2 files changed, 47 insertions, 41 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index a176964fc2f..62fd0207715 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -12283,9 +12283,36 @@ Sema::CheckForFunctionRedefinition(FunctionDecl *FD,
                                    const FunctionDecl *EffectiveDefinition,
                                    SkipBodyInfo *SkipBody) {
   const FunctionDecl *Definition = EffectiveDefinition;
+  if (!Definition && !FD->isDefined(Definition) && !FD->isCXXClassMember()) {
+    // If this is a friend function defined in a class template, it does not
+    // have a body until it is used, nevertheless it is a definition, see
+    // [temp.inst]p2:
+    //
+    // ... for the purpose of determining whether an instantiated redeclaration
+    // is valid according to [basic.def.odr] and [class.mem], a declaration that
+    // corresponds to a definition in the template is considered to be a
+    // definition.
+    //
+    // The following code must produce redefinition error:
+    //
+    //     template<typename T> struct C20 { friend void func_20() {} };
+    //     C20<int> c20i;
+    //     void func_20() {}
+    //
+    for (auto I : FD->redecls()) {
+      if (I != FD && !I->isInvalidDecl() &&
+          I->getFriendObjectKind() != Decl::FOK_None) {
+        if (FunctionDecl *Original = I->getInstantiatedFromMemberFunction()) {
+          if (Original->isThisDeclarationADefinition()) {
+            Definition = I;
+            break;
+          }
+        }
+      }
+    }
+  }
   if (!Definition)
-    if (!FD->isDefined(Definition))
-      return;
+    return;
 
   if (canRedefineFunction(Definition, getLangOpts()))
     return;
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d87f9dbbbea..800956dd535 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1806,45 +1806,24 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D,
     //   apply to non-template function declarations and definitions also apply
     //   to these implicit definitions.
     if (D->isThisDeclarationADefinition()) {
-      // Check for a function body.
-      const FunctionDecl *Definition = nullptr;
-      if (Function->isDefined(Definition) &&
-          Definition->getTemplateSpecializationKind() == TSK_Undeclared) {
-        SemaRef.Diag(Function->getLocation(), diag::err_redefinition)
-            << Function->getDeclName();
-        SemaRef.Diag(Definition->getLocation(), diag::note_previous_definition);
-      }
-      // Check for redefinitions due to other instantiations of this or
-      // a similar friend function.
-      else for (auto R : Function->redecls()) {
-        if (R == Function)
-          continue;
-
-        // If some prior declaration of this function has been used, we need
-        // to instantiate its definition.
-        if (!QueuedInstantiation && R->isUsed(false)) {
-          if (MemberSpecializationInfo *MSInfo =
-                  Function->getMemberSpecializationInfo()) {
-            if (MSInfo->getPointOfInstantiation().isInvalid()) {
-              SourceLocation Loc = R->getLocation(); // FIXME
-              MSInfo->setPointOfInstantiation(Loc);
-              SemaRef.PendingLocalImplicitInstantiations.push_back(
-                                               std::make_pair(Function, Loc));
-              QueuedInstantiation = true;
-            }
-          }
-        }
-
-        // If some prior declaration of this function was a friend with an
-        // uninstantiated definition, reject it.
-        if (R->getFriendObjectKind()) {
-          if (const FunctionDecl *RPattern =
-                  R->getTemplateInstantiationPattern()) {
-            if (RPattern->isDefined(RPattern)) {
-              SemaRef.Diag(Function->getLocation(), diag::err_redefinition)
-                << Function->getDeclName();
-              SemaRef.Diag(R->getLocation(), diag::note_previous_definition);
-              break;
+      SemaRef.CheckForFunctionRedefinition(Function);
+      if (!Function->isInvalidDecl()) {
+        for (auto R : Function->redecls()) {
+          if (R == Function)
+            continue;
+
+          // If some prior declaration of this function has been used, we need
+          // to instantiate its definition.
+          if (!QueuedInstantiation && R->isUsed(false)) {
+            if (MemberSpecializationInfo *MSInfo =
+                Function->getMemberSpecializationInfo()) {
+              if (MSInfo->getPointOfInstantiation().isInvalid()) {
+                SourceLocation Loc = R->getLocation(); // FIXME
+                MSInfo->setPointOfInstantiation(Loc);
+                SemaRef.PendingLocalImplicitInstantiations.push_back(
+                    std::make_pair(Function, Loc));
+                QueuedInstantiation = true;
+              }
             }
           }
         }