Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -8314,6 +8314,8 @@
   "variable length arrays are not supported in OpenCL">;
 def err_bad_kernel_param_type : Error<
   "%0 cannot be used as the type of a kernel parameter">;
+def err_opencl_implicit_function_decl : Error<
+  "implicit declaration of function %0 is invalid in OpenCL">;
 def err_record_with_pointers_kernel_param : Error<
   "%select{struct|union}0 kernel parameters may not contain pointers">;
 def note_within_field_of_type : Note<
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -12512,6 +12512,9 @@
   unsigned diag_id;
   if (II.getName().startswith("__builtin_"))
     diag_id = diag::warn_builtin_unknown;
+  // OpenCL v2.0 s6.9.u - Implicit function declaration is not supported.
+  else if (getLangOpts().OpenCL)
+    diag_id = diag::err_opencl_implicit_function_decl;
   else if (getLangOpts().C99)
     diag_id = diag::ext_implicit_function_decl;
   else
Index: test/SemaOpenCL/clang-builtin-version.cl
===================================================================
--- test/SemaOpenCL/clang-builtin-version.cl
+++ test/SemaOpenCL/clang-builtin-version.cl
@@ -4,13 +4,13 @@
 
 kernel void dse_builtins() {
   int tmp;
-  enqueue_kernel(tmp, tmp, tmp, ^(void) { // expected-warning{{implicit declaration of function 'enqueue_kernel' is invalid in C99}}
+  enqueue_kernel(tmp, tmp, tmp, ^(void) { // expected-error{{implicit declaration of function 'enqueue_kernel' is invalid in OpenCL}}
     return;
   });
-  unsigned size = get_kernel_work_group_size(^(void) { // expected-warning{{implicit declaration of function 'get_kernel_work_group_size' is invalid in C99}}
+  unsigned size = get_kernel_work_group_size(^(void) { // expected-error{{implicit declaration of function 'get_kernel_work_group_size' is invalid in OpenCL}}
     return;
   });
-  size = get_kernel_preferred_work_group_size_multiple(^(void) { // expected-warning{{implicit declaration of function 'get_kernel_preferred_work_group_size_multiple' is invalid in C99}}
+  size = get_kernel_preferred_work_group_size_multiple(^(void) { // expected-error{{implicit declaration of function 'get_kernel_preferred_work_group_size_multiple' is invalid in OpenCL}}
     return;
   });
 }
@@ -18,27 +18,48 @@
 void pipe_builtins() {
   int tmp;
 
-  read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'read_pipe' is invalid in C99}}
-  write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'write_pipe' is invalid in C99}}
+  foo(void); // expected-error{{implicit declaration of function 'foo' is invalid in OpenCL}}
+  // expected-note@-1{{'foo' declared here}}
+  // expected-error@-2{{expected expression}}
+  boo(); // expected-error{{implicit declaration of function 'boo' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'foo'?}}
 
-  reserve_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'reserve_read_pipe' is invalid in C99}}
-  reserve_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'reserve_write_pipe' is invalid in C99}}
+  read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'read_pipe' is invalid in OpenCL}}
+  write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'write_pipe' is invalid in OpenCL}}
 
-  work_group_reserve_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'work_group_reserve_read_pipe' is invalid in C99}}
-  work_group_reserve_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'work_group_reserve_write_pipe' is invalid in C99}}
+  reserve_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'reserve_read_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{'reserve_read_pipe' declared here}}
+  reserve_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'reserve_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'reserve_read_pipe'?}}
 
-  sub_group_reserve_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'sub_group_reserve_write_pipe' is invalid in C99}}
-  sub_group_reserve_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'sub_group_reserve_read_pipe' is invalid in C99}}
+  work_group_reserve_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'work_group_reserve_read_pipe' is invalid in OpenCL}}
+  // expected-note@-1 2{{'work_group_reserve_read_pipe' declared here}}
+  work_group_reserve_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'work_group_reserve_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'work_group_reserve_read_pipe'?}}
+  // expected-note@-2{{'work_group_reserve_write_pipe' declared here}}
 
-  commit_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'commit_read_pipe' is invalid in C99}}
-  commit_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'commit_write_pipe' is invalid in C99}}
+  sub_group_reserve_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'sub_group_reserve_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'work_group_reserve_write_pipe'?}}
+  sub_group_reserve_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'sub_group_reserve_read_pipe' is invalid in OpenCL}}
 
-  work_group_commit_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'work_group_commit_read_pipe' is invalid in C99}}
-  work_group_commit_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'work_group_commit_write_pipe' is invalid in C99}}
+  commit_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'commit_read_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{'commit_read_pipe' declared here}}
+  commit_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'commit_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'commit_read_pipe'?}}
 
-  sub_group_commit_write_pipe(tmp, tmp); // expected-warning{{implicit declaration of function 'sub_group_commit_write_pipe' is invalid in C99}}
-  sub_group_commit_read_pipe(tmp, tmp);  // expected-warning{{implicit declaration of function 'sub_group_commit_read_pipe' is invalid in C99}}
+  work_group_commit_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'work_group_commit_read_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{'work_group_commit_read_pipe' declared here}}
+  // expected-note@-2{{did you mean 'work_group_reserve_read_pipe'?}}
+  work_group_commit_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'work_group_commit_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{'work_group_commit_write_pipe' declared here}}
+  // expected-note@-2{{did you mean 'work_group_commit_read_pipe'?}}
 
-  get_pipe_num_packets(tmp); // expected-warning{{implicit declaration of function 'get_pipe_num_packets' is invalid in C99}}
-  get_pipe_max_packets(tmp); // expected-warning{{implicit declaration of function 'get_pipe_max_packets' is invalid in C99}}
+  sub_group_commit_write_pipe(tmp, tmp); // expected-error{{implicit declaration of function 'sub_group_commit_write_pipe' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'work_group_commit_write_pipe'?}}
+  sub_group_commit_read_pipe(tmp, tmp);  // expected-error{{implicit declaration of function 'sub_group_commit_read_pipe' is invalid in OpenCL}}
+
+  get_pipe_num_packets(tmp); // expected-error{{implicit declaration of function 'get_pipe_num_packets' is invalid in OpenCL}}
+  // expected-note@-1{{'get_pipe_num_packets' declared here}}
+  get_pipe_max_packets(tmp); // expected-error{{implicit declaration of function 'get_pipe_max_packets' is invalid in OpenCL}}
+  // expected-note@-1{{did you mean 'get_pipe_num_packets'?}}
 }
Index: test/SemaOpenCL/to_addr_builtin.cl
===================================================================
--- test/SemaOpenCL/to_addr_builtin.cl
+++ test/SemaOpenCL/to_addr_builtin.cl
@@ -10,7 +10,7 @@
 
   glob = to_global(glob, loc);
 #if __OPENCL_C_VERSION__ < CL_VERSION_2_0
-  // expected-warning@-2{{implicit declaration of function 'to_global' is invalid in C99}}
+  // expected-error@-2{{implicit declaration of function 'to_global' is invalid in OpenCL}}
   // expected-warning@-3{{incompatible integer to pointer conversion assigning to '__global int *' from 'int'}}
 #else
   // expected-error@-5{{invalid number of arguments to function: 'to_global'}}