Index: lib/Sema/SemaLookup.cpp
===================================================================
--- lib/Sema/SemaLookup.cpp
+++ lib/Sema/SemaLookup.cpp
@@ -690,6 +690,16 @@
             S.Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID))
           return false;
 
+        // OpenCL v1.2 s6.9.f:
+        // The library functions defined in the C99 standard headers assert.h,
+        // ctype.h, complex.h, errno.h, fenv.h, float.h, inttypes.h, limits.h,
+        // locale.h, setjmp.h, signal.h, stdarg.h, stdio.h, stdlib.h, string.h,
+        // tgmath.h, time.h, wchar.h and wctype.h are not available and cannot
+        // be included by a program.
+        if (S.getLangOpts().OpenCL &&
+            S.Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID))
+          return false;
+
         if (NamedDecl *D = S.LazilyCreateBuiltin((IdentifierInfo *)II,
                                                  BuiltinID, S.TUScope,
                                                  R.isForRedeclaration(),
Index: test/SemaOpenCL/builtin.cl
===================================================================
--- /dev/null
+++ test/SemaOpenCL/builtin.cl
@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 %s -verify -pedantic -fsyntax-only
+
+float __attribute__((overloadable)) acos(float); // expected-no-diagnostics