diff --git a/src/bindgen/ir/generic_path.rs b/src/bindgen/ir/generic_path.rs
index 53b6bdec..e97488d4 100644
--- a/src/bindgen/ir/generic_path.rs
+++ b/src/bindgen/ir/generic_path.rs
@@ -21,6 +21,7 @@ pub enum GenericParamType {
 pub struct GenericParam {
     name: Path,
     ty: GenericParamType,
+    default: Option<GenericArgument>,
 }
 
 impl GenericParam {
@@ -28,20 +29,36 @@ impl GenericParam {
         GenericParam {
             name: Path::new(name),
             ty: GenericParamType::Type,
+            default: None,
         }
     }
 
     pub fn load(param: &syn::GenericParam) -> Result<Option<Self>, String> {
         match *param {
-            syn::GenericParam::Type(syn::TypeParam { ref ident, .. }) => Ok(Some(GenericParam {
-                name: Path::new(ident.unraw().to_string()),
-                ty: GenericParamType::Type,
-            })),
+            syn::GenericParam::Type(syn::TypeParam {
+                ref ident,
+                ref default,
+                ..
+            }) => {
+                let default = match default.as_ref().map(Type::load).transpose()? {
+                    None => None,
+                    Some(None) => Err(format!("unsupported generic type default: {:?}", default))?,
+                    Some(Some(ty)) => Some(GenericArgument::Type(ty)),
+                };
+                Ok(Some(GenericParam {
+                    name: Path::new(ident.unraw().to_string()),
+                    ty: GenericParamType::Type,
+                    default,
+                }))
+            }
 
             syn::GenericParam::Lifetime(_) => Ok(None),
 
             syn::GenericParam::Const(syn::ConstParam {
-                ref ident, ref ty, ..
+                ref ident,
+                ref ty,
+                ref default,
+                ..
             }) => match Type::load(ty)? {
                 None => {
                     // A type that evaporates, like PhantomData.
@@ -50,6 +67,11 @@ impl GenericParam {
                 Some(ty) => Ok(Some(GenericParam {
                     name: Path::new(ident.unraw().to_string()),
                     ty: GenericParamType::Const(ty),
+                    default: default
+                        .as_ref()
+                        .map(ConstExpr::load)
+                        .transpose()?
+                        .map(GenericArgument::Const),
                 })),
             },
         }
@@ -81,17 +103,32 @@ impl GenericParams {
         item_name: &str,
         arguments: &'out [GenericArgument],
     ) -> Vec<(&'out Path, &'out GenericArgument)> {
-        assert!(self.len() > 0, "{} is not generic", item_name);
         assert!(
-            self.len() == arguments.len(),
+            self.len() >= arguments.len(),
             "{} has {} params but is being instantiated with {} values",
             item_name,
             self.len(),
             arguments.len(),
         );
         self.iter()
-            .map(|param| param.name())
-            .zip(arguments.iter())
+            .enumerate()
+            .map(|(i, param)| {
+                // Fall back to the GenericParam default if no GenericArgument is available.
+                let arg = arguments
+                    .get(i)
+                    .or(param.default.as_ref())
+                    .unwrap_or_else(|| {
+                        panic!(
+                            "{} with {} params is being instantiated with only {} values, \
+                             and param {} lacks a default value",
+                            item_name,
+                            self.len(),
+                            arguments.len(),
+                            i
+                        )
+                    });
+                (param.name(), arg)
+            })
             .collect()
     }
 
