diff --git a/clang/lib/Headers/opencl-c-base.h b/clang/lib/Headers/opencl-c-base.h
--- a/clang/lib/Headers/opencl-c-base.h
+++ b/clang/lib/Headers/opencl-c-base.h
@@ -568,4 +568,7 @@
 #pragma OPENCL EXTENSION cl_intel_device_side_avc_motion_estimation : end
 #endif // cl_intel_device_side_avc_motion_estimation
 
+// Disable any extensions we may have enabled previously.
+#pragma OPENCL EXTENSION all : disable
+
 #endif //_OPENCL_BASE_H_
diff --git a/clang/lib/Headers/opencl-c.h b/clang/lib/Headers/opencl-c.h
--- a/clang/lib/Headers/opencl-c.h
+++ b/clang/lib/Headers/opencl-c.h
@@ -4633,6 +4633,7 @@
 // Conversions with double data type parameters or return value.
 
 #ifdef cl_khr_fp64
+#pragma OPENCL EXTENSION cl_khr_fp64 : enable
 char __ovld __cnfn convert_char(double);
 char __ovld __cnfn convert_char_rte(double);
 char __ovld __cnfn convert_char_rtn(double);
@@ -5455,6 +5456,7 @@
 #endif //cl_khr_fp64
 
 #ifdef cl_khr_fp16
+#pragma OPENCL EXTENSION cl_khr_fp16 : enable
 // Convert half types to non-double types.
 uchar __ovld __cnfn convert_uchar(half);
 uchar __ovld __cnfn convert_uchar_rte(half);
diff --git a/clang/test/SemaOpenCL/half.cl b/clang/test/SemaOpenCL/half.cl
--- a/clang/test/SemaOpenCL/half.cl
+++ b/clang/test/SemaOpenCL/half.cl
@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 %s -verify -pedantic -fsyntax-only -Wno-unused-value -triple spir-unknown-unknown
+// RUN: %clang_cc1 %s -verify -pedantic -fsyntax-only -Wno-unused-value -triple spir-unknown-unknown -fdeclare-opencl-builtins -finclude-default-header
 
-#pragma OPENCL EXTENSION cl_khr_fp16 : disable
 constant float f = 1.0h; // expected-error{{half precision constant requires cl_khr_fp16}}
 
 half half_disabled(half *p, // expected-error{{declaring function return value of type 'half' is not allowed}}