special_member_kind}0 with a type "
+  "different from the implicit type is incompatible with C++ standards before "
+  "C++2a">, InGroup<CXXPre2aCompat>, DefaultIgnore;
 def warn_vbase_moved_multiple_times : Warning<
   "defaulted move assignment operator of %0 will move assign virtual base "
   "class %1 multiple times">, InGroup<DiagGroup<"multiple-move-vbase">>;

clang/lib/Sema/SemaDeclCXX.cpp

@@ -6451,20 +6451,29 @@ void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
   //        copy operation can take a non-const reference) as an implicit
   //        declaration, and
   //     -- not have default arguments.
+  // C++2a changes the second bullet to instead delete the function if it's
+  // defaulted on its first declaration, unless it's "an assignment operator,
+  // and its return type differs or its parameter type is not a reference".
+  bool DeleteOnTypeMismatch = getLangOpts().CPlusPlus2a && First;
+  bool ShouldDeleteForTypeMismatch = false;
   unsigned ExpectedParams = 1;
   if (CSM == CXXDefaultConstructor || CSM == CXXDestructor)
     ExpectedParams = 0;
   if (MD->getNumParams() != ExpectedParams) {
-    // This also checks for default arguments: a copy or move constructor with a
+    // This checks for default arguments: a copy or move constructor with a
     // default argument is classified as a default constructor, and assignment
     // operations and destructors can't have default arguments.
     Diag(MD->getLocation(), diag::err_defaulted_special_member_params)
       << CSM << MD->getSourceRange();
     HadError = true;
   } else if (MD->isVariadic()) {
-    Diag(MD->getLocation(), diag::err_defaulted_special_member_variadic)
-      << CSM << MD->getSourceRange();
-    HadError = true;
+    if (DeleteOnTypeMismatch)
+      ShouldDeleteForTypeMismatch = true;
+    else {
+      Diag(MD->getLocation(), diag::err_defaulted_special_member_variadic)
+        << CSM << MD->getSourceRange();
+      HadError = true;
+    }
   }
 
   const FunctionProtoType *Type = MD->getType()->getAs<FunctionProtoType>();
@@ -6489,9 +6498,13 @@ void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
 
     // A defaulted special member cannot have cv-qualifiers.
     if (Type->getTypeQuals()) {
-      Diag(MD->getLocation(), diag::err_defaulted_special_member_quals)
-        << (CSM == CXXMoveAssignment) << getLangOpts().CPlusPlus14;
-      HadError = true;
+      if (DeleteOnTypeMismatch)
+        ShouldDeleteForTypeMismatch = true;
+      else {
+        Diag(MD->getLocation(), diag::err_defaulted_special_member_quals)
+          << (CSM == CXXMoveAssignment) << getLangOpts().CPlusPlus14;
+        HadError = true;
+      }
     }
   }
 
@@ -6504,23 +6517,30 @@ void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
     HasConstParam = ReferentType.isConstQualified();
 
     if (ReferentType.isVolatileQualified()) {
-      Diag(MD->getLocation(),
-           diag::err_defaulted_special_member_volatile_param) << CSM;
-      HadError = true;
+      if (DeleteOnTypeMismatch)
+        ShouldDeleteForTypeMismatch = true;
+      else {
+        Diag(MD->getLocation(),
+             diag::err_defaulted_special_member_volatile_param) << CSM;
+        HadError = true;
+      }
     }
 
     if (HasConstParam && !CanHaveConstParam) {
-      if (CSM == CXXCopyConstructor || CSM == CXXCopyAssignment) {
+      if (DeleteOnTypeMismatch)
+        ShouldDeleteForTypeMismatch = true;
+      else if (CSM == CXXCopyConstructor || CSM == CXXCopyAssignment) {
         Diag(MD->getLocation(),
              diag::err_defaulted_special_member_copy_const_param)
           << (CSM == CXXCopyAssignment);
         // FIXME: Explain why this special member can't be const.
+        HadError = true;
       } else {
         Diag(MD->getLocation(),
              diag::err_defaulted_special_member_move_const_param)
           << (CSM == CXXMoveAssignment);
+        HadError = true;
       }
-      HadError = true;
     }
   } else if (ExpectedParams) {
     // A copy assignment operator can take its argument by value, but a
@@ -6582,14 +6602,27 @@ void Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD) {
                                         EPI));
   }
 
