Enable declaring constexpr funcs/vars with ==, improve enum/flag_enum

Simplifying `enum` and `flag_enum` needed:

- evolving the `==` compile-time alias design to be the way we we express compile-time `constexpr` functions and objects

- including to work around Cpp1's limitation on declaring `constexpr`/`inline`/`static` members of a type that have an incomplete type, including the same type being defined, by using the dance described in https://stackoverflow.com/questions/11928089/

- enabling emitting `operator<<` as a `friend` function

Author: hsutter

Diff for: include/cpp2util.h

@@ -1586,59 +1586,20 @@ constexpr auto unsafe_narrow( X&& x ) noexcept -> decltype(auto)
 
 //-----------------------------------------------------------------------
 //
-//  strict_value: a strong typedef-like helper for value types
+//  has_flags:  query whether a flag_enum value has all flags in 'flags' set
 //
-//  Intended for use as an underlying type for types/variables where you
-//  don't want implicit conversions or comparisons to happen between
-//  values even if they may share the same underlying type (e.g.,
-//  Color::Red and CardGame::Poker may both be represented as an `int`
-//  but shouldn't be interconvertible or intercomparable)
-//
-//  Used by the `enum` and `flag_enum` metafunctions
+//  flags       set of flags to check
 //
+//  Returns a function object that takes a 'value' of the same type as
+//  'flags', and evaluates to true if and only if 'value' has set all of
+//  the bits set in 'flags'
+// 
 //-----------------------------------------------------------------------
 //
-template <typename T, typename Tag, bool BitwiseOps>
-class strict_value {
-    T t = {};
-public:
-    explicit constexpr strict_value() { }
-
-    template <typename U>
-    explicit constexpr strict_value(U const& u) : t{unsafe_narrow<T>(u)} { }
-
-    template <typename U> requires std::is_convertible_v<T,U>
-    explicit constexpr operator U() const { return t; }
-
-    template <typename U> requires std::is_convertible_v<T,U>
-    explicit constexpr operator U()       { return t; }
-
-    constexpr auto operator<=>( strict_value const& ) const -> std::strong_ordering = default;
-
-    auto to_string() const -> std::string { return Tag::to_string(*this); }
-
-    friend auto operator<<(std::ostream& o, strict_value const& v) -> std::ostream& { return o << v.to_string(); }
-
-    //  Bitwise operations
-
-    constexpr auto operator|=( strict_value const& that )       -> strict_value requires BitwiseOps { t |= that.t; return *this; }
-    constexpr auto operator&=( strict_value const& that )       -> strict_value requires BitwiseOps { t &= that.t; return *this; }
-    constexpr auto operator^=( strict_value const& that )       -> strict_value requires BitwiseOps { t ^= that.t; return *this; }
-
-    constexpr auto operator| ( strict_value const& that ) const -> strict_value requires BitwiseOps { return strict_value(t | that.t); }
-    constexpr auto operator& ( strict_value const& that ) const -> strict_value requires BitwiseOps { return strict_value(t & that.t); }
-    constexpr auto operator^ ( strict_value const& that ) const -> strict_value requires BitwiseOps { return strict_value(t ^ that.t); }
-
-    constexpr auto has       ( strict_value const& that ) const -> bool         requires BitwiseOps { return t & that.t; }
-
-    constexpr auto set       ( strict_value const& that )       -> void         requires BitwiseOps { t |=  that.t; }
-    constexpr auto clear     ( strict_value const& that )       -> void         requires BitwiseOps { t &= ~that.t; }
-};
-
-template <typename T, typename Tag>
-auto has_flags(strict_value<T, Tag, true> flags)
+template <typename T>
+auto has_flags(T flags)
 {
-    return [=](strict_value<T, Tag, true> value) { return (value & flags) == flags; };
+    return [=](T value) { return (value & flags) == flags; };
 }
 
 

