Add example for C2129

docs/error-messages/compiler-errors-1/compiler-error-c2129.md

@@ -13,3 +13,27 @@ static function 'function' declared but not defined
 A forward reference is made to a **`static`** function that is never defined.
 
 A **`static`** function must be defined within file scope. If the function is defined in another file, it must be declared **`extern`**.
+
+The following sample generates C2129:
+
+```cpp
+// C2129.cpp
+static void foo();   // C2129
+
+int main() {
+   foo();
+}
+```
+
+Possible resolution:
+
+```cpp
+// C2129b.cpp
+static void foo();
+
+int main() {
+   foo();
+}
+
+static void foo() {}
+```