diff --git a/src/bindgen/ir/annotation.rs b/src/bindgen/ir/annotation.rs
index 6625fe1..79e4233 100644
--- a/src/bindgen/ir/annotation.rs
+++ b/src/bindgen/ir/annotation.rs
@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use std::collections::hash_map::Entry;
 use std::collections::HashMap;
 use std::str::FromStr;
 
@@ -115,8 +116,11 @@ impl AnnotationSet {
         })
     }
 
-    pub fn add(&mut self, name: &str, value: AnnotationValue) {
-        self.annotations.insert(name.to_string(), value);
+    /// Adds an annotation value if none is specified.
+    pub fn add_default(&mut self, name: &str, value: AnnotationValue) {
+        if let Entry::Vacant(e) = self.annotations.entry(name.to_string()) {
+            e.insert(value);
+        }
     }
 
     pub fn list(&self, name: &str) -> Option<Vec<String>> {
diff --git a/src/bindgen/ir/enumeration.rs b/src/bindgen/ir/enumeration.rs
index 9437097..ae846f0 100644
--- a/src/bindgen/ir/enumeration.rs
+++ b/src/bindgen/ir/enumeration.rs
@@ -145,7 +145,7 @@ impl EnumVariant {
         let variant_cfg = Cfg::append(mod_cfg, Cfg::load(&variant.attrs));
         let mut annotations = AnnotationSet::load(&variant.attrs)?;
         if let Some(b) = enum_annotations.bool("derive-ostream") {
-            annotations.add("derive-ostream", AnnotationValue::Bool(b));
+            annotations.add_default("derive-ostream", AnnotationValue::Bool(b));
         }
         let body = match variant.fields {
             syn::Fields::Unit => VariantBody::Empty(annotations),
diff --git a/tests/expectations/both/derive_ostream.c b/tests/expectations/both/derive_ostream.c
index 9508230..f90d0a8 100644
--- a/tests/expectations/both/derive_ostream.c
+++ b/tests/expectations/both/derive_ostream.c
@@ -72,4 +72,22 @@ typedef struct H {
   };
 } H;
 
-void root(A a, B b, C c, D d, F f, H h);
+enum I_Tag {
+  ThereAgain,
+  SomethingElse,
+};
+typedef uint8_t I_Tag;
+
+typedef struct ThereAgain_Body {
+  uint8_t x;
+  int16_t y;
+} ThereAgain_Body;
+
+typedef struct I {
+  I_Tag tag;
+  union {
+    ThereAgain_Body there_again;
+  };
+} I;
+
+void root(A a, B b, C c, D d, F f, H h, I i);
diff --git a/tests/expectations/both/derive_ostream.compat.c b/tests/expectations/both/derive_ostream.compat.c
index 8b1f191..35b8996 100644
--- a/tests/expectations/both/derive_ostream.compat.c
+++ b/tests/expectations/both/derive_ostream.compat.c
@@ -90,11 +90,35 @@ typedef struct H {
   };
 } H;
 
+enum I_Tag
+#ifdef __cplusplus
+  : uint8_t
+#endif // __cplusplus
+ {
+  ThereAgain,
+  SomethingElse,
+};
+#ifndef __cplusplus
+typedef uint8_t I_Tag;
+#endif // __cplusplus
+
+typedef struct ThereAgain_Body {
+  uint8_t x;
+  int16_t y;
+} ThereAgain_Body;
+
+typedef struct I {
+  I_Tag tag;
+  union {
+    ThereAgain_Body there_again;
+  };
+} I;
+
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus
 
-void root(A a, B b, C c, D d, F f, H h);
+void root(A a, B b, C c, D d, F f, H h, I i);
 
 #ifdef __cplusplus
 } // extern "C"
diff --git a/tests/expectations/derive_ostream.c b/tests/expectations/derive_ostream.c
index 210590f..69b76e1 100644
--- a/tests/expectations/derive_ostream.c
+++ b/tests/expectations/derive_ostream.c
@@ -72,4 +72,22 @@ typedef struct {
   };
 } H;
 
-void root(A a, B b, C c, D d, F f, H h);
+enum I_Tag {
+  ThereAgain,
+  SomethingElse,
+};
+typedef uint8_t I_Tag;
+
+typedef struct {
+  uint8_t x;
+  int16_t y;
+} ThereAgain_Body;
+
+typedef struct {
+  I_Tag tag;
+  union {
+    ThereAgain_Body there_again;
+  };
+} I;
+
+void root(A a, B b, C c, D d, F f, H h, I i);
diff --git a/tests/expectations/derive_ostream.compat.c b/tests/expectations/derive_ostream.compat.c
index 6e20467..59bc476 100644
--- a/tests/expectations/derive_ostream.compat.c
+++ b/tests/expectations/derive_ostream.compat.c
@@ -90,11 +90,35 @@ typedef struct {
   };
 } H;
 
+enum I_Tag
+#ifdef __cplusplus
+  : uint8_t
+#endif // __cplusplus
+ {
+  ThereAgain,
+  SomethingElse,
+};
+#ifndef __cplusplus
+typedef uint8_t I_Tag;
+#endif // __cplusplus
+
+typedef struct {
+  uint8_t x;
+  int16_t y;
+} ThereAgain_Body;
+
+typedef struct {
+  I_Tag tag;
+  union {
+    ThereAgain_Body there_again;
+  };
+} I;
+
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus
 