-  if (ShouldDeleteSpecialMember(MD, CSM)) {
+  if (ShouldDeleteForTypeMismatch || ShouldDeleteSpecialMember(MD, CSM)) {
     if (First) {
       SetDeclDeleted(MD, MD->getLocation());
+      if (!inTemplateInstantiation() && !HadError) {
+        Diag(MD->getLocation(), diag::warn_defaulted_method_deleted) << CSM;
+        if (ShouldDeleteForTypeMismatch) {
+          Diag(MD->getLocation(), diag::note_deleted_type_mismatch) << CSM;
+        } else {
+          ShouldDeleteSpecialMember(MD, CSM, nullptr, /*Diagnose*/true);
+        }
+      }
+      if (ShouldDeleteForTypeMismatch && !HadError) {
+        Diag(MD->getLocation(),
+             diag::warn_cxx17_compat_defaulted_method_type_mismatch) << CSM;
+      }
     } else {
       // C++11 [dcl.fct.def.default]p4:
       //   [For a] user-provided explicitly-defaulted function [...] if such a
       //   function is implicitly defined as deleted, the program is ill-formed.
       Diag(MD->getLocation(), diag::err_out_of_line_default_deletes) << CSM;
+      assert(!ShouldDeleteForTypeMismatch && "deleted non-first decl");
       ShouldDeleteSpecialMember(MD, CSM, nullptr, /*Diagnose*/true);
       HadError = true;
     }

clang/test/CXX/class.derived/class.abstract/p16.cpp

@@ -44,8 +44,8 @@ struct G : D {};
 // expected-error@-3 {{deleted function 'operator=' cannot override a non-deleted function}}
 // expected-note@-4 {{while declaring the implicit move assignment operator for 'G'}}
 // expected-note@-5 {{move assignment operator of 'G' is implicitly deleted because base class 'D' has an inaccessible move assignment operator}}
-struct H : D {
-  H &operator=(H&&) = default;
+struct H : D { // expected-note {{deleted because base class 'D' has an inaccessible move assignment}}
+  H &operator=(H&&) = default; // expected-warning {{implicitly deleted}}
   // expected-error@-1 {{deleted function 'operator=' cannot override a non-deleted function}}
   // expected-note@-3 {{move assignment operator of 'H' is implicitly deleted because base class 'D' has an inaccessible move assignment operator}}
   ~H();

clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p1.cpp

@@ -1,15 +1,30 @@
 // RUN: %clang_cc1 -verify %s -std=c++11
+// RUN: %clang_cc1 -verify %s -std=c++17
+// RUN: %clang_cc1 -verify %s -std=c++2a
 
 // A function that is explicitly defaulted shall
 struct A {
   // -- be a special member function,
   A(int) = default; // expected-error {{only special member functions may be defaulted}}
+  A(A) = default; // expected-error {{must pass its first argument by reference}}
 
   // -- have the same declared function type as if it had been implicitly
   //    declared
   void operator=(const A &) = default; // expected-error {{must return 'A &'}}
-  A(...) = default; // expected-error {{cannot be variadic}}
-  A(const A &, ...) = default; // expected-error {{cannot be variadic}}
+  A(...) = default;
+  A(const A &, ...) = default;
+  A &operator=(const A&) const = default;
+  A &operator=(A) const = default; // expected-error {{must be an lvalue refe}}
+#if __cplusplus <= 201703L
+  // expected-error@-5 {{cannot be variadic}}
+  // expected-error@-5 {{cannot be variadic}}
+  // expected-error@-5 {{may not have 'const'}}
+  // expected-error@-5 {{may not have 'const'}}
+#else
+  // expected-warning@-10 {{implicitly deleted}} expected-note@-10 {{declared type does not match the type of an implicit default constructor}}
+  // expected-warning@-10 {{implicitly deleted}} expected-note@-10 {{declared type does not match the type of an implicit copy constructor}}
+  // expected-warning@-10 {{implicitly deleted}} expected-note@-10 {{declared type does not match the type of an implicit copy assignment}}
+#endif
 
