Change cpp2::to_string catchall overload from C ... variadic to auto const&

This avoids errors about passing non-PODs to variadics (even though the parameter is never used), and ensures the catchall is used only for a single parameter (not inadvertently for a format string)

Closes #1136

Author: hsutter

include/cpp2util.h

@@ -264,6 +264,7 @@
     #endif
     #include <functional>
     #include <iostream>
+    #include <sstream>
     #include <iterator>
     #include <limits>
     #include <memory>
@@ -1189,7 +1190,7 @@ struct nonesuch_ {
 };
 constexpr inline nonesuch_ nonesuch;
 
-inline auto to_string(...) -> std::string
+inline auto to_string([[maybe_unused]] auto const& _) -> std::string
 {
     return "(customize me - no cpp2::to_string overload exists for this type)";
 }
@@ -1216,6 +1217,25 @@ inline auto to_string(T const& t) -> std::string
     return std::to_string(t);
 }
 
+////  We'd like to add this convenience, which would support std::filesystem::path() and similar.
+////
+////  TODO: As written, this one would make some cases ambiguous. We should rewrite these
+////  common to_string functions as a single template with 'if constexpr' alternatives,
+////  as was done with 'is', and then this will be simpler.
+//template<typename T>
+//inline auto to_string(T const& t) -> std::string
+//    requires std::is_convertible_v<decltype((std::stringstream() << t).str()), std::string>
+//{
+//    return (std::stringstream() << t).str();
+//}
+
+template<typename T>
+inline auto to_string(T const& t) -> std::string
+    requires std::is_convertible_v<decltype(t.to_string()), std::string>
+{
+    return t.to_string();
+}
+
 inline auto to_string(char const& t) -> std::string
 {
     return std::string{t};

regression-tests/test-results/clang-12-c++20/mixed-bugfix-for-ufcs-non-local.cpp.output

@@ -1,118 +1,118 @@
 mixed-bugfix-for-ufcs-non-local.cpp2:13:12: error: a lambda expression cannot appear in this context
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_1> bool inline constexpr v0{ false };// Fails on GCC ([GCC109781][]) and Clang 12 (a lambda expression cannot appear in this context)
            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:15:3: error: a lambda expression cannot appear in this context
 t<CPP2_UFCS_NONLOCAL(f)(o)> inline constexpr v1{ t<true>() };// Fails on Clang 12 (lambda in unevaluated context).
   ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:21:12: error: a lambda expression cannot appear in this context
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_2> auto g() -> void;
            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:23:42: error: a lambda expression cannot appear in this context
 auto g([[maybe_unused]] cpp2::impl::in<t<CPP2_UFCS_NONLOCAL(f)(o)>> unnamed_param_1) -> void;
                                          ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:27:29: error: a lambda expression cannot appear in this context
 [[nodiscard]] auto h() -> t<CPP2_UFCS_NONLOCAL(f)(o)>;
                             ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:31:12: error: a lambda expression cannot appear in this context
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_3> using a = bool;// Fails on GCC ([GCC109781][]) and Clang 12 (a lambda expression cannot appear in this context)
            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:33:12: error: a lambda expression cannot appear in this context
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_4> auto inline constexpr b{ false };// Fails on GCC ([GCC109781][]).
            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:35:13: error: a lambda expression cannot appear in this context
 using c = t<CPP2_UFCS_NONLOCAL(f)(o)>;// Fails on Clang 12 (lambda in unevaluated context) and Clang 12 (a lambda expression cannot appear in this context)
             ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:37:28: error: a lambda expression cannot appear in this context
 auto inline constexpr d{ t<CPP2_UFCS_NONLOCAL(f)(o)>() };// Fails on Clang 12 (lambda in unevaluated context).
                            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:21:12: error: a lambda expression cannot appear in this context
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_2> auto g() -> void{}// Fails on GCC ([GCC109781][]) and Clang 12 (a lambda expression cannot appear in this context)
            ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:23:42: error: a lambda expression cannot appear in this context
 auto g([[maybe_unused]] cpp2::impl::in<t<CPP2_UFCS_NONLOCAL(f)(o)>> unnamed_param_1) -> void{}// Fails on Clang 12 (lambda in unevaluated context).
                                          ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:27:29: error: a lambda expression cannot appear in this context
 [[nodiscard]] auto h() -> t<CPP2_UFCS_NONLOCAL(f)(o)> { return o;  }// Fails on Clang 12 (lambda in unevaluated context).
                             ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 mixed-bugfix-for-ufcs-non-local.cpp2:41:84: error: lambda expression in an unevaluated operand
   inline CPP2_CONSTEXPR bool u::c{ [](cpp2::impl::in<std::type_identity_t<decltype(CPP2_UFCS_NONLOCAL(f)(o))>> x) mutable -> auto { return x; }(true) };// Fails on Clang 12 (lambda in unevaluated context).
                                                                                    ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 13 errors generated.

regression-tests/test-results/clang-12-c++20/pure2-bugfix-for-ufcs-noexcept.cpp.output

@@ -1,10 +1,10 @@
 pure2-bugfix-for-ufcs-noexcept.cpp2:5:26: error: lambda expression in an unevaluated operand
   static_assert(noexcept(CPP2_UFCS(swap)(t(), t())));// Fails on Clang 12 (lambda in unevaluated context) and GCC 10 (static assertion failed)
                          ^
-../../../include/cpp2util.h:1166:59: note: expanded from macro 'CPP2_UFCS'
+../../../include/cpp2util.h:1167:59: note: expanded from macro 'CPP2_UFCS'
 #define CPP2_UFCS(...)                                    CPP2_UFCS_(&,CPP2_UFCS_EMPTY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 1 error generated.

regression-tests/test-results/clang-12-c++20/pure2-bugfix-for-ufcs-sfinae.cpp.output

@@ -1,19 +1,19 @@
 pure2-bugfix-for-ufcs-sfinae.cpp2:1:78: error: lambda expression in an unevaluated operand
 template<typename T> [[nodiscard]] auto f() -> std::type_identity_t<decltype(CPP2_UFCS_NONLOCAL(a)(T()))>;
                                                                              ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 pure2-bugfix-for-ufcs-sfinae.cpp2:1:78: error: lambda expression in an unevaluated operand
 template<typename T> [[nodiscard]] auto f() -> std::type_identity_t<decltype(CPP2_UFCS_NONLOCAL(a)(T()))>{}// Fails on Clang 12 (lambda in unevaluated context).
                                                                              ^
-../../../include/cpp2util.h:1171:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
+../../../include/cpp2util.h:1172:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^
-../../../include/cpp2util.h:1133:66: note: expanded from macro 'CPP2_UFCS_'
+../../../include/cpp2util.h:1134:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
 2 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:92&&) 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:93&&) 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.cpp2:5:11: note: candidates are: ‘element::element(const element&)’
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:20: note:                 ‘template<class auto:90> element::element(auto:90&&)’
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:20: note:                 ‘template<class auto:91> element::element(auto:91&&)’
 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:93&&) 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:94&&) 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.cpp2:6:16: note: candidates are: ‘void element::operator=(const element&)’
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:16: note:                 ‘template<class auto:91> element& element::operator=(auto:91&&)’
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:16: note:                 ‘template<class auto:92> element& element::operator=(auto:92&&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:1:7: note: ‘class element’ defined here