Diff for: regression-tests/pure2-enum.cpp2

@@ -65,10 +65,17 @@ main: () = {
 
     x = skat_game::diamonds;        // ok, can assign one skat_game from another
 
+    std::cout << "file_attributes::cached.get_raw_value() is (file_attributes::cached.get_raw_value())$\n";
+    std::cout << "file_attributes::current.get_raw_value() is (file_attributes::current.get_raw_value())$\n";
+    std::cout << "file_attributes::obsolete.get_raw_value() is (file_attributes::obsolete.get_raw_value())$\n";
+    std::cout << "file_attributes::cached_and_current.get_raw_value() is (file_attributes::cached_and_current.get_raw_value())$\n";
+
     f: file_attributes = file_attributes::cached_and_current;
-    f &= file_attributes::cached  | file_attributes::obsolete;
+    f &= file_attributes::cached | file_attributes::obsolete;
+    std::cout << "f. get_raw_value() is (f. get_raw_value())$\n";
 
     f2 := file_attributes::cached;
+    std::cout << "f2.get_raw_value() is (f2.get_raw_value())$\n";
 
     std::cout << "f  is " << f  << "\n";
     std::cout << "f2 is " << f2 << "\n";
@@ -78,8 +85,8 @@ main: () = {
     f2.set(file_attributes::cached);
     std::cout << "f2 is " << f2 << "\n";
 
-    std::cout << "f  as int  is (f  as int )$\n";
-    std::cout << "f2 as int  is (f2 as int )$\n";
+    std::cout << "f. get_raw_value() is (f. get_raw_value())$\n";
+    std::cout << "f2.get_raw_value() is (f2.get_raw_value())$\n";
 
     std::cout << "f  is (f2) is (f  is (f2))$\n";
     std::cout << "f2 is (f ) is (f2 is (f ))$\n\n";
@@ -91,14 +98,14 @@ main: () = {
 
     std::cout << "f  is " << f  << "\n";
     std::cout << "f2 is " << f2 << "\n";
-    std::cout << "f  as int  is (f  as int )$\n";
-    std::cout << "f2 as int  is (f2 as int )$\n";
+    std::cout << "f. get_raw_value() is (f. get_raw_value())$\n";
+    std::cout << "f2.get_raw_value() is (f2.get_raw_value())$\n";
     std::cout << "f  == f2   is (f  == f2  )$\n";
     std::cout << "f  is (f2) is (f  is (f2))$\n";
     std::cout << "f2 is (f ) is (f2 is (f ))$\n";
     std::cout << "(f & f2) == f2 is ((f & f2) == f2)$\n";
 
-    std::cout << "inspecting: " << inspect f -> std::string {
+    std::cout << "inspecting f: " << inspect f -> std::string {
         is (file_attributes::current) = "exactly 'current'";
         is (cpp2::has_flags(f2))      = "includes all f2's flags ('cached' and 'current')";
         is _                          = "something else";

Diff for: regression-tests/test-results/clang-12/pure2-enum.cpp.execution

@@ -4,21 +4,27 @@ using << prints clubs
 with if else: clubs
 with inspect: clubs
 
+file_attributes::cached.get_raw_value() is 1
+file_attributes::current.get_raw_value() is 2
+file_attributes::obsolete.get_raw_value() is 4
+file_attributes::cached_and_current.get_raw_value() is 3
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (cached)
 f2 is (cached)
 f2 is (none)
 f2 is (cached)
-f  as int  is 1
-f2 as int  is 1
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (f2) is true
 f2 is (f ) is true
 
 f  is (cached, current, obsolete, cached_and_current)
 f2 is (cached, current, cached_and_current)
-f  as int  is 7
-f2 as int  is 3
+f. get_raw_value() is 7
+f2.get_raw_value() is 3
 f  == f2   is false
 f  is (f2) is false
 f2 is (f ) is false
 (f & f2) == f2 is true
-inspecting: includes all f2's flags ('cached' and 'current')
+inspecting f: includes all f2's flags ('cached' and 'current')

Diff for: regression-tests/test-results/gcc-10/pure2-enum.cpp.execution

@@ -4,21 +4,27 @@ using << prints clubs
 with if else: clubs
 with inspect: clubs
 
