Example for Niebloid.

HappyCerberus · HappyCerberus · commit f31788f44b5a · 2022-09-28T18:34:15.000+02:00
diff --git a/code_examples/theory/CMakeLists.txt b/code_examples/theory/CMakeLists.txt
@@ -24,3 +24,6 @@ add_executable(adl_nonfunc adl_nonfunc.cpp)
 
 add_executable(adl_shutdown adl_shutdown.cpp)
 # Compile only
+
+add_executable(adl_niebloid adl_niebloid.cpp)
+# Compile only
diff --git a/code_examples/theory/adl_niebloid.cpp b/code_examples/theory/adl_niebloid.cpp
@@ -0,0 +1,6 @@
+#include <utility>
+
+#include "adl_niebloid_code.h"
+int main() {
+    calling_site();
+}
diff --git a/code_examples/theory/adl_niebloid_code.h b/code_examples/theory/adl_niebloid_code.h
@@ -0,0 +1,29 @@
+namespace dflt {
+namespace impl {
+    template <typename T> 
+    concept HasCustomImpl = requires(T a) { do_something(a); };
+
+    struct DoSomethingFn { 
+        template <typename T> void operator()(T&& arg) const
+        requires HasCustomImpl<T> { do_something(std::forward<T>(arg)); }
+        template <typename T> void operator()(T&&) const
+        requires (!HasCustomImpl<T>) { /* default implementation */ }
+    };
+}
+
+inline namespace var {
+    constexpr inline auto do_something = impl::DoSomethingFn{};
+}
+}
+
+namespace custom { 
+    struct X { friend void do_something(const X&){}; }; 
+    struct Y {};
+}
+
+void calling_site() {
+    custom::X x;
+    custom::Y y;
+    dflt::do_something(x); // calls custom::do_something(const X&)
+    dflt::do_something(y); // calls default implementation
+}
diff --git a/code_examples/theory/adl_shutdown_code.h b/code_examples/theory/adl_shutdown_code.h
@@ -9,5 +9,5 @@ constexpr inline auto some_func = [](const auto&) {};
 void calling_site() {
     Custom::X x;
     some_func(x); // calls ::some_func
-    // Because ::some_func prevents ADL, Custom::some_func cannot be called.
+    // Because ADL is skipped, Custom::some_func cannot be called.
 }

Original file line number	Diff line number	Diff line change
`@@ -9,5 +9,5 @@ constexpr inline auto some_func = [](const auto&) {};`
`9`	`9`	`void calling_site() {`
`10`	`10`	`Custom::X x;`
`11`	`11`	`some_func(x); // calls ::some_func`
`12`		`- // Because ::some_func prevents ADL, Custom::some_func cannot be called.`
	`12`	`+ // Because ADL is skipped, Custom::some_func cannot be called.`
`13`	`13`	`}`