7 files changed, 43 insertions, 3 deletions
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index b1816c23860..019518204ac 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4681,6 +4681,9 @@ def warn_strict_prototypes : Warning<
 def warn_missing_variable_declarations : Warning<
   "no previous extern declaration for non-static variable %0">,
   InGroup<DiagGroup<"missing-variable-declarations">>, DefaultIgnore;
+def note_static_for_internal_linkage : Note<
+  "declare 'static' if the %select{variable|function}0 is not intended to be "
+  "used outside of this translation unit">;
 def err_static_data_member_reinitialization :
   Error<"static data member %0 already has an initializer">;
 def err_redefinition : Error<"redefinition of %0">;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index e92c3d0a19a..03b6cc91657 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11899,8 +11899,11 @@ void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
     while (prev && prev->isThisDeclarationADefinition())
       prev = prev->getPreviousDecl();
 
-    if (!prev)
+    if (!prev) {
       Diag(var->getLocation(), diag::warn_missing_variable_declarations) << var;
+      Diag(var->getTypeSpecStartLoc(), diag::note_static_for_internal_linkage)
+          << /* variable */ 0;
+    }
   }
 
   // Cache the result of checking for constant initialization.
@@ -13364,6 +13367,13 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
                         ? FixItHint::CreateInsertion(FTL.getRParenLoc(), "void")
                         : FixItHint{});
         }
+      } else {
+        Diag(FD->getTypeSpecStartLoc(), diag::note_static_for_internal_linkage)
+            << /* function */ 1
+            << (FD->getStorageClass() == SC_None
+                    ? FixItHint::CreateInsertion(FD->getTypeSpecStartLoc(),
+                                                 "static ")
+                    : FixItHint{});
       }
 
       // GNU warning -Wstrict-prototypes
diff --git a/clang/test/Sema/warn-missing-prototypes.c b/clang/test/Sema/warn-missing-prototypes.c
index 892e0664318..5940a49572b 100644
--- a/clang/test/Sema/warn-missing-prototypes.c
+++ b/clang/test/Sema/warn-missing-prototypes.c
@@ -9,11 +9,17 @@ int f(int x) { return x; } // expected-warning{{no previous prototype for functi
 static int g(int x) { return x; }
 
 int h(int x) { return x; } // expected-warning{{no previous prototype for function 'h'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:1-[[@LINE-2]]:1}:"static "
 
 static int g2();
 
 int g2(int x) { return x; }
 
+extern int g3(int x) { return x; } // expected-warning{{no previous prototype for function 'g3'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
+// CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:{{.*}}-[[@LINE-2]]:{{.*}}}:"{{.*}}"
+
 void test(void);
 
 int h3(); // expected-note{{this declaration is not a prototype; add parameter declarations to make it one}}
@@ -28,6 +34,7 @@ void test(void) {
 }
 
 int h2(int x) { return x; } // expected-warning{{no previous prototype for function 'h2'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
 int h3(int x) { return x; } // expected-warning{{no previous prototype for function 'h3'}}
 int h4(int x) { return x; }
 
diff --git a/clang/test/Sema/warn-missing-variable-declarations.c b/clang/test/Sema/warn-missing-variable-declarations.c
index e5ce97da216..f9644bec603 100644
--- a/clang/test/Sema/warn-missing-variable-declarations.c
+++ b/clang/test/Sema/warn-missing-variable-declarations.c
@@ -1,16 +1,19 @@
 // RUN: %clang_cc1 -Wmissing-variable-declarations -fsyntax-only -verify %s
 
 int vbad1; // expected-warning{{no previous extern declaration for non-static variable 'vbad1'}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 
 int vbad2;
 int vbad2 = 10; // expected-warning{{no previous extern declaration for non-static variable 'vbad2'}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 
-struct {
+struct { // expected-note{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
   int mgood1;
 } vbad3; // expected-warning{{no previous extern declaration for non-static variable 'vbad3'}}
 
