[Concepts] Implement subsection [dcl.spec.concept]p7 of the Concepts TS

[nwilson]

Summary

A program shall not declare an explicit instantiation (14.8.2), an explicit specialization (14.8.3), or a partial specialization of a concept definition.

[nwilson]

lib/Sema/SemaDecl.cpp

7846

Please let me know if there are thoughts about better error recovery here. I did this because we don't have the specifier information when checking an explicit specialization. So, we still diagnose for the explication specialization (one of the checks in this patch), when concept is specified, e.g.:
template<typename T> concept bool C() { return true; }
template <> concept bool C<int>() { return true; }

Would it be okay to pass Declarator to Sema::CheckFunctionTemplateSpecialization?

[aaron.ballman]

Generally looks good to me, but you should wait for @rsmith as he may have other ideas on the error recovery.

lib/Sema/SemaDecl.cpp

6298

Can you put comments near the magic numbers to describe what they mean? e.g., 1 /*variable*/

Same comment applies elsewhere in the patch as well.

[nwilson]

• Address Aaron's comments by putting comments next to the magic numbers

include/clang/Basic/DiagnosticSemaKinds.td

def err_function_concept_with_params : Error<
   "function concept cannot have any parameters">;
 def err_function_concept_bool_ret : Error<
   "declared return type of function concept must be 'bool'">;
 def err_variable_concept_bool_decl : Error<
   "declared type of variable concept must be 'bool'">;
 def err_concept_specified_specialization : Error<
   "'concept' cannot be applied on an "
   "%select{explicit instantiation|explicit specialization|partial specialization}0">;
+def err_concept_specialized : Error<
+  "%select{function|variable}0 concept cannot be "
+  "%select{explicitly instantiated|explicitly specialized|partially specialized}1">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
   "'%0' type specifier is incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 
 // __make_integer_seq
 def err_integer_sequence_negative_length : Error<

lib/Sema/SemaDecl.cpp

if (!Previous.empty()) {
         << Name << computeDeclContext(D.getCXXScopeSpec(), true)
         << D.getCXXScopeSpec().getRange();
       NewVD->setInvalidDecl();
     }
 
     if (!IsVariableTemplateSpecialization)
       D.setRedeclaration(CheckVariableDeclaration(NewVD, Previous));
 
+    // C++ Concepts TS [dcl.spec.concept]p7: A program shall not declare [...]
+    // an explicit specialization (14.8.3) or a partial specialization of a
+    // concept definition.
+    if (IsVariableTemplateSpecialization &&
+        !D.getDeclSpec().isConceptSpecified() && !Previous.empty() &&
+        Previous.isSingleResult()) {
+      NamedDecl *PreviousDecl = Previous.getFoundDecl();
+      if (VarTemplateDecl *VarTmpl = dyn_cast<VarTemplateDecl>(PreviousDecl)) {
+        if (VarTmpl->isConcept()) {
+          Diag(NewVD->getLocation(), diag::err_concept_specialized)
+              << 1                            /*variable*/
+              << (IsPartialSpecialization ? 2 /*partially specialized*/
+                                          : 1 /*explicitly specialized*/);
+          Diag(VarTmpl->getLocation(), diag::note_previous_declaration);
+          NewVD->setInvalidDecl();
+        }
+      }
+    }
+
     if (NewTemplate) {
       VarTemplateDecl *PrevVarTemplate =
           NewVD->getPreviousDecl()
               ? NewVD->getPreviousDecl()->getDescribedVarTemplate()
               : nullptr;
 
       // Check the template parameter list of this declaration, possibly
       // merging in the template parameter list from the previous variable
if (isConcept) {
       }
 
       // C++ Concepts TS [dcl.spec.concept]p1: The concept specifier shall be
       // applied only to the definition of a function template or variable
       // template, declared in namespace scope.
       if (isFunctionTemplateSpecialization) {
         Diag(D.getDeclSpec().getConceptSpecLoc(),
              diag::err_concept_specified_specialization) << 1;
+        NewFD->setInvalidDecl(true);
+        return NewFD;
       }
     }
 
     // If __module_private__ was specified, mark the function accordingly.
     if (D.getDeclSpec().isModulePrivateSpecified()) {
       if (isFunctionTemplateSpecialization) {
         SourceLocation ModulePrivateLoc
           = D.getDeclSpec().getModulePrivateSpecLoc();

lib/Sema/SemaTemplate.cpp

UnresolvedSetIterator Result = getMostSpecialized(
       PDiag(diag::note_function_template_spec_matched));
 
   if (Result == Candidates.end())
     return true;
 
   // Ignore access information;  it doesn't figure into redeclaration checking.
   FunctionDecl *Specialization = cast<FunctionDecl>(*Result);
 
