Make forward parameters of concrete type accept convertible/derived arguments

A `forward` parameter with a concrete type used to require an exact match, but can now accept arguments of convertible/derived types.

The previous commit's change to `@struct` requires this change, so that code like this continues to work after the previous commit:

    data: @struct type = {  name: std::string;  city: std::string;  }

    x: data = ( "Henry", "Memphis" );   // ok, uses conversions

Added `@struct<noforward>` option to allow using `@struct` with GCC 10, which doesn't support `forward` parameters. And use that in cppfront's own sources, just so cppfront itself continues building under GCC10. (Someday we'll drop GCC 10 as a supported compiler, but for now it's still doing pretty well except for the won't-backport `requires`-clause bug...)

Note that allowing the generated forwarding reference to bind to arguments of derived types was also suggested by @brevzin of @jumptrading in https://wg21.link/p2481 -- thanks Barry, for the Cpp2 suggestion and for the ISO C++ 'forward' parameter proposal paper!

More examples:

    Base: @polymorphic_base type = { }  // a Base type
    Derived: type = { this: Base; }     // a Base-derived type

    b: Base        = ();
    d: Derived     = ();
    s: std::string = ();

    f:(forward _: Base       ) = { }    // forward a Base object
    g:(forward _: std::string) = { }    // forward a std::string object

    main: () = {
        f( b );         // ok, b is-a Base
        f( d );         // ok, d is-a Base
        f( s );         // ERROR, s is-not-a Base

        g( s );         // ok, s is-a std::string
        g( "xyzzy" );   // ok, "xyzzy" is-convertible-to std::string
        g( b );         // ERROR, b is-not-a std::string
    }

Author: hsutter

include/cpp2regex.h2

@@ -40,7 +40,7 @@ bview  : <CharT> type == std::basic_string_view<CharT>;
 
 //  Structure for storing group information.
 //
-match_group: @struct <Iter> type = 
+match_group: @struct<noforward> <Iter> type = 
 {
     start: Iter = ();
     end: Iter = ();
@@ -50,7 +50,7 @@ match_group: @struct <Iter> type =
 
 //  Return value for every matcher.
 // 
-match_return: @struct <Iter> type = 
+match_return: @struct<noforward> <Iter> type = 
 {
     matched: bool = false;
     pos: Iter = ();
@@ -119,15 +119,15 @@ match_context: <CharT, Iter, max_groups: int> type =
 
 //  End function that returns a valid match.
 //
-true_end_func: @struct type = 
+true_end_func: @struct<noforward> type = 
 {
     operator(): (in this, cur, inout ctx) ctx..pass(cur);
 }
 
 
 //  Empty group reset function.
 //
-no_reset: @struct type = 
+no_reset: @struct<noforward> type = 
 {
     operator(): (this, inout _:) = {}
 }

