c++: improve void_t explanation

3 files changed, 52 insertions, 9 deletions

diff --git a/src/development/c++.md b/src/development/c++.md
index 9dc8fb0..6cdc251 100644
--- a/src/development/c++.md
+++ b/src/development/c++.md
@@ -58,17 +58,13 @@ of the `decltype(std:declval<T>...` expressions is ill-formed, the template
 specialization for `is_valid` will be removed from the candidate set due to
 [SFINAE][sfinae].
 ```cpp
-template<typename T, typename = void>
-struct is_valid : std::false_type {};
-
-template<typename T>
-struct is_valid<T, std::void_t<
-                       decltype(std::declval<T>().some_fun1()),
-                       decltype(std::declval<T>().some_fun2())
-               >> : std::true_type {};
+{{#include c++/tmpl-void_t.cc:3:45}}
 ```
 > `std::declval<T>()` creates an instance of type T in an unevaluated context.
 
+A more detailed description is available in the SO discussion [How does
+`void_t` work](https://stackoverflow.com/a/27688405).
+
 ## Template selection with partially / fully specializations.
 ```cpp
 {{#include c++/tmpl-pair.cc:3:}}
diff --git a/src/development/c++/Makefile b/src/development/c++/Makefile
index 1d5172b..ca752d9 100644
--- a/src/development/c++/Makefile
+++ b/src/development/c++/Makefile
@@ -1,4 +1,4 @@
-SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc
+SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc tmpl-void_t.cc
 BIN = $(SRC:.cc=)
 
 all: $(BIN)
diff --git a/src/development/c++/tmpl-void_t.cc b/src/development/c++/tmpl-void_t.cc
new file mode 100644
index 0000000..fd95315
--- /dev/null
+++ b/src/development/c++/tmpl-void_t.cc
@@ -0,0 +1,47 @@
+// Copyright (C) 2023 johannst
+
+#include <type_traits>
+
+// (1) Primary template.
+template<typename T, typename = void>
+struct is_valid : std::false_type {};
+
+// (2) Partial template specialization.
+template<typename T>
+struct is_valid<T, std::void_t<decltype(std::declval<T>().some_fun1()),
+                               decltype(std::declval<T>().some_fun2())>> : std::true_type {};
+struct A {
+    void some_fun1() {}
+    void some_fun2() {}
+};
+
+struct B {};
+
+static_assert(is_valid<A>::value, "is true");
+// * Compare template arg list with primary template, we only supplied one
+//   arg, the second one will be defaulted as
+//   is_valid<A, void>
+// * Compare template arg list against available specializations, this will
+//   try to match the pattern <A, void> against the patterns defined in the
+//   partial specializations.
+// * Try specialization (2)
+//   * T -> A
+//   * Evaluate std::void_t -> decltype's are well-formed
+//     std::void_t<...> -> void
+//   * Specialization (2) matches <A, void>
+// * Pick the most specialized version -> (2)
+
+static_assert(!is_valid<B>::value, "is false");
+// * Compare template arg list with primary template, we only supplied one
+//   arg, the second one will be defaulted as
+//   is_valid<A, void>
+// * Compare template arg list against available specializations, this will
+//   try to match the pattern <B, void> against the patterns defined in the
+//   partial specializations.
+// * Try specialization (2)
+//   * T -> B
+//   * Evaluate std::void_t -> decltype's are ill-formed
+//   * Specialization (2) is removed from candidate set, no hard error (SFINAE)
+// * No specialization matches, take the primary template.
+
+int main() {}