-void root(A a, B b, C c, D d, F f, H h);
+void root(A a, B b, C c, D d, F f, H h, I i);
 
 #ifdef __cplusplus
 } // extern "C"
diff --git a/tests/expectations/derive_ostream.cpp b/tests/expectations/derive_ostream.cpp
index 6f2923e..4831cad 100644
--- a/tests/expectations/derive_ostream.cpp
+++ b/tests/expectations/derive_ostream.cpp
@@ -155,8 +155,30 @@ struct H {
   };
 };
 
+struct I {
+  enum class Tag : uint8_t {
+    ThereAgain,
+    SomethingElse,
+  };
+
+  struct ThereAgain_Body {
+    uint8_t x;
+    int16_t y;
+
+    friend std::ostream& operator<<(std::ostream& stream, const ThereAgain_Body& instance) {
+      return stream << "{ " << "x=" << instance.x << ", "
+                            << "y=" << instance.y << " }";
+    }
+  };
+
+  Tag tag;
+  union {
+    ThereAgain_Body there_again;
+  };
+};
+
 extern "C" {
 
-void root(A a, B b, C c, D d, F f, H h);
+void root(A a, B b, C c, D d, F f, H h, I i);
 
 } // extern "C"
diff --git a/tests/expectations/tag/derive_ostream.c b/tests/expectations/tag/derive_ostream.c
index c8342b6..a81f01a 100644
--- a/tests/expectations/tag/derive_ostream.c
+++ b/tests/expectations/tag/derive_ostream.c
@@ -72,4 +72,22 @@ struct H {
   };
 };
 
-void root(struct A a, struct B b, C c, struct D d, union F f, struct H h);
+enum I_Tag {
+  ThereAgain,
+  SomethingElse,
+};
+typedef uint8_t I_Tag;
+
+struct ThereAgain_Body {
+  uint8_t x;
+  int16_t y;
+};
+
+struct I {
+  I_Tag tag;
+  union {
+    struct ThereAgain_Body there_again;
+  };
+};
+
+void root(struct A a, struct B b, C c, struct D d, union F f, struct H h, struct I i);
diff --git a/tests/expectations/tag/derive_ostream.compat.c b/tests/expectations/tag/derive_ostream.compat.c
index c080f7f..7bb4b64 100644
--- a/tests/expectations/tag/derive_ostream.compat.c
+++ b/tests/expectations/tag/derive_ostream.compat.c
@@ -90,11 +90,35 @@ struct H {
   };
 };
 
+enum I_Tag
+#ifdef __cplusplus
+  : uint8_t
+#endif // __cplusplus
+ {
+  ThereAgain,
+  SomethingElse,
+};
+#ifndef __cplusplus
+typedef uint8_t I_Tag;
+#endif // __cplusplus
+
+struct ThereAgain_Body {
+  uint8_t x;
+  int16_t y;
+};
+
+struct I {
+  I_Tag tag;
+  union {
+    struct ThereAgain_Body there_again;
+  };
+};
+
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus
 
-void root(struct A a, struct B b, C c, struct D d, union F f, struct H h);
+void root(struct A a, struct B b, C c, struct D d, union F f, struct H h, struct I i);
 
 #ifdef __cplusplus
 } // extern "C"
diff --git a/tests/rust/derive_ostream.rs b/tests/rust/derive_ostream.rs
index 64e170f..e7e33fa 100644
--- a/tests/rust/derive_ostream.rs
+++ b/tests/rust/derive_ostream.rs
@@ -1,22 +1,22 @@
 /// cbindgen:derive-ostream
 #[repr(C)]
-struct A(i32);
+pub struct A(i32);
 
 /// cbindgen:field-names=[x, y]
 /// cbindgen:derive-ostream
 #[repr(C)]
-struct B(i32, f32);
+pub struct B(i32, f32);
 
 /// cbindgen:derive-ostream
 #[repr(u32)]
-enum C {
+pub enum C {
     X = 2,
     Y,
 }
 
 /// cbindgen:derive-ostream
 #[repr(C)]
-struct D {
+pub struct D {
     List: u8,
     Of: usize,
     Things: B,
@@ -24,7 +24,7 @@ struct D {
 
 /// cbindgen:derive-ostream
 #[repr(u8)]
-enum F {
+pub enum F {
     Foo(i16),
     Bar { x: u8, y: i16 },
     Baz
@@ -32,12 +32,20 @@ enum F {
 
 /// cbindgen:derive-ostream
 #[repr(C, u8)]
-enum H {
+pub enum H {
     Hello(i16),
     There { x: u8, y: i16 },
     Everyone
 }
 
+/// cbindgen:derive-ostream=false
+#[repr(C, u8)]
+pub enum I {
+    /// cbindgen:derive-ostream=true
+    ThereAgain { x: u8, y: i16 },
+    SomethingElse
+}
+
 #[no_mangle]
 pub extern "C" fn root(
     a: A,
@@ -46,5 +54,6 @@ pub extern "C" fn root(
     d: D,
     f: F,
     h: H,
+    i: I,
 ) { }