regression-tests/test-results/clang-12-c++20/pure2-last-use.cpp.output

@@ -10,12 +10,18 @@ pure2-last-use.cpp2:273:30: note: to match this '<'
 pure2-last-use.cpp2:344:16: error: no template named 'move_only_function' in namespace 'std'
   public: std::move_only_function<int()> b; 
           ~~~~~^
-pure2-last-use.cpp2:348:94: error: no member named 'move_only_function' in namespace 'std'
-CPP2_REQUIRES_ (std::is_same_v<std::unique_ptr<int>, CPP2_TYPEOF(a_)> && std::is_same_v<std::move_only_function<int()>, CPP2_TYPEOF(b_)> && std::is_same_v<std::add_lvalue_reference_t<int>, CPP2_TYPEOF(c_)>) ;
-                                                                                        ~~~~~^
+pure2-last-use.cpp2:348:161: error: no member named 'move_only_function' in namespace 'std'
+CPP2_REQUIRES_ (std::is_convertible_v<CPP2_TYPEOF(a_), std::add_const_t<std::unique_ptr<int>>&> && std::is_convertible_v<CPP2_TYPEOF(b_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(c_), std::add_const_t<std::add_lvalue_reference_t<int>>&>) ;
+                                                                                                                                                           ~~~~~^
 ../../../include/cpp2util.h:10008:43: note: expanded from macro 'CPP2_REQUIRES_'
     #define CPP2_REQUIRES_(...) requires (__VA_ARGS__)
                                           ^~~~~~~~~~~
+pure2-last-use.cpp2:348:188: error: expected expression
+CPP2_REQUIRES_ (std::is_convertible_v<CPP2_TYPEOF(a_), std::add_const_t<std::unique_ptr<int>>&> && std::is_convertible_v<CPP2_TYPEOF(b_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(c_), std::add_const_t<std::add_lvalue_reference_t<int>>&>) ;
+                                                                                                                                                                                           ^
+pure2-last-use.cpp2:348:193: error: use of address-of-label extension outside of a function body
+CPP2_REQUIRES_ (std::is_convertible_v<CPP2_TYPEOF(a_), std::add_const_t<std::unique_ptr<int>>&> && std::is_convertible_v<CPP2_TYPEOF(b_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(c_), std::add_const_t<std::add_lvalue_reference_t<int>>&>) ;
+                                                                                                                                                                                                ^
 pure2-last-use.cpp2:773:69: error: no template named 'move_only_function' in namespace 'std'
 auto issue_888_1([[maybe_unused]] std::string unnamed_param_1, std::move_only_function<int([[maybe_unused]] cpp2::impl::in<int> unnamed_param_1)> size) -> void;
                                                                ~~~~~^
@@ -43,9 +49,15 @@ class issue_857_4 {
 pure2-last-use.cpp2:905:1: note: still within definition of 'issue_857_4' here
 namespace captures {
 ^
-pure2-last-use.cpp2:279:178: error: no member named 'move_only_function' in namespace 'std'
-requires (std::is_same_v<std::add_pointer_t<int()>, CPP2_TYPEOF(f_)> && std::is_same_v<std::add_pointer_t<int(cpp2::impl::in<int> in_)>, CPP2_TYPEOF(g_)> && std::is_same_v<std::move_only_function<int()>, CPP2_TYPEOF(mf_)> && std::is_same_v<std::move_only_function<int(cpp2::impl::in<int> in_)>, CPP2_TYPEOF(mg_)>) 
-                                                                                                                                                                            ~~~~~^
+pure2-last-use.cpp2:279:272: error: no member named 'move_only_function' in namespace 'std'
+requires (std::is_convertible_v<CPP2_TYPEOF(f_), std::add_const_t<std::add_pointer_t<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(g_), std::add_const_t<std::add_pointer_t<int(cpp2::impl::in<int> in_)>>&> && std::is_convertible_v<CPP2_TYPEOF(mf_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(mg_), std::add_const_t<std::move_only_function<int(cpp2::impl::in<int> in_)>>&>) 
+                                                                                                                                                                                                                                                                          ~~~~~^
+pure2-last-use.cpp2:279:299: error: expected expression
+requires (std::is_convertible_v<CPP2_TYPEOF(f_), std::add_const_t<std::add_pointer_t<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(g_), std::add_const_t<std::add_pointer_t<int(cpp2::impl::in<int> in_)>>&> && std::is_convertible_v<CPP2_TYPEOF(mf_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(mg_), std::add_const_t<std::move_only_function<int(cpp2::impl::in<int> in_)>>&>) 
+                                                                                                                                                                                                                                                                                                          ^
+pure2-last-use.cpp2:279:304: error: use of address-of-label extension outside of a function body
+requires (std::is_convertible_v<CPP2_TYPEOF(f_), std::add_const_t<std::add_pointer_t<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(g_), std::add_const_t<std::add_pointer_t<int(cpp2::impl::in<int> in_)>>&> && std::is_convertible_v<CPP2_TYPEOF(mf_), std::add_const_t<std::move_only_function<int()>>&> && std::is_convertible_v<CPP2_TYPEOF(mg_), std::add_const_t<std::move_only_function<int(cpp2::impl::in<int> in_)>>&>) 
+                                                                                                                                                                                                                                                                                                               ^
 pure2-last-use.cpp2:278:14: error: out-of-line definition of 'issue_857_4' does not match any declaration in 'issue_857_4'
 issue_857_4::issue_857_4(auto&& f_, auto&& g_, auto&& mf_, auto&& mg_)
              ^~~~~~~~~~~
@@ -58,29 +70,5 @@ pure2-last-use.cpp2:282:272: error: member initializer 'mf' does not name a non-
 pure2-last-use.cpp2:283:272: error: member initializer 'mg' does not name a non-static data member or base class
                                                                                                                                                                                                                                                                              , mg{ CPP2_FORWARD(mg_) }{}
                                                                                                                                                                                                                                                                                ^~~~~~~~~~~~~~~~~~~~~~~
-pure2-last-use.cpp2:332:8: error: incomplete type 'issue_857_5' named in nested name specifier
-  auto issue_857_5::f() && -> void { f_copy(std::move(cpp2::move(*this).a));  }
-       ^~~~~~~~~~~~~
-pure2-last-use.cpp2:331:7: note: forward declaration of 'issue_857_5'
-class issue_857_5;
-      ^
-pure2-last-use.cpp2:334:3: error: incomplete type 'issue_857_5' named in nested name specifier
-  issue_857_5::issue_857_5(auto&& a_)
-  ^~~~~~~~~~~~~
-pure2-last-use.cpp2:331:7: note: forward declaration of 'issue_857_5'
-class issue_857_5;
-      ^
-pure2-last-use.cpp2:334:3: error: incomplete type 'issue_857_5' named in nested name specifier
-  issue_857_5::issue_857_5(auto&& a_)
-  ^~~~~~~~~~~~~
-pure2-last-use.cpp2:331:7: note: forward declaration of 'issue_857_5'
-class issue_857_5;
-      ^
-pure2-last-use.cpp2:338:6: error: incomplete type 'issue_857_5' named in nested name specifier
-auto issue_857_5::operator=(auto&& a_) -> issue_857_5& 
-     ^~~~~~~~~~~~~
-pure2-last-use.cpp2:331:7: note: forward declaration of 'issue_857_5'
-class issue_857_5;
-      ^
 fatal error: too many errors emitted, stopping now [-ferror-limit=]
 20 errors generated.

