[clang] Add the exclude_from_explicit_instantiation attribute

Summary:
This attribute allows excluding a member of a class template from being part
of an explicit template instantiation of that class template. This also makes
sure that code using such a member will not take for granted that an external
instantiation exists in another translation unit. The attribute was discussed
on cfe-dev at [1] and is primarily motivated by the removal of always_inline
in libc++ to control what's part of the ABI (see links in [1]).

[1]: http://lists.llvm.org/pipermail/cfe-dev/2018-August/059024.html

rdar://problem/43428125

Reviewers: rsmith

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51789

llvm-svn: 343790

3 files changed, 21 insertions, 6 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index ccbbe50093b..a9c5f7d58bf 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -644,7 +644,8 @@ void Sema::getUndefinedButUsed(
         continue;
       if (FD->isExternallyVisible() &&
           !isExternalWithNoLinkageType(FD) &&
-          !FD->getMostRecentDecl()->isInlined())
+          !FD->getMostRecentDecl()->isInlined() &&
+          !FD->hasAttr<ExcludeFromExplicitInstantiationAttr>())
         continue;
       if (FD->getBuiltinID())
         continue;
@@ -654,7 +655,8 @@ void Sema::getUndefinedButUsed(
         continue;
       if (VD->isExternallyVisible() &&
           !isExternalWithNoLinkageType(VD) &&
-          !VD->getMostRecentDecl()->isInline())
+          !VD->getMostRecentDecl()->isInline() &&
+          !VD->hasAttr<ExcludeFromExplicitInstantiationAttr>())
         continue;
 
       // Skip VarDecls that lack formal definitions but which we know are in
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index a437f159968..c0fb3356bae 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6512,6 +6512,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_InternalLinkage:
     handleInternalLinkageAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_ExcludeFromExplicitInstantiation:
+    handleSimpleAttribute<ExcludeFromExplicitInstantiationAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_LTOVisibilityPublic:
     handleSimpleAttribute<LTOVisibilityPublicAttr>(S, D, AL);
     break;
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 5666cf04a24..cfedf0a0994 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2574,10 +2574,14 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
   for (auto *D : Instantiation->decls()) {
     bool SuppressNew = false;
     if (auto *Function = dyn_cast<FunctionDecl>(D)) {
-      if (FunctionDecl *Pattern
-            = Function->getInstantiatedFromMemberFunction()) {
-        MemberSpecializationInfo *MSInfo
-          = Function->getMemberSpecializationInfo();
+      if (FunctionDecl *Pattern =
+              Function->getInstantiatedFromMemberFunction()) {
+
+        if (Function->hasAttr<ExcludeFromExplicitInstantiationAttr>())
+          continue;
+
+        MemberSpecializationInfo *MSInfo =
+            Function->getMemberSpecializationInfo();
         assert(MSInfo && "No member specialization information?");
         if (MSInfo->getTemplateSpecializationKind()
                                                  == TSK_ExplicitSpecialization)
@@ -2618,6 +2622,9 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
         continue;
 
       if (Var->isStaticDataMember()) {
+        if (Var->hasAttr<ExcludeFromExplicitInstantiationAttr>())
+          continue;
+
         MemberSpecializationInfo *MSInfo = Var->getMemberSpecializationInfo();
         assert(MSInfo && "No member specialization information?");
         if (MSInfo->getTemplateSpecializationKind()
@@ -2649,6 +2656,9 @@ Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
         }
       }
     } else if (auto *Record = dyn_cast<CXXRecordDecl>(D)) {
+      if (Record->hasAttr<ExcludeFromExplicitInstantiationAttr>())
+        continue;
+
       // Always skip the injected-class-name, along with any
       // redeclarations of nested classes, since both would cause us
       // to try to instantiate the members of a class twice.