Merge bitcoin/bitcoin#24416: doc: Avoid ADL for function calls

52a797bfe5ced4329f2272be417c35730ec8839f doc: Avoid ADL for function calls (Hennadii Stepanov)

Pull request description:

  It happened two times recently, when [ADL](https://en.cppreference.com/w/cpp/language/adl) popped up unexpectedly and brought some confusion:
  - https://github.com/bitcoin/bitcoin/pull/24338/files#r805989994
  > Any idea why this even compiles?
  - https://www.erisian.com.au/bitcoin-core-dev/log-2022-02-18.html#l-51:
  > 2022-02-18T03:24:14  \<dongcarl\> Does anyone know why this compiles? 6d3d2caa37
  > 2022-02-18T03:24:14  \<dongcarl\> GetUTXOStatsWithHasher and MakeUTXOHasher are both in the `kernel::` namespace and I never added a `using` declaration on top...
  > 2022-02-18T03:25:53  \<sipa\> https://en.cppreference.com/w/cpp/language/adl ?

  Let's document our intention to avoid similar cases in the future.

ACKs for top commit:
  laanwj:
    Anyhow, ACK 52a797bfe5ced4329f2272be417c35730ec8839f, there is no need to hold merge up on this, documenting it is a step forward.

Tree-SHA512: f52688b5d8f6130302185206ec6ea4731b099a75294ea2d477901a52d6d58473e3427e658aea408c140c2824c37a0399ec7376aded2a91197895ea52d51f0018

doc/developer-notes.md

@@ -106,6 +106,28 @@ code.
   - `static_assert` is preferred over `assert` where possible. Generally; compile-time checking is preferred over run-time checking.
   - Align pointers and references to the left i.e. use `type& var` and not `type &var`.
 
+For function calls a namespace should be specified explicitly, unless such functions have been declared within it.
+Otherwise, [argument-dependent lookup](https://en.cppreference.com/w/cpp/language/adl), also known as ADL, could be
+triggered that makes code harder to maintain and reason about:
+```c++
+#include <filesystem>
+
+namespace fs {
+class path : public std::filesystem::path
+{
+};
+// The intention is to disallow this function.
+bool exists(const fs::path& p) = delete;
+} // namespace fs
+
+int main()
+{
+    //fs::path p; // error
+    std::filesystem::path p; // compiled
+    exists(p); // ADL being used for unqualified name lookup
+}
+```
+
 Block style example:
 ```c++
 int g_count = 0;