From 21425c05e0a91a2b16503958bbf0201c108df967 Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Date: Mon, 5 Dec 2022 12:59:10 +0100
Subject: [PATCH] Add a centralized dependent_false type

The main use case is to static_assert on it at the end of a
if constexpr / else if constexpr chain.

See P2593 for a discussion about why this is pretty much the
only "allowed" way of doing so, short of running into IFNDR.

I'm actually adding two versions: one for TTP and one for NTTP,
as Qt code uses both.

Apply it in QFlatMap.

Change-Id: Iaff97e350784683d0c3994020b1352d5188931d6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/global/qttypetraits.h | 7 +++++++
 src/corelib/tools/qflatmap_p.h    | 3 +--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/corelib/global/qttypetraits.h b/src/corelib/global/qttypetraits.h
index e8939e7d5f..7e991ab8ca 100644
--- a/src/corelib/global/qttypetraits.h
+++ b/src/corelib/global/qttypetraits.h
@@ -45,6 +45,13 @@ noexcept(std::conjunction_v<std::is_nothrow_move_constructible<T>,
     return old;
 }
 
+namespace QtPrivate {
+// helper to be used to trigger a "dependent static_assert(false)"
+// (for instance, in a final `else` branch of a `if constexpr`.)
+template <typename T> struct type_dependent_false : std::false_type {};
+template <auto T> struct value_dependent_false : std::false_type {};
+}
+
 QT_END_NAMESPACE
 
 #endif // QTTYPETRAITS_H
diff --git a/src/corelib/tools/qflatmap_p.h b/src/corelib/tools/qflatmap_p.h
index 6937d21544..68ab567439 100644
--- a/src/corelib/tools/qflatmap_p.h
+++ b/src/corelib/tools/qflatmap_p.h
@@ -848,7 +848,6 @@ public:
     size_type remove_if(Predicate pred)
     {
         const auto indirect_call_to_pred = [pred = std::move(pred)](iterator it) {
-            [[maybe_unused]] auto dependent_false = [](auto &&...) { return false; };
             using Pair = decltype(*it);
             using K = decltype(it.key());
             using V = decltype(it.value());
@@ -860,7 +859,7 @@ public:
             } else if constexpr (std::is_invocable_v<P, K> && !std::is_invocable_v<P, Pair>) {
                 return pred(it.key());
             } else {
-                static_assert(dependent_false(pred),
+                static_assert(QtPrivate::type_dependent_false<Predicate>(),
                     "Don't know how to call the predicate.\n"
                     "Options:\n"
                     "- pred(*it)\n"