+  // C++ Concepts TS [dcl.spec.concept]p7: A program shall not declare [...]
+  // an explicit specialization (14.8.3) [...] of a concept definition.
+  if (Specialization->getPrimaryTemplate()->isConcept()) {
+    Diag(FD->getLocation(), diag::err_concept_specialized)
+        << 0 /*function*/ << 1 /*explicitly specialized*/;
+    Diag(Specialization->getLocation(), diag::note_previous_declaration);
+    return true;
+  }
+
   FunctionTemplateSpecializationInfo *SpecInfo
     = Specialization->getTemplateSpecializationInfo();
   assert(SpecInfo && "Function template specialization info missing?");
 
   // Note: do not overwrite location info if previous template
   // specialization kind was explicit.
   TemplateSpecializationKind TSK = SpecInfo->getTemplateSpecializationKind();
   if (TSK == TSK_Undeclared || TSK == TSK_ImplicitInstantiation) {
if (!PrevTemplate) {
         Diag(D.getIdentifierLoc(),
              diag::err_explicit_instantiation_without_template_id)
           << PrevTemplate;
         Diag(PrevTemplate->getLocation(),
              diag::note_explicit_instantiation_here);
         return true;
       }
 
+      // C++ Concepts TS [dcl.spec.concept]p7: A program shall not declare an
+      // explicit instantiation (14.8.2) [...] of a concept definition.
+      if (PrevTemplate->isConcept()) {
+        Diag(D.getIdentifierLoc(), diag::err_concept_specialized)
+            << 1 /*variable*/ << 0 /*explicitly instantiated*/;
+        Diag(PrevTemplate->getLocation(), diag::note_previous_declaration);
+        return true;
+      }
+
       // Translate the parser's template argument list into our AST format.
       TemplateArgumentListInfo TemplateArgs =
           makeTemplateArgumentListInfo(*this, *D.getName().TemplateId);
 
       DeclResult Res = CheckVarTemplateId(PrevTemplate, TemplateLoc,
                                           D.getIdentifierLoc(), TemplateArgs);
       if (Res.isInvalid())
         return true;
DeclResult Sema::ActOnExplicitInstantiation(Scope *S,
   FunctionTemplateDecl *FunTmpl = Specialization->getPrimaryTemplate();
   if (D.getName().getKind() != UnqualifiedId::IK_TemplateId && !FunTmpl &&
       D.getCXXScopeSpec().isSet() &&
       !ScopeSpecifierHasTemplateId(D.getCXXScopeSpec()))
     Diag(D.getIdentifierLoc(),
          diag::ext_explicit_instantiation_without_qualified_id)
     << Specialization << D.getCXXScopeSpec().getRange();
 
+  // C++ Concepts TS [dcl.spec.concept]p7: A program shall not declare an
+  // explicit instantiation (14.8.2) [...] of a concept definition.
+  if (FunTmpl && FunTmpl->isConcept() &&
+      !D.getDeclSpec().isConceptSpecified()) {
+    Diag(D.getIdentifierLoc(), diag::err_concept_specialized)
+        << 0 /*function*/ << 0 /*explicitly instantiated*/;
+    Diag(FunTmpl->getLocation(), diag::note_previous_declaration);
+    return true;
+  }
+
   CheckExplicitInstantiationScope(*this,
                    FunTmpl? (NamedDecl *)FunTmpl
                           : Specialization->getInstantiatedFromMemberFunction(),
                                   D.getIdentifierLoc(),
                                   D.getCXXScopeSpec().isSet());
 
   // FIXME: Create some kind of ExplicitInstantiationDecl here.
   return (Decl*) nullptr;

test/CXX/concepts-ts/dcl.dcl/dcl.spec/dcl.spec.concept/p7.cpp

+// RUN:  %clang_cc1 -std=c++14 -fconcepts-ts -x c++ -verify %s
+
+template <typename T> concept bool FCEI() { return true; } // expected-note {{previous declaration is here}} expected-note {{previous declaration is here}}
+template bool FCEI<int>(); // expected-error {{function concept cannot be explicitly instantiated}}
+extern template bool FCEI<double>(); // expected-error {{function concept cannot be explicitly instantiated}}
+
+template <typename T> concept bool FCES() { return true; } // expected-note {{previous declaration is here}}
+template <> bool FCES<int>() { return true; } // expected-error {{function concept cannot be explicitly specialized}}
+
+template <typename T> concept bool VC { true }; // expected-note {{previous declaration is here}} expected-note {{previous declaration is here}}
+template bool VC<int>; // expected-error {{variable concept cannot be explicitly instantiated}}
+extern template bool VC<double>; // expected-error {{variable concept cannot be explicitly instantiated}}
+
+template <typename T> concept bool VCES { true }; // expected-note {{previous declaration is here}}
+template <> bool VCES<int> { true }; // expected-error {{variable concept cannot be explicitly specialized}}
+
+template <typename T> concept bool VCPS { true }; // expected-note {{previous declaration is here}}
+template <typename T> bool VCPS<T *> { true }; // expected-error {{variable concept cannot be partially specialized}}