diff --git a/src/bindgen/ir/generic_path.rs b/src/bindgen/ir/generic_path.rs
index 9a89d04..8db3d3f 100644
--- a/src/bindgen/ir/generic_path.rs
+++ b/src/bindgen/ir/generic_path.rs
@@ -27,6 +27,32 @@ impl GenericParams {
                 .collect(),
         )
     }
+
+    fn write_internal<F: Write>(
+        &self,
+        config: &Config,
+        out: &mut SourceWriter<F>,
+        with_default: bool,
+    ) {
+        if !self.0.is_empty() && config.language == Language::Cxx {
+            out.write("template<");
+            for (i, item) in self.0.iter().enumerate() {
+                if i != 0 {
+                    out.write(", ");
+                }
+                write!(out, "typename {}", item);
+                if with_default {
+                    write!(out, " = void");
+                }
+            }
+            out.write(">");
+            out.new_line();
+        }
+    }
+
+    pub fn write_with_default<F: Write>(&self, config: &Config, out: &mut SourceWriter<F>) {
+        self.write_internal(config, out, true);
+    }
 }
 
 impl Deref for GenericParams {
@@ -39,17 +65,7 @@ impl Deref for GenericParams {
 
 impl Source for GenericParams {
     fn write<F: Write>(&self, config: &Config, out: &mut SourceWriter<F>) {
-        if !self.0.is_empty() && config.language == Language::Cxx {
-            out.write("template<");
-            for (i, item) in self.0.iter().enumerate() {
-                if i != 0 {
-                    out.write(", ");
-                }
-                write!(out, "typename {}", item);
-            }
-            out.write(">");
-            out.new_line();
-        }
+        self.write_internal(config, out, false);
     }
 }
 
diff --git a/src/bindgen/ir/opaque.rs b/src/bindgen/ir/opaque.rs
index a46dcee..2efd397 100644
--- a/src/bindgen/ir/opaque.rs
+++ b/src/bindgen/ir/opaque.rs
@@ -105,12 +105,16 @@ impl Item for OpaqueItem {
         out: &mut Monomorphs,
     ) {
         assert!(
-            self.generic_params.len() > 0,
+            !self.generic_params.is_empty(),
             "{} is not generic",
             self.path
         );
+
+        // We can be instantiated with less generic params because of default
+        // template parameters, or because of empty types that we remove during
+        // parsing (`()`).
         assert!(
-            self.generic_params.len() == generic_values.len(),
+            self.generic_params.len() >= generic_values.len(),
             "{} has {} params but is being instantiated with {} values",
             self.path,
             self.generic_params.len(),
@@ -137,7 +141,7 @@ impl Source for OpaqueItem {
 
         self.documentation.write(config, out);
 
-        self.generic_params.write(config, out);
+        self.generic_params.write_with_default(config, out);
 
         if config.style.generate_typedef() && config.language == Language::C {
             write!(
diff --git a/src/bindgen/parser.rs b/src/bindgen/parser.rs
index 55280f2..831c37f 100644
--- a/src/bindgen/parser.rs
+++ b/src/bindgen/parser.rs
@@ -391,7 +391,7 @@ impl Parse {
         add_opaque("Option", vec!["T"]);
         add_opaque("NonNull", vec!["T"]);
         add_opaque("Vec", vec!["T"]);
-        add_opaque("HashMap", vec!["K", "V"]);
+        add_opaque("HashMap", vec!["K", "V", "Hasher"]);
         add_opaque("BTreeMap", vec!["K", "V"]);
         add_opaque("HashSet", vec!["T"]);
         add_opaque("BTreeSet", vec!["T"]);
diff --git a/tests/expectations/both/opaque.c b/tests/expectations/both/opaque.c
new file mode 100644
index 0000000..75614b6
--- /dev/null
+++ b/tests/expectations/both/opaque.c
@@ -0,0 +1,28 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+typedef struct Result_Foo Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef Result_Foo Bar;
+
+void root(const Foo *a, const Bar *b);
diff --git a/tests/expectations/both/opaque.compat.c b/tests/expectations/both/opaque.compat.c
new file mode 100644
index 0000000..a467628
--- /dev/null
+++ b/tests/expectations/both/opaque.compat.c
@@ -0,0 +1,36 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+typedef struct Result_Foo Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef Result_Foo Bar;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void root(const Foo *a, const Bar *b);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif // __cplusplus
diff --git a/tests/expectations/cell.cpp b/tests/expectations/cell.cpp
index 2c16e26..443fa6f 100644
--- a/tests/expectations/cell.cpp
+++ b/tests/expectations/cell.cpp
@@ -3,10 +3,10 @@
 #include <cstdlib>
 #include <new>
 
-template<typename T>
+template<typename T = void>
 struct NotReprC;
 
-template<typename T>
+template<typename T = void>
 struct RefCell;
 
 using Foo = NotReprC<RefCell<int32_t>>;
diff --git a/tests/expectations/monomorph-1.cpp b/tests/expectations/monomorph-1.cpp
index 7d56cc6..e4c94c0 100644
--- a/tests/expectations/monomorph-1.cpp
+++ b/tests/expectations/monomorph-1.cpp
@@ -3,7 +3,7 @@
 #include <cstdlib>
 #include <new>
 