   //    (except for possibly differing ref-qualifiers
   A &operator=(A &&) & = default;
@@ -23,3 +38,35 @@ struct A {
   A(double = 0.0) = default; // expected-error {{cannot have default arguments}}
   A(const A & = 0) = default; // expected-error {{cannot have default arguments}}
 };
+
+struct A2 {
+  A2(...);
+  A2(const A2 &, ...);
+  A2 &operator=(const A2&) const;
+};
+A2::A2(...) = default; // expected-error {{cannot be variadic}}
+A2::A2(const A2&, ...) = default; // expected-error {{cannot be variadic}}
+A2 &A2::operator=(const A2&) const = default; // expected-error {{may not have 'const'}}
+
+struct B {
+  B(B&);
+  B &operator=(B&);
+};
+struct C : B {
+  C(const C&) = default;
+  C &operator=(const C&) = default;
+#if __cplusplus <= 201703L
+  // expected-error@-3 {{is const, but a member or base requires it to be non-const}}
+  // expected-error@-3 {{is const, but a member or base requires it to be non-const}}
+#else
+  // expected-warning@-6 {{implicitly deleted}} expected-note@-6 {{type does not match}}
+  // expected-warning@-6 {{implicitly deleted}} expected-note@-6 {{type does not match}}
+#endif
+};
+
+struct D : B { // expected-note 2{{base class}}
+  D(const D&);
+  D &operator=(const D&);
+};
+D::D(const D&) = default; // expected-error {{would delete}} expected-error {{is const, but}}
+D &D::operator=(const D&) = default; // expected-error {{would delete}} expected-error {{is const, but}}

clang/test/CXX/drs/dr6xx.cpp

@@ -757,8 +757,8 @@ namespace dr666 { // dr666: yes
 #if __cplusplus >= 201103L
 namespace dr667 { // dr667: yes
   struct A {
-    A() = default;
-    int &r;
+    A() = default; // expected-warning {{explicitly defaulted default constructor is implicitly deleted}}
+    int &r; // expected-note {{because field 'r' of reference type 'int &' would not be initialized}}
   };
   static_assert(!__is_trivially_constructible(A), "");
 

clang/test/CXX/special/class.copy/p12-0x.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++11 -verify %s
+// RUN: %clang_cc1 -std=c++11 -verify %s -Wno-defaulted-function-deleted
 
 // expected-no-diagnostics
 

clang/test/CXX/special/class.copy/p23-cxx11.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -verify %s -std=c++11
+// RUN: %clang_cc1 -verify %s -std=c++11 -Wno-defaulted-function-deleted
 
 struct Trivial {};
 

clang/test/CXX/special/class.ctor/p5-0x.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -Wno-defaulted-function-deleted
 
 struct DefaultedDefCtor1 {};
 struct DefaultedDefCtor2 { DefaultedDefCtor2() = default; };

clang/test/CXX/special/class.dtor/p5-0x.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -verify -std=c++11 %s
+// RUN: %clang_cc1 -verify -std=c++11 %s -Wno-defaulted-function-deleted
 
 struct NonTrivDtor {
   ~NonTrivDtor();

clang/test/SemaCUDA/implicit-member-target.cu

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=gnu++11 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=gnu++11 -fsyntax-only -verify %s -Wno-defaulted-function-deleted
 
 #include "Inputs/cuda.h"
 

clang/test/SemaCXX/cxx0x-deleted-default-ctor.cpp

@@ -59,7 +59,7 @@ struct good_const {
 good_const gc;
 
 struct no_default {
-  no_default() = delete; // expected-note 4{{deleted here}}
+  no_default() = delete; // expected-note 5{{deleted here}}
 };
 struct no_dtor {
   ~no_dtor() = delete; // expected-note 2{{deleted here}}
@@ -108,8 +108,8 @@ struct has_friend {
 has_friend hf;
 
