diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13319,8 +13319,10 @@
     return false;
 
   // Don't warn about 'main'.
-  if (FD->isMain())
-    return false;
+  if (isa<TranslationUnitDecl>(FD->getDeclContext()->getRedeclContext()))
+    if (IdentifierInfo *II = FD->getIdentifier())
+      if (II->isStr("main"))
+        return false;
 
   // Don't warn about inline functions.
   if (FD->isInlined())
diff --git a/clang/test/Sema/no-warn-missing-prototype.c b/clang/test/Sema/no-warn-missing-prototype.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/no-warn-missing-prototype.c
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsyntax-only -Wmissing-prototypes-x c -ffreestanding -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wmissing-prototypes -x c++ -ffreestanding -verify %s
+// expected-no-diagnostics
+int main() {
+  return 0;
+}