diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -523,6 +523,41 @@
     return a + b * c;
   }
 
+The matrix type extension supports compound assignments for addition, subtraction, and multiplication, provided
+their types are consistent.
+
+.. code-block:: c++
+
+  typedef float m4x4_t __attribute__((matrix_type(4, 4)));
+
+  void f(m4x4_t a, m4x4_t b) {
+    a += b;
+    a -= b;
+    a *= b;
+  }
+
+The matrix type extension supports explicit casts. The casts we support are C-style casts in C and C++ and
+static casts. Implicit type conversion between matrix types is not allowed.
+
+.. code-block: c++
+
+  typedef int ix5x5 __attribute__((matrix_type(5, 5)));
+  typedef float fx5x5 __attribute__((matrix_type(5, 5)));
+
+  void f1(ix5x5 i, fx5x5 f) {
+    f = (fx5x5)i;
+  }
+
+
+  template <typename X>
+  using matrix_4_4 = X __attribute__((matrix_type(4, 4)));
+
+  void f2() {
+    matrix_5_5<double> d;
+    matrix_5_5<int> i;
+    i = (matrix_5_5<int>)d;
+    i = static_cast<matrix_5_5<int>>(d);
+  }
 
 Half-Precision Floating Point
 =============================