 struct defaulted_delete {
-  no_default nd; // expected-note {{because field 'nd' has a deleted default constructor}}
-  defaulted_delete() = default; // expected-note{{implicitly deleted here}}
+  no_default nd; // expected-note 2{{because field 'nd' has a deleted default constructor}}
+  defaulted_delete() = default; // expected-note{{implicitly deleted here}} expected-warning {{implicitly deleted}}
 };
 defaulted_delete dd; // expected-error {{call to implicitly-deleted default constructor}}
 

clang/test/SemaCXX/cxx17-compat.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -std=c++17 -pedantic -verify %s
-// RUN: %clang_cc1 -fsyntax-only -std=c++2a -Wc++17-compat-pedantic -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++2a -Wc++17-compat-pedantic -verify %s -Wno-defaulted-function-deleted
 
 struct A {};
 int (A::*pa)() const&;
@@ -42,3 +42,15 @@ void copy_lambda() { Lambda = Lambda; }
 #else
     // expected-warning@-4 {{assignment of lambda is incompatible with C++ standards before C++2a}}
 #endif
+
+struct DefaultDeleteWrongTypeBase {
+  DefaultDeleteWrongTypeBase(DefaultDeleteWrongTypeBase&);
+};
+struct DefaultDeleteWrongType : DefaultDeleteWrongTypeBase {
+  DefaultDeleteWrongType(const DefaultDeleteWrongType&) = default;
+#if __cplusplus <= 201703L
+    // expected-error@-2 {{a member or base requires it to be non-const}}
+#else
+    // expected-warning@-4 {{explicitly defaulting this copy constructor with a type different from the implicit type is incompatible with C++ standards before C++2a}}
+#endif
+};

clang/test/SemaCXX/dr1301.cpp

@@ -6,7 +6,7 @@ struct A { // expected-note 2{{candidate}}
 int a = A().n; // expected-error {{no matching constructor}}
 
 struct B {
-  B() = delete; // expected-note 3{{here}}
+  B() = delete; // expected-note 4{{here}}
   int n;
 };
 int b = B().n; // expected-error {{call to deleted}}
@@ -17,8 +17,8 @@ struct C {
 int c = C().b.n; // expected-error {{call to implicitly-deleted default}}
 
 struct D {
-  D() = default; // expected-note {{here}}
-  B b; // expected-note {{'b' has a deleted default constructor}}
+  D() = default; // expected-note {{here}} expected-warning {{implicitly deleted}}
+  B b; // expected-note 2{{'b' has a deleted default constructor}}
 };
 int d = D().b.n; // expected-error {{call to implicitly-deleted default}}
 

clang/test/SemaCXX/microsoft-dtor-lookup-cxx11.cpp

@@ -6,8 +6,8 @@ struct S {
   void operator delete(void*, double);
 } s; // expected-error {{attempt to use a deleted function}}
 
-struct T { // expected-note{{virtual destructor requires an unambiguous, accessible 'operator delete'}}
-  virtual ~T() = default; // expected-note {{explicitly defaulted function was implicitly deleted here}}
+struct T { // expected-note 2{{virtual destructor requires an unambiguous, accessible 'operator delete'}}
+  virtual ~T() = default; // expected-note {{explicitly defaulted function was implicitly deleted here}} expected-warning {{implicitly deleted}}
   void operator delete(void*, int);
   void operator delete(void*, double);
 } t; // expected-error {{attempt to use a deleted function}}

clang/test/SemaTemplate/exception-spec-crash.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -std=c++11 -fcxx-exceptions -DCXX_EXCEPTIONS -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s -Wno-defaulted-function-deleted
+// RUN: %clang_cc1 -std=c++11 -fcxx-exceptions -DCXX_EXCEPTIONS -fsyntax-only -verify %s -Wno-defaulted-function-deleted
 
 template <class _Tp> struct is_nothrow_move_constructible {
   static const bool value = false;

clang/www/cxx_status.html

@@ -884,7 +884,7 @@ as the draft C++2a standard evolves.
     <tr>
       <td><tt>const</tt> mismatch with defaulted copy constructor</td>
       <td><a href="http://wg21.link/p0641r2">P0641R2</a></td>
-      <td class="none" align="center">No</td>
+      <td class="svn" align="center">SVN</td>
     </tr>
     <tr>
       <td rowspan="3">Consistent comparison (<tt>operator&lt;=&gt;</tt>)</td>