Improve field count typical case performance

test/core/Jamfile.v2

@@ -70,6 +70,10 @@ test-suite pfr_tests
     [ run ../../example/sample_printing.cpp  : : : : auto_engine_sample_printing ]
     [ run ../../example/get.cpp              : : : : auto_engine_get ]
     [ run ../../example/quick_examples.cpp   : : : : auto_engine_quick ]
+
+    [ compile-fail fields_count_on_incomplete_type.cpp : <define>BOOST_PFR_RUN_TEST_ON=void : fields_count_on_incomplete_type_void ]
+    [ compile-fail fields_count_on_incomplete_type.cpp : <define>BOOST_PFR_RUN_TEST_ON="void()" : fields_count_on_incomplete_type_function ]
+    [ compile-fail fields_count_on_incomplete_type.cpp : <define>BOOST_PFR_RUN_TEST_ON="struct Foo" : fields_count_on_incomplete_type_struct ]
   ;
 
 local BLACKLIST_TESTS_FOR_LOOPHOLE =

test/core/compile-fail/constructible_0_or_more_args.cpp

@@ -0,0 +1,15 @@
+// Copyright (c) 2023-2024 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/tuple_size.hpp>
+
+struct A {
+    template <typename... Args>
+    explicit A(Args&&...) {}
+};
+
+int main() {
+    (void)boost::pfr::tuple_size<A>::value; // Must be a compile time error
+}

test/core/compile-fail/constructible_1_or_more_args.cpp

@@ -0,0 +1,15 @@
+// Copyright (c) 2023-2024 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/tuple_size.hpp>
+
+struct A {
+    template <typename Arg0, typename... Args>
+    explicit A(Arg0&&, Args&&...) {}
+};
+
+int main() {
+    (void)boost::pfr::tuple_size<A>::value; // Must be a compile time error
+}

test/core/fields_count_on_incomplete_type.cpp

@@ -0,0 +1,10 @@
+// Copyright (c) 2023-2024 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/detail/fields_count.hpp>
+
+int main() {
+    return static_cast<int>(boost::pfr::detail::fields_count<BOOST_PFR_RUN_TEST_ON>());
+}

test/core/run/bitfields_count.cpp

@@ -5,17 +5,82 @@
 
 #include <boost/pfr/tuple_size.hpp>
 
-struct bf {
-    unsigned int i1: 1;
-    unsigned int i2: 1;
-    unsigned int i3: 1;
-    unsigned int i4: 1;
-    unsigned int i5: 1;
-    unsigned int i6: 1;
+#include <cstdint>
+
+struct bf7 {
+    uint8_t b0 : 1;
+    uint8_t b1 : 1;
+    uint8_t b2 : 1;
+    uint8_t b3 : 1;
+    uint8_t b4 : 1;
+    uint8_t b5 : 1;
+    uint8_t b6 : 1;
 };
+static_assert(sizeof(bf7) == 1, "");
 
-int main() {
-    static_assert(boost::pfr::tuple_size<bf>::value == 6, "");
-}
+struct bf8 {
+    uint8_t b0 : 1;
+    uint8_t b1 : 1;
+    uint8_t b2 : 1;
+    uint8_t b3 : 1;
+    uint8_t b4 : 1;
+    uint8_t b5 : 1;
+    uint8_t b6 : 1;
+    uint8_t b7 : 1;
+};
+static_assert(sizeof(bf8) == 1, "");
 
+struct bf16 {
+    uint8_t b0 : 1;
+    uint8_t b1 : 1;
+    uint8_t b2 : 1;
+    uint8_t b3 : 1;
+    uint8_t b4 : 1;
+    uint8_t b5 : 1;
+    uint8_t b6 : 1;
+    uint8_t b7 : 1;
+    uint8_t b8 : 1;
+    uint8_t b9 : 1;
+    uint8_t b10 : 1;
+    uint8_t b11 : 1;
+    uint8_t b12 : 1;
+    uint8_t b13 : 1;
+    uint8_t b14 : 1;
+    uint8_t b15 : 1;
+};
+static_assert(sizeof(bf16) == 2, "");
 
+struct bf24 {
+    uint8_t b0 : 1;
+    uint8_t b1 : 1;
+    uint8_t b2 : 1;
+    uint8_t b3 : 1;
+    uint8_t b4 : 1;
+    uint8_t b5 : 1;
+    uint8_t b6 : 1;
+    uint8_t b7 : 1;
+    uint8_t b8 : 1;
+    uint8_t b9 : 1;
+    uint8_t b10 : 1;
+    uint8_t b11 : 1;
+    uint8_t b12 : 1;
+    uint8_t b13 : 1;
+    uint8_t b14 : 1;
+    uint8_t b15 : 1;
+    uint8_t b16 : 1;
+    uint8_t b17 : 1;
+    uint8_t b18 : 1;
+    uint8_t b19 : 1;
+    uint8_t b20 : 1;
+    uint8_t b21 : 1;
+    uint8_t b22 : 1;
+    uint8_t b23 : 1;
+};
+static_assert(sizeof(bf24) == 3, "");
+
+int main() {
+    static_assert(boost::pfr::tuple_size_v<bf7> == 7, "");
+    static_assert(boost::pfr::tuple_size_v<bf8> == 8, "");
+    static_assert(boost::pfr::tuple_size_v<bf16> == 16, "");
+    static_assert(boost::pfr::tuple_size_v<bf24> == 24, "");
+}

test/core/run/huge_count.cpp

@@ -0,0 +1,63 @@
+// Copyright (c) 2024 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/tuple_size.hpp>
+
+#include <climits>
+#include <cstdint>
+
+#if defined(__clang__)
+#   if SIZE_MAX > (1ULL << 32) - 1
+#       define ARRAY_MAX (SIZE_MAX >> 3)
+#   else
+#       define ARRAY_MAX SIZE_MAX
+#   endif
+#   define OBJECT_MAX SIZE_MAX
+#elif defined(__GNUC__)
+#   define ARRAY_MAX INT_MAX
+#   define OBJECT_MAX (SIZE_MAX >> 1)
+#elif defined(_MSC_VER)
+#   define ARRAY_MAX INT_MAX
+#   define OBJECT_MAX UINT_MAX
+#else  // Let's play it safe
+#   define ARRAY_MAX INT_MAX
+#   define OBJECT_MAX INT_MAX
+#endif
+
+#pragma pack(1)
+struct A {
+    char x[ARRAY_MAX <= (OBJECT_MAX >> 3) ? ARRAY_MAX : OBJECT_MAX >> 3];
+};
+
+struct B {
+    A a;
+    A b;
+    A c;
+    A d;
+    A e;
+    A f;
+    A g;
+    A h;
+};
+
+struct C {
+    A& a;
+    A b;
+    A c;
+    A d;
+    A e;
+    A f;
+    A g;
+    A h;
+};
+#pragma pack()
+
+int main() {
+#ifndef _MSC_VER
+    static_assert(boost::pfr::tuple_size_v<char[ARRAY_MAX]> == ARRAY_MAX, "");
+#endif
+    static_assert(boost::pfr::tuple_size_v<B> == 8, "");
+    static_assert(boost::pfr::tuple_size_v<C> == 8, "");
+}