diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 9865e1d0a3eb..699c3db7b835 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -98,7 +98,7 @@ def note_previous_builtin_declaration : Note<"%0 is a builtin with type %1">;
 def err_implicit_decl_requires_stdio : Error<
   "implicit declaration of '%0' requires inclusion of the header <stdio.h>">;
 def warn_redecl_library_builtin : Warning<
-  "incompatible redeclaration of library function %0 will be ignored">;
+  "incompatible redeclaration of library function %0">;
 def err_builtin_definition : Error<"definition of builtin function %0">;
 
 /// parser diagnostics
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 89375d7066e7..bcb21c33ce9b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -747,11 +747,14 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, Decl *OldD) {
     if (Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID)) {
       // The function the user is redeclaring is a library-defined
       // function like 'malloc' or 'printf'. Warn about the
-      // redeclaration, then ignore it.
+      // redeclaration, then pretend that we don't know about this
+      // library built-in.
       Diag(New->getLocation(), diag::warn_redecl_library_builtin) << New;
       Diag(Old->getLocation(), diag::note_previous_builtin_declaration)
         << Old << Old->getType();
-      return true;
+      New->getIdentifier()->setBuiltinID(Builtin::NotBuiltin);
+      Old->setInvalidDecl();
+      return false;
     }
 
     PrevDiag = diag::note_previous_builtin_declaration;
diff --git a/clang/test/Sema/implicit-builtin-decl.c b/clang/test/Sema/implicit-builtin-decl.c
index e1f24e35f3b2..6477a5723ffa 100644
--- a/clang/test/Sema/implicit-builtin-decl.c
+++ b/clang/test/Sema/implicit-builtin-decl.c
@@ -7,7 +7,7 @@ void f() {
 
 void *alloca(__SIZE_TYPE__); // redeclaration okay
 
-int *calloc(__SIZE_TYPE__, __SIZE_TYPE__); // expected-warning{{incompatible redeclaration of library function 'calloc' will be ignored}} \
+int *calloc(__SIZE_TYPE__, __SIZE_TYPE__); // expected-warning{{incompatible redeclaration of library function 'calloc'}} \
                     // expected-note{{'calloc' is a builtin with type 'void *}}
 
 
@@ -16,8 +16,8 @@ void g(int malloc) { // okay: these aren't functions
 }
 
 void h() {
-  int malloc(int); // expected-warning{{incompatible redeclaration of library function 'malloc' will be ignored}}
-  int strcpy(int); // expected-warning{{incompatible redeclaration of library function 'strcpy' will be ignored}} \
+  int malloc(int); // expected-warning{{incompatible redeclaration of library function 'malloc'}}
+  int strcpy(int); // expected-warning{{incompatible redeclaration of library function 'strcpy'}} \
   // expected-note{{'strcpy' is a builtin with type 'char *(char *, char const *)'}}
 }
 
@@ -35,7 +35,19 @@ int f0() {
   return __builtin_object_size(&a); // expected-error {{too few arguments to function}}
 }
 
-void * realloc(void *p, int size) { // expected-warning{{incompatible redeclaration of library function 'realloc' will be ignored}} \
+void * realloc(void *p, int size) { // expected-warning{{incompatible redeclaration of library function 'realloc'}} \
 // expected-note{{'realloc' is a builtin with type 'void *(void *,}}
   return p;
 }
+
+// PR3855
+void snprintf(); // expected-warning{{incompatible redeclaration of library function 'snprintf'}} \
+    // expected-note{{'snprintf' is a builtin with type 'int (char *, unsigned long, char const *, ...)'}}
+
+int
+main(int argc, char *argv[])
+{
+  snprintf();
+}
+
+void snprintf() { }
diff --git a/clang/test/Sema/implicit-builtin-redecl.c b/clang/test/Sema/implicit-builtin-redecl.c
index c9b38558b3e2..2ad35185b252 100644
--- a/clang/test/Sema/implicit-builtin-redecl.c
+++ b/clang/test/Sema/implicit-builtin-redecl.c
@@ -10,5 +10,5 @@ void *calloc(int, int, int); // expected-warning{{incompatible redeclaration of
 // expected-note{{'calloc' is a builtin with type 'void *}}
 
 void f1(void) { 
-  return calloc(0, 0, 0);  // expected-error{{too many arguments to function call}}
+  calloc(0, 0, 0);
 }