 int vbad4;
 int vbad4 = 10; // expected-warning{{no previous extern declaration for non-static variable 'vbad4'}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 extern int vbad4;
 
 extern int vgood1;
diff --git a/clang/test/SemaCXX/warn-missing-prototypes.cpp b/clang/test/SemaCXX/warn-missing-prototypes.cpp
index cb419336020..bb71aa8b142 100644
--- a/clang/test/SemaCXX/warn-missing-prototypes.cpp
+++ b/clang/test/SemaCXX/warn-missing-prototypes.cpp
@@ -1,9 +1,13 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wmissing-prototypes -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -Wmissing-prototypes -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
 
 void f() { } // expected-warning {{no previous prototype for function 'f'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-2]]:1-[[@LINE-2]]:1}:"static "
 
 namespace NS {
   void f() { } // expected-warning {{no previous prototype for function 'f'}}
+  // expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
 }
 
 namespace {
@@ -32,3 +36,7 @@ class I {
 
 // Don't warn on explicitly deleted functions.
 void j() = delete;
+
+extern void k() {} // expected-warning {{no previous prototype for function 'k'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
+// CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-2]]:{{.*}}-[[@LINE-2]]:{{.*}}}:"{{.*}}"
diff --git a/clang/test/SemaCXX/warn-missing-variable-declarations.cpp b/clang/test/SemaCXX/warn-missing-variable-declarations.cpp
index 5b71f38c811..e2480fd663b 100644
--- a/clang/test/SemaCXX/warn-missing-variable-declarations.cpp
+++ b/clang/test/SemaCXX/warn-missing-variable-declarations.cpp
@@ -1,11 +1,15 @@
-// RUN: %clang -Wmissing-variable-declarations -fsyntax-only -Xclang -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wmissing-variable-declarations -std=c++17 %s
 
 // Variable declarations that should trigger a warning.
 int vbad1; // expected-warning{{no previous extern declaration for non-static variable 'vbad1'}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
+
 int vbad2 = 10; // expected-warning{{no previous extern declaration for non-static variable 'vbad2'}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 
 namespace x {
   int vbad3; // expected-warning{{no previous extern declaration for non-static variable 'vbad3'}}
+  // expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 }
 
 // Variable declarations that should not trigger a warning.
@@ -58,7 +62,9 @@ const int const_var = 0;
 constexpr int constexpr_var = 0;
 inline constexpr int inline_constexpr_var = 0;
 extern const int extern_const_var = 0; // expected-warning {{no previous extern declaration}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 extern constexpr int extern_constexpr_var = 0; // expected-warning {{no previous extern declaration}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 
 template<typename> int var_template = 0;
 template<typename> constexpr int const_var_template = 0;
@@ -69,7 +75,9 @@ int use_var_template() { return var_template<int[2]>; }
 template int var_template<int[3]>;
 extern template int var_template<int[4]>;
 template<> int var_template<int[5]>; // expected-warning {{no previous extern declaration}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
 
 // FIXME: We give this specialization internal linkage rather than inheriting
 // the linkage from the template! We should not warn here.
 template<> int static_var_template<int[5]>; // expected-warning {{no previous extern declaration}}
+// expected-note@-1{{declare 'static' if the variable is not intended to be used outside of this translation unit}}
diff --git a/clang/test/SemaOpenCL/warn-missing-prototypes.cl b/clang/test/SemaOpenCL/warn-missing-prototypes.cl
index 487cb28399a..1b2eed555d4 100644
--- a/clang/test/SemaOpenCL/warn-missing-prototypes.cl
+++ b/clang/test/SemaOpenCL/warn-missing-prototypes.cl
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wmissing-prototypes %s
 
 void f() { } // expected-warning {{no previous prototype for function 'f'}}
+// expected-note@-1{{declare 'static' if the function is not intended to be used outside of this translation unit}}
 
 // Don't warn about kernel functions.
 kernel void g() { }