@@ -111,13 +148,18 @@ impl GenericParams {
                 match item.ty {
                     GenericParamType::Type => {
                         write!(out, "typename {}", item.name);
-                        if with_default {
+                        if let Some(GenericArgument::Type(ref ty)) = item.default {
+                            write!(out, " = ");
+                            cdecl::write_type(language_backend, out, ty, config);
+                        } else if with_default {
                             write!(out, " = void");
                         }
                     }
                     GenericParamType::Const(ref ty) => {
                         cdecl::write_field(language_backend, out, ty, item.name.name(), config);
-                        if with_default {
+                        if let Some(GenericArgument::Const(ref expr)) = item.default {
+                            write!(out, " = {}", expr.as_str());
+                        } else if with_default {
                             write!(out, " = 0");
                         }
                     }
diff --git a/tests/expectations/generic_defaults.c b/tests/expectations/generic_defaults.c
new file mode 100644
index 00000000..72fbb417
--- /dev/null
+++ b/tests/expectations/generic_defaults.c
@@ -0,0 +1,19 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+typedef struct {
+  Foo_i32 f;
+  uint32_t p;
+} Bar_i32__u32;
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+void foo_root(Foo_i16 f, Bar_i32__u32 b, Baz_i64 z);
diff --git a/tests/expectations/generic_defaults.compat.c b/tests/expectations/generic_defaults.compat.c
new file mode 100644
index 00000000..d08662a3
--- /dev/null
+++ b/tests/expectations/generic_defaults.compat.c
@@ -0,0 +1,27 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+typedef struct {
+  Foo_i32 f;
+  uint32_t p;
+} Bar_i32__u32;
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void foo_root(Foo_i16 f, Bar_i32__u32 b, Baz_i64 z);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus
diff --git a/tests/expectations/generic_defaults.cpp b/tests/expectations/generic_defaults.cpp
new file mode 100644
index 00000000..afc28fb8
--- /dev/null
+++ b/tests/expectations/generic_defaults.cpp
@@ -0,0 +1,23 @@
+#include <cstdarg>
+#include <cstdint>
+#include <cstdlib>
+#include <ostream>
+#include <new>
+
+template<typename T, typename P = void>
+using Foo = T;
+
+template<typename T, typename P>
+struct Bar {
+  Foo<T> f;
+  P p;
+};
+
+template<typename T>
+using Baz = Foo<T>;
+
+extern "C" {
+
+void foo_root(Foo<int16_t> f, Bar<int32_t, uint32_t> b, Baz<int64_t> z);
+
+}  // extern "C"
diff --git a/tests/expectations/generic_defaults.pyx b/tests/expectations/generic_defaults.pyx
new file mode 100644
index 00000000..0c17c1b6
--- /dev/null
+++ b/tests/expectations/generic_defaults.pyx
@@ -0,0 +1,21 @@
+from libc.stdint cimport int8_t, int16_t, int32_t, int64_t, intptr_t
+from libc.stdint cimport uint8_t, uint16_t, uint32_t, uint64_t, uintptr_t
+cdef extern from *:
+  ctypedef bint bool
+  ctypedef struct va_list
+
+cdef extern from *:
+
+  ctypedef int16_t Foo_i16;
+
+  ctypedef int32_t Foo_i32;
+
+  ctypedef struct Bar_i32__u32:
+    Foo_i32 f;
+    uint32_t p;
+
+  ctypedef int64_t Foo_i64;
+
+  ctypedef Foo_i64 Baz_i64;
+
+  void foo_root(Foo_i16 f, Bar_i32__u32 b, Baz_i64 z);
diff --git a/tests/expectations/generic_defaults_both.c b/tests/expectations/generic_defaults_both.c
new file mode 100644
index 00000000..1e7e7c5f
--- /dev/null
+++ b/tests/expectations/generic_defaults_both.c
@@ -0,0 +1,19 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+typedef struct Bar_i32__u32 {
+  Foo_i32 f;
+  uint32_t p;
+} Bar_i32__u32;
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+void foo_root(Foo_i16 f, struct Bar_i32__u32 b, Baz_i64 z);
diff --git a/tests/expectations/generic_defaults_both.compat.c b/tests/expectations/generic_defaults_both.compat.c
new file mode 100644
index 00000000..e7aa9d44
--- /dev/null
+++ b/tests/expectations/generic_defaults_both.compat.c
@@ -0,0 +1,27 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+typedef struct Bar_i32__u32 {
+  Foo_i32 f;
+  uint32_t p;
+} Bar_i32__u32;
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void foo_root(Foo_i16 f, struct Bar_i32__u32 b, Baz_i64 z);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus
diff --git a/tests/expectations/generic_defaults_tag.c b/tests/expectations/generic_defaults_tag.c
new file mode 100644
index 00000000..b3fa4988
--- /dev/null
+++ b/tests/expectations/generic_defaults_tag.c
@@ -0,0 +1,19 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+struct Bar_i32__u32 {
+  Foo_i32 f;
+  uint32_t p;
+};
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+void foo_root(Foo_i16 f, struct Bar_i32__u32 b, Baz_i64 z);
diff --git a/tests/expectations/generic_defaults_tag.compat.c b/tests/expectations/generic_defaults_tag.compat.c
new file mode 100644
index 00000000..9fcc7e3c
--- /dev/null
+++ b/tests/expectations/generic_defaults_tag.compat.c
@@ -0,0 +1,27 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef int16_t Foo_i16;
+
+typedef int32_t Foo_i32;
+
+struct Bar_i32__u32 {
+  Foo_i32 f;
+  uint32_t p;
+};
+
+typedef int64_t Foo_i64;
+
+typedef Foo_i64 Baz_i64;
+
+#ifdef __cplusplus
+extern "C" {
+#endif // __cplusplus
+
+void foo_root(Foo_i16 f, struct Bar_i32__u32 b, Baz_i64 z);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif  // __cplusplus
diff --git a/tests/expectations/generic_defaults_tag.pyx b/tests/expectations/generic_defaults_tag.pyx
new file mode 100644
index 00000000..9b880749
--- /dev/null
+++ b/tests/expectations/generic_defaults_tag.pyx
@@ -0,0 +1,21 @@
+from libc.stdint cimport int8_t, int16_t, int32_t, int64_t, intptr_t
+from libc.stdint cimport uint8_t, uint16_t, uint32_t, uint64_t, uintptr_t
+cdef extern from *:
+  ctypedef bint bool
+  ctypedef struct va_list
+
+cdef extern from *:
+
+  ctypedef int16_t Foo_i16;
+
+  ctypedef int32_t Foo_i32;
+
+  cdef struct Bar_i32__u32:
+    Foo_i32 f;
+    uint32_t p;
+
+  ctypedef int64_t Foo_i64;
+
+  ctypedef Foo_i64 Baz_i64;
+
+  void foo_root(Foo_i16 f, Bar_i32__u32 b, Baz_i64 z);
diff --git a/tests/rust/generic_defaults.rs b/tests/rust/generic_defaults.rs
new file mode 100644
index 00000000..7b7a8357
--- /dev/null
+++ b/tests/rust/generic_defaults.rs
@@ -0,0 +1,16 @@
+#[repr(transparent)]
+pub struct Foo<T, P = c_void> {
+    field: T,
+    _phantom: std::marker::PhantomData<P>,
+}
+
+#[repr(C)]
+pub struct Bar<T, P> {
+    f: Foo<T>,
+    p: P,
+}
+
+pub type Baz<T> = Foo<T>;
+
+#[no_mangle]
+pub extern "C" fn foo_root(f: Foo<i16>, b: Bar<i32, u32>, z: Baz<i64>) {}