diff --git a/include/ap_common.h b/include/ap_common.h
index 8a53924..4d2886c 100644
--- a/include/ap_common.h
+++ b/include/ap_common.h
@@ -22,10 +22,12 @@
 // Forward declaration of all AP types.
 #include <ap_decl.h>
 
-// Automatically enable half if FPO exists
-#if ! defined _AP_ENABLE_HALF_ && defined __SIM_FPO__
-#define _AP_ENABLE_HALF_ 1
-#endif
+
+#ifdef __SYNTHESIS__
+#error "The open-source version of AP types does not support synthesis."
+#endif // ifdef __SYNTHESIS__
+#define _AP_ENABLE_HALF_ 0
+
 
 #if _AP_ENABLE_HALF_ == 1
 // Before ap_private definition.
@@ -191,9 +193,7 @@ struct remove_const<_Tp const> {
 // ----------------------------------------------------------------------
 
 // Define ssdm_int and _ssdm_op.
-#ifdef __SYNTHESIS__
-#error "This header does not sypport synthesis"
-#endif // ifdef __SYNTHESIS__
+// XXX deleted in open-source version
 
 #ifndef NON_C99STRING
 #define _AP_C99 true
diff --git a/include/ap_fixed_base.h b/include/ap_fixed_base.h
index 5427f77..1d94b93 100644
--- a/include/ap_fixed_base.h
+++ b/include/ap_fixed_base.h
@@ -18,9 +18,7 @@
 #define __AP_FIXED_BASE_H__
 
 #ifndef __AP_FIXED_H__
-// TODO make this an error
-#pragma message \
-    "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
 #endif
 
 // for ap_int_base and its reference types.
diff --git a/include/ap_fixed_ref.h b/include/ap_fixed_ref.h
index 0fe5220..aefda0a 100644
--- a/include/ap_fixed_ref.h
+++ b/include/ap_fixed_ref.h
@@ -18,9 +18,7 @@
 #define __AP_FIXED_REF_H__
 
 #ifndef __AP_FIXED_H__
-// TODO make this an error
-#pragma message \
-    "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
 #endif
 
 #ifndef __cplusplus
diff --git a/include/ap_fixed_special.h b/include/ap_fixed_special.h
index 4484d44..0f7a9f7 100644
--- a/include/ap_fixed_special.h
+++ b/include/ap_fixed_special.h
@@ -16,6 +16,11 @@
 
 #ifndef __AP_FIXED_SPECIAL_H__
 #define __AP_FIXED_SPECIAL_H__
+
+#ifndef __AP_FIXED_H__
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#endif
+
 #ifndef __SYNTHESIS__
 #include <cstdio>
 #include <cstdlib>
diff --git a/include/ap_int_base.h b/include/ap_int_base.h
index f801585..091552a 100644
--- a/include/ap_int_base.h
+++ b/include/ap_int_base.h
@@ -18,9 +18,7 @@
 #define __AP_INT_BASE_H__
 
 #ifndef __AP_INT_H__
-// TODO make this an error
-#pragma message \
-    "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
 #endif
 
 #ifndef __cplusplus
diff --git a/include/ap_int_ref.h b/include/ap_int_ref.h
index a9b24ee..421f09f 100644
--- a/include/ap_int_ref.h
+++ b/include/ap_int_ref.h
@@ -18,9 +18,7 @@
 #define __AP_INT_REF_H__
 
 #ifndef __AP_INT_H__
-// TODO make this an error
-#pragma message \
-    "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
 #endif
 
 #ifndef __cplusplus
diff --git a/include/ap_int_special.h b/include/ap_int_special.h
index e45f3d0..3afc619 100644
--- a/include/ap_int_special.h
+++ b/include/ap_int_special.h
@@ -16,6 +16,11 @@
 
 #ifndef __AP_INT_SPECIAL_H__
 #define __AP_INT_SPECIAL_H__
+
+#ifndef __AP_INT_H__
+#error "Only ap_fixed.h and ap_int.h can be included directly in user code."
+#endif
+
 #ifndef __SYNTHESIS__
 #include <cstdio>
 #include <cstdlib>