+file_attributes::cached.get_raw_value() is 1
+file_attributes::current.get_raw_value() is 2
+file_attributes::obsolete.get_raw_value() is 4
+file_attributes::cached_and_current.get_raw_value() is 3
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (cached)
 f2 is (cached)
 f2 is (none)
 f2 is (cached)
-f  as int  is 1
-f2 as int  is 1
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (f2) is true
 f2 is (f ) is true
 
 f  is (cached, current, obsolete, cached_and_current)
 f2 is (cached, current, cached_and_current)
-f  as int  is 7
-f2 as int  is 3
+f. get_raw_value() is 7
+f2.get_raw_value() is 3
 f  == f2   is false
 f  is (f2) is false
 f2 is (f ) is false
 (f & f2) == f2 is true
-inspecting: includes all f2's flags ('cached' and 'current')
+inspecting f: includes all f2's flags ('cached' and 'current')

Diff for: regression-tests/test-results/gcc-10/pure2-print.cpp.output

@@ -3,6 +3,8 @@ In file included from pure2-print.cpp:7:
 pure2-print.cpp2:7:1: note: in expansion of macro ‘CPP2_REQUIRES_’
 ../../../include/cpp2util.h:10005:33: error: expected initializer before ‘static_assert’
 pure2-print.cpp2:89:1: note: in expansion of macro ‘CPP2_REQUIRES_’
+pure2-print.cpp2:6:24: error: ‘constexpr const T outer::object_alias’ is not a static data member of ‘class outer’
+pure2-print.cpp2:6:31: error: template definition of non-template ‘constexpr const T outer::object_alias’
 pure2-print.cpp2:88:37: error: no declaration matches ‘void outer::print(std::ostream&, const Args& ...) requires  cpp2::cmp_greater_eq(sizeof (Args)..., 0)’
 pure2-print.cpp2:88:37: note: no functions named ‘void outer::print(std::ostream&, const Args& ...) requires  cpp2::cmp_greater_eq(sizeof (Args)..., 0)’
 pure2-print.cpp2:4:7: note: ‘class outer’ defined here

Diff for: regression-tests/test-results/gcc-13/pure2-enum.cpp.execution

@@ -4,21 +4,27 @@ using << prints clubs
 with if else: clubs
 with inspect: clubs
 
+file_attributes::cached.get_raw_value() is 1
+file_attributes::current.get_raw_value() is 2
+file_attributes::obsolete.get_raw_value() is 4
+file_attributes::cached_and_current.get_raw_value() is 3
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (cached)
 f2 is (cached)
 f2 is (none)
 f2 is (cached)
-f  as int  is 1
-f2 as int  is 1
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (f2) is true
 f2 is (f ) is true
 
 f  is (cached, current, obsolete, cached_and_current)
 f2 is (cached, current, cached_and_current)
-f  as int  is 7
-f2 as int  is 3
+f. get_raw_value() is 7
+f2.get_raw_value() is 3
 f  == f2   is false
 f  is (f2) is false
 f2 is (f ) is false
 (f & f2) == f2 is true
-inspecting: includes all f2's flags ('cached' and 'current')
+inspecting f: includes all f2's flags ('cached' and 'current')

Diff for: regression-tests/test-results/msvc-2022/pure2-enum.cpp.execution

@@ -4,21 +4,27 @@ using << prints clubs
 with if else: clubs
 with inspect: clubs
 
+file_attributes::cached.get_raw_value() is 1
+file_attributes::current.get_raw_value() is 2
+file_attributes::obsolete.get_raw_value() is 4
+file_attributes::cached_and_current.get_raw_value() is 3
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (cached)
 f2 is (cached)
 f2 is (none)
 f2 is (cached)
-f  as int  is 1
-f2 as int  is 1
+f. get_raw_value() is 1
+f2.get_raw_value() is 1
 f  is (f2) is true
 f2 is (f ) is true
 
 f  is (cached, current, obsolete, cached_and_current)
 f2 is (cached, current, cached_and_current)
-f  as int  is 7
-f2 as int  is 3
+f. get_raw_value() is 7
+f2.get_raw_value() is 3
 f  == f2   is false
 f  is (f2) is false
 f2 is (f ) is false
 (f & f2) == f2 is true
-inspecting: includes all f2's flags ('cached' and 'current')
+inspecting f: includes all f2's flags ('cached' and 'current')

Diff for: regression-tests/test-results/pure2-bugfix-for-max-munch.cpp

@@ -10,7 +10,7 @@
 
 //=== Cpp2 type definitions and function declarations ===========================
 
-template<typename T> auto static constexpr v = 0;
+template<typename T> auto inline constexpr v = 0;
 auto main() -> int;