Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2988,6 +2988,10 @@
   "'gnu_inline' attribute requires function to be marked 'inline',"
   " attribute ignored">,
   InGroup<IgnoredAttributes>;
+def warn_gnu_inline_cplusplus_without_extern : Warning<
+  "'gnu_inline' attribute without 'extern' in C++ treated as externally"
+  " available, this changed in Clang 10">,
+  InGroup<DiagGroup<"gnu-inline-cpp-without-extern">>;
 def err_attribute_vecreturn_only_vector_member : Error<
   "the vecreturn attribute can only be used on a class or structure with one member, which must be a vector">;
 def err_attribute_vecreturn_only_pod_record : Error<
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -3251,6 +3251,9 @@
       return true;
   }
 
+  if (Context.getLangOpts().CPlusPlus)
+    return false;
+
   if (Context.getLangOpts().GNUInline || hasAttr<GNUInlineAttr>()) {
     // With GNU inlining, a declaration with 'inline' but not 'extern', forces
     // an externally visible definition.
@@ -3279,9 +3282,6 @@
     return FoundBody;
   }
 
-  if (Context.getLangOpts().CPlusPlus)
-    return false;
-
   // C99 6.7.4p6:
   //   [...] If all of the file scope declarations for a function in a
   //   translation unit include the inline function specifier without extern,
@@ -3360,6 +3360,8 @@
     // If it's not the case that both 'inline' and 'extern' are
     // specified on the definition, then this inline definition is
     // externally visible.
+    if (Context.getLangOpts().CPlusPlus)
+      return false;
     if (!(isInlineSpecified() && getStorageClass() == SC_Extern))
       return true;
 
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -4423,6 +4423,9 @@
     return;
   }
 
+  if (S.LangOpts.CPlusPlus && Fn->getStorageClass() != SC_Extern)
+    S.Diag(AL.getLoc(), diag::warn_gnu_inline_cplusplus_without_extern);
+
   D->addAttr(::new (S.Context)
              GNUInlineAttr(AL.getRange(), S.Context,
                            AL.getAttributeSpellingListIndex()));
Index: clang/test/CodeGen/inline.c
===================================================================
--- clang/test/CodeGen/inline.c
+++ clang/test/CodeGen/inline.c
@@ -52,7 +52,7 @@
 // CHECK3-LABEL: define i32 @_Z3barv()
 // CHECK3-LABEL: define linkonce_odr i32 @_Z3foov()
 // CHECK3-NOT: unreferenced
-// CHECK3-LABEL: define void @_Z10gnu_inlinev()
+// CHECK3-LABEL: define available_externally void @_Z10gnu_inlinev()
 // CHECK3-LABEL: define available_externally void @_Z13gnu_ei_inlinev()
 // CHECK3-NOT: @_Z5testCv
 // CHECK3-LABEL: define linkonce_odr i32 @_Z2eiv()
@@ -85,6 +85,7 @@
 extern __inline void unreferenced2() {}
 
 __inline __attribute((__gnu_inline__)) void gnu_inline() {}
+void (*P1)() = gnu_inline;
 
 // PR3988
 extern __inline __attribute__((gnu_inline)) void gnu_ei_inline() {}
Index: clang/test/SemaCUDA/gnu-inline.cu
===================================================================
--- clang/test/SemaCUDA/gnu-inline.cu
+++ clang/test/SemaCUDA/gnu-inline.cu
@@ -2,9 +2,7 @@
 
 #include "Inputs/cuda.h"
 
-// expected-no-diagnostics
-
 // Check that we can handle gnu_inline functions when compiling in CUDA mode.
 
 void foo();
-inline __attribute__((gnu_inline)) void bar() { foo(); }
+inline __attribute__((gnu_inline)) void bar() { foo(); } // expected-warning {{'gnu_inline' attribute without 'extern' in C++ treated as externally available, this changed in Clang 10}}
Index: clang/test/SemaCXX/gnu_inline.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/gnu_inline.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+extern inline
+__attribute__((__gnu_inline__))
+void gnu_inline1() {}
+
+inline
+__attribute__((__gnu_inline__)) // expected-warning {{'gnu_inline' attribute without 'extern' in C++ treated as externally available, this changed in Clang 10}}
+void gnu_inline2() {}
Index: clang/test/SemaCXX/undefined-inline.cpp
===================================================================
--- clang/test/SemaCXX/undefined-inline.cpp
+++ clang/test/SemaCXX/undefined-inline.cpp
@@ -40,20 +40,20 @@
 }
 
 namespace test8 {
-  inline void foo() __attribute__((gnu_inline));
+  inline void foo() __attribute__((gnu_inline)); // expected-warning {{'gnu_inline' attribute without 'extern' in C++ treated as externally available, this changed in Clang 10}}
   void test() { foo(); }
 }
 
 namespace test9 {
   void foo();
   void test() { foo(); }
-  inline void foo() __attribute__((gnu_inline));
+  inline void foo() __attribute__((gnu_inline)); // expected-warning {{'gnu_inline' attribute without 'extern' in C++ treated as externally available, this changed in Clang 10}}
 }
 
 namespace test10 {
   inline void foo();
   void test() { foo(); }
-  inline void foo() __attribute__((gnu_inline));
+  inline void foo() __attribute__((gnu_inline)); // expected-warning {{'gnu_inline' attribute without 'extern' in C++ treated as externally available, this changed in Clang 10}}
 }
 
 namespace test11 {