Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -4652,7 +4652,8 @@
   "no previous prototype for function %0">,
   InGroup<DiagGroup<"missing-prototypes">>, DefaultIgnore;
 def note_declaration_not_a_prototype : Note<
-  "this declaration is not a prototype; add 'void' to make it a prototype for a zero-parameter function">;
+  "this declaration is not a prototype; add %select{'void'|parameter declarations}0 "
+  "to make it %select{a prototype for a zero-parameter function|one}0">;
 def warn_strict_prototypes : Warning<
   "this %select{function declaration is not|block declaration is not|"
   "old-style function definition is not preceded by}0 a prototype">,
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -12747,8 +12747,9 @@
   Consumer.HandleInlineFunctionDefinition(D);
 }
 
-static bool ShouldWarnAboutMissingPrototype(const FunctionDecl *FD,
-                             const FunctionDecl*& PossibleZeroParamPrototype) {
+static bool
+ShouldWarnAboutMissingPrototype(const FunctionDecl *FD,
+                                const FunctionDecl *&PossiblePrototype) {
   // Don't warn about invalid declarations.
   if (FD->isInvalidDecl())
     return false;
@@ -12785,7 +12786,6 @@
   if (FD->isDeleted())
     return false;
 
-  bool MissingPrototype = true;
   for (const FunctionDecl *Prev = FD->getPreviousDecl();
        Prev; Prev = Prev->getPreviousDecl()) {
     // Ignore any declarations that occur in function or method
@@ -12793,13 +12793,11 @@
     if (Prev->getLexicalDeclContext()->isFunctionOrMethod())
       continue;
 
-    MissingPrototype = !Prev->getType()->isFunctionProtoType();
-    if (FD->getNumParams() == 0)
-      PossibleZeroParamPrototype = Prev;
-    break;
+    PossiblePrototype = Prev;
+    return Prev->getType()->isFunctionNoProtoType();
   }
 
-  return MissingPrototype;
+  return true;
 }
 
 void
@@ -13319,21 +13317,22 @@
     //   prototype declaration. This warning is issued even if the
     //   definition itself provides a prototype. The aim is to detect
     //   global functions that fail to be declared in header files.
-    const FunctionDecl *PossibleZeroParamPrototype = nullptr;
-    if (ShouldWarnAboutMissingPrototype(FD, PossibleZeroParamPrototype)) {
+    const FunctionDecl *PossiblePrototype = nullptr;
+    if (ShouldWarnAboutMissingPrototype(FD, PossiblePrototype)) {
       Diag(FD->getLocation(), diag::warn_missing_prototype) << FD;
 
-      if (PossibleZeroParamPrototype) {
+      if (PossiblePrototype) {
         // We found a declaration that is not a prototype,
         // but that could be a zero-parameter prototype
-        if (TypeSourceInfo *TI =
-                PossibleZeroParamPrototype->getTypeSourceInfo()) {
+        if (TypeSourceInfo *TI = PossiblePrototype->getTypeSourceInfo()) {
           TypeLoc TL = TI->getTypeLoc();
           if (FunctionNoProtoTypeLoc FTL = TL.getAs<FunctionNoProtoTypeLoc>())
-            Diag(PossibleZeroParamPrototype->getLocation(),
+            Diag(PossiblePrototype->getLocation(),
                  diag::note_declaration_not_a_prototype)
-                << PossibleZeroParamPrototype
-                << FixItHint::CreateInsertion(FTL.getRParenLoc(), "void");
+                << (FD->getNumParams() != 0)
+                << (FD->getNumParams() == 0
+                        ? FixItHint::CreateInsertion(FTL.getRParenLoc(), "void")
+                        : FixItHint{});
         }
       }
 
Index: test/Sema/warn-missing-prototypes.c
===================================================================
--- test/Sema/warn-missing-prototypes.c
+++ test/Sema/warn-missing-prototypes.c
@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -Wdocumentation -Wmissing-prototypes -verify %s
 // RUN: %clang_cc1 -fsyntax-only -Wdocumentation -Wmissing-prototypes -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
 
-int f();
+int f(); // expected-note{{this declaration is not a prototype; add parameter declarations to make it one}}
 
 int f(int x) { return x; } // expected-warning{{no previous prototype for function 'f'}}
 
@@ -15,7 +15,7 @@
 
 void test(void);
 
-int h3();
+int h3(); // expected-note{{this declaration is not a prototype; add parameter declarations to make it one}}
 int h4(int);
 int h4();