Updated test case documentation

docs/test_cases/t20012.md

@@ -0,0 +1,97 @@
+# t20012 - Lambda expression call sequence diagram test case
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20012_sequence:
+    type: sequence
+    glob:
+      - ../../tests/t20012/t20012.cc
+    include:
+      namespaces:
+        - clanguml::t20012
+    using_namespace:
+      - clanguml::t20012
+    start_from:
+      - function: "clanguml::t20012::tmain()"
+```
+## Source code
+File t20012.cc
+```cpp
+#include <functional>
+#include <utility>
+
+namespace clanguml {
+namespace t20012 {
+struct A {
+    void a() { aa(); }
+
+    void aa() { aaa(); }
+
+    void aaa() { }
+};
+
+struct B {
+    void b() { bb(); }
+
+    void bb() { bbb(); }
+
+    void bbb() { }
+};
+
+struct C {
+    void c() { cc(); }
+
+    void cc() { ccc(); }
+
+    void ccc() { }
+};
+
+template <typename F> struct R {
+    R(F &&f)
+        : f_{std::move(f)}
+    {
+    }
+
+    void r() { f_(); }
+
+    F f_;
+};
+
+void tmain()
+{
+    A a;
+    B b;
+    C c;
+
+    // The activity shouldn't be marked at the lambda definition, but
+    // wherever it is actually called...
+    auto alambda = [&a, &b]() {
+        a.a();
+        b.b();
+    };
+
+    // ...like here
+    alambda();
+
+    // There should be no call to B in the sequence diagram as the blambda
+    // is never called
+    [[maybe_unused]] auto blambda = [&b]() { b.b(); };
+
+    // Nested lambdas should also work
+    auto clambda = [alambda, &c]() {
+        c.c();
+        alambda();
+    };
+    clambda();
+
+    R r{[&c]() { c.c(); }};
+
+    r.r();
+}
+}
+}
+```
+## Generated UML diagrams
+![t20012_sequence](./t20012_sequence.svg "Lambda expression call sequence diagram test case")