-template<typename T>
+template<typename T = void>
 struct Bar;
 
 template<typename T>
diff --git a/tests/expectations/monomorph-3.cpp b/tests/expectations/monomorph-3.cpp
index 05dc8a6..e15c407 100644
--- a/tests/expectations/monomorph-3.cpp
+++ b/tests/expectations/monomorph-3.cpp
@@ -3,7 +3,7 @@
 #include <cstdlib>
 #include <new>
 
-template<typename T>
+template<typename T = void>
 struct Bar;
 
 template<typename T>
diff --git a/tests/expectations/opaque.c b/tests/expectations/opaque.c
new file mode 100644
index 0000000..75614b6
--- /dev/null
+++ b/tests/expectations/opaque.c
@@ -0,0 +1,28 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+typedef struct Result_Foo Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef Result_Foo Bar;
+
+void root(const Foo *a, const Bar *b);
diff --git a/tests/expectations/opaque.compat.c b/tests/expectations/opaque.compat.c
new file mode 100644
index 0000000..a467628
--- /dev/null
+++ b/tests/expectations/opaque.compat.c
@@ -0,0 +1,36 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+typedef struct Result_Foo Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef Result_Foo Bar;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void root(const Foo *a, const Bar *b);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif // __cplusplus
diff --git a/tests/expectations/opaque.cpp b/tests/expectations/opaque.cpp
new file mode 100644
index 0000000..e604fe6
--- /dev/null
+++ b/tests/expectations/opaque.cpp
@@ -0,0 +1,33 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <cstdarg>
+#include <cstdint>
+#include <cstdlib>
+#include <new>
+
+template<typename K = void, typename V = void, typename Hasher = void>
+struct HashMap;
+
+template<typename T = void, typename E = void>
+struct Result;
+
+/// Fast hash map used internally.
+template<typename K, typename V>
+using FastHashMap = HashMap<K, V, BuildHasherDefault<DefaultHasher>>;
+
+using Foo = FastHashMap<int32_t, int32_t>;
+
+using Bar = Result<Foo>;
+
+extern "C" {
+
+void root(const Foo *a, const Bar *b);
+
+} // extern "C"
diff --git a/tests/expectations/std_lib.cpp b/tests/expectations/std_lib.cpp
index 261522e..c756dce 100644
--- a/tests/expectations/std_lib.cpp
+++ b/tests/expectations/std_lib.cpp
@@ -3,15 +3,15 @@
 #include <cstdlib>
 #include <new>
 
-template<typename T>
+template<typename T = void>
 struct Option;
 
-template<typename T, typename E>
+template<typename T = void, typename E = void>
 struct Result;
 
 struct String;
 
-template<typename T>
+template<typename T = void>
 struct Vec;
 
 extern "C" {
diff --git a/tests/expectations/swift_name.cpp b/tests/expectations/swift_name.cpp
index 739ca6f..89d4442 100644
--- a/tests/expectations/swift_name.cpp
+++ b/tests/expectations/swift_name.cpp
@@ -5,7 +5,7 @@
 #include <cstdlib>
 #include <new>
 
-template<typename T>
+template<typename T = void>
 struct Box;
 
 struct Opaque;
diff --git a/tests/expectations/tag/opaque.c b/tests/expectations/tag/opaque.c
new file mode 100644
index 0000000..5728f2e
--- /dev/null
+++ b/tests/expectations/tag/opaque.c
@@ -0,0 +1,28 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+struct Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef struct Result_Foo Bar;
+
+void root(const Foo *a, const Bar *b);
diff --git a/tests/expectations/tag/opaque.compat.c b/tests/expectations/tag/opaque.compat.c
new file mode 100644
index 0000000..ae56420
--- /dev/null
+++ b/tests/expectations/tag/opaque.compat.c
@@ -0,0 +1,36 @@
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher;
+
+struct Result_Foo;
+
+/**
+ * Fast hash map used internally.
+ */
+typedef struct HashMap_i32__i32__BuildHasherDefault_DefaultHasher FastHashMap_i32__i32;
+
+typedef FastHashMap_i32__i32 Foo;
+
+typedef struct Result_Foo Bar;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void root(const Foo *a, const Bar *b);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif // __cplusplus
diff --git a/tests/rust/opaque.rs b/tests/rust/opaque.rs
new file mode 100644
index 0000000..7e1c6be
--- /dev/null
+++ b/tests/rust/opaque.rs
@@ -0,0 +1,10 @@
+/// Fast hash map used internally.
+type FastHashMap<K, V> =
+    std::collections::HashMap<K, V, std::hash::BuildHasherDefault<std::collections::hash_map::DefaultHasher>>;
+
+pub type Foo = FastHashMap<i32, i32>;
+
+pub type Bar = Result<Foo, ()>;
+
+#[no_mangle]
+pub extern "C" fn root(a: &Foo, b: &Bar) {}
diff --git a/tests/rust/opaque.toml b/tests/rust/opaque.toml
new file mode 100644
index 0000000..d7219e1
--- /dev/null
+++ b/tests/rust/opaque.toml
@@ -0,0 +1,9 @@
+header = """
+#ifdef __cplusplus
+// These could be added as opaque types I guess.
+template <typename T>
+struct BuildHasherDefault;
+
+struct DefaultHasher;
+#endif
+"""