regression-tests/test-results/gcc-10-c++20/pure2-bugfix-for-requires-clause-in-forward-declaration.cpp.output

@@ -6,12 +6,12 @@ pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:46: error: expect
 In file included from pure2-bugfix-for-requires-clause-in-forward-declaration.cpp:7:
 ../../../include/cpp2util.h:10005:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:4:1: note: in expansion of macro ‘CPP2_REQUIRES_’
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:3: error: no declaration matches ‘element::element(auto:98&&) requires  is_same_v<std::__cxx11::string, typename std::remove_cv<typename std::remove_reference<decltype(element::__ct ::n)>::type>::type>’
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:3: error: no declaration matches ‘element::element(auto:98&&) requires  is_convertible_v<typename std::remove_cv<typename std::remove_reference<decltype(element::__ct ::n)>::type>::type, const std::__cxx11::add_const_t<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&>’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:5:11: note: candidates are: ‘element::element(const element&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:20: note:                 ‘template<class auto:96> element::element(auto:96&&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:1:7: note: ‘class element’ defined here
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:5:78: error: expected unqualified-id before ‘{’ token
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:8: error: no declaration matches ‘element& element::operator=(auto:99&&) requires  is_same_v<std::__cxx11::string, typename std::remove_cv<typename std::remove_reference<decltype(element::operator=::n)>::type>::type>’
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:8: error: no declaration matches ‘element& element::operator=(auto:99&&) requires  is_convertible_v<typename std::remove_cv<typename std::remove_reference<decltype(element::operator=::n)>::type>::type, const std::__cxx11::add_const_t<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&>’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:6:16: note: candidates are: ‘void element::operator=(const element&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:16: note:                 ‘template<class auto:97> element& element::operator=(auto:97&&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:1:7: note: ‘class element’ defined here

regression-tests/test-results/gcc-10-c++20/pure2-bugfix-for-ufcs-arguments.cpp.output

@@ -6,13 +6,13 @@ pure2-bugfix-for-ufcs-arguments.cpp2:74:39: error: expected ‘;’ at end of me
 In file included from pure2-bugfix-for-ufcs-arguments.cpp:7:
 ../../../include/cpp2util.h:10005:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
 pure2-bugfix-for-ufcs-arguments.cpp2:75:1: note: in expansion of macro ‘CPP2_REQUIRES_’
-pure2-bugfix-for-ufcs-arguments.cpp2:72:3: error: no declaration matches ‘B::B(auto:116&&) requires  is_same_v<A, typename std::remove_cv<typename std::remove_reference<decltype(B::__ct ::m_)>::type>::type>’
+pure2-bugfix-for-ufcs-arguments.cpp2:72:3: error: no declaration matches ‘B::B(auto:116&&) requires  is_convertible_v<typename std::remove_cv<typename std::remove_reference<decltype(B::__ct ::m_)>::type>::type, const add_const_t<A>&>’
 pure2-bugfix-for-ufcs-arguments.cpp2:68:7: note: candidates are: ‘constexpr B::B(B&&)’
 pure2-bugfix-for-ufcs-arguments.cpp2:68:7: note:                 ‘constexpr B::B(const B&)’
 pure2-bugfix-for-ufcs-arguments.cpp2:71:11: note:                 ‘template<class auto:104> B::B(auto:104&&)’
 pure2-bugfix-for-ufcs-arguments.cpp2:68:7: note: ‘class B’ defined here
 pure2-bugfix-for-ufcs-arguments.cpp2:74:81: error: expected unqualified-id before ‘{’ token
-pure2-bugfix-for-ufcs-arguments.cpp2:76:6: error: no declaration matches ‘B& B::operator=(auto:117&&) requires  is_same_v<A, typename std::remove_cv<typename std::remove_reference<decltype(B::operator=::m_)>::type>::type>’
+pure2-bugfix-for-ufcs-arguments.cpp2:76:6: error: no declaration matches ‘B& B::operator=(auto:117&&) requires  is_convertible_v<typename std::remove_cv<typename std::remove_reference<decltype(B::operator=::m_)>::type>::type, const add_const_t<A>&>’
 pure2-bugfix-for-ufcs-arguments.cpp2:68:7: note: candidates are: ‘constexpr B& B::operator=(B&&)’
 pure2-bugfix-for-ufcs-arguments.cpp2:68:7: note:                 ‘constexpr B& B::operator=(const B&)’
 pure2-bugfix-for-ufcs-arguments.cpp2:74:14: note:                 ‘template<class auto:105> B& B::operator=(auto:105&&)’