reduce use of SFINAE to shorten error messages (#44)

Remove SFINAE from user-facing functions to reduce the length of compiler
errors. For example, the error for `eq(a == b)` instead of `eq(a, b)` is

<details><summary>before</summary>

```
external/toolchains_llvm++llvm+llvm_toolchain_llvm/bin/../include/c++/v1/__type_traits/invoke.h:314:1: error: no type named 'type' in 'std::invoke_result<const (lambda at sel/test/constant_test.cpp:26:7) &, int>'
  314 | using invoke_result_t = typename invoke_result<_Fn, _Args...>::type;
      | ^~~~~
external/skytest+/src/test.hpp:27:16: note: in instantiation of template type alias 'invoke_result_t' requested here
   27 |           std::invoke_result_t<const remove_cvref_t<F>&, Args...>,
      |                ^
external/toolchains_llvm++llvm+llvm_toolchain_llvm/bin/../include/c++/v1/__type_traits/conjunction.h:60:58: note: in instantiation of template class 'skytest::detail::returns_result<(lambda at sel/test/constant_test.cpp:26:7), int>' requested here
   60 | struct conjunction<_Arg, _Args...> : conditional_t<!bool(_Arg::value), _Arg, conjunction<_Args...>> {};
      |                                                          ^
external/skytest+/src/test_param.hpp:117:7: note: in instantiation of template class 'std::conjunction<skytest::detail::returns_result<(lambda at sel/test/constant_test.cpp:26:7), int>, skytest::detail::returns_result<(lambda at sel/test/constant_test.cpp:26:7), float>, skytest::detail::returns_result<(lambda at sel/test/constant_test.cpp:26:7), double>>' requested here
  117 |     : std::conjunction<returns_result<F, param_reference_t<Is, Params>>...>
      |       ^
external/skytest+/src/test_param.hpp:127:7: note: in instantiation of template class 'skytest::detail::param_invocable_<std::integer_sequence<unsigned long, 0, 1, 2>, (lambda at sel/test/constant_test.cpp:26:7), skytest::constexpr_params_t<1, 2.000000e+00, 3.000000e+00>>' requested here
  127 |     : param_invocable_<param_sequence_t<Params>, F, param_resolve_t<Params>>
      |       ^
external/skytest+/src/test_param.hpp:131:43: note: in instantiation of template class 'skytest::detail::param_invocable<(lambda at sel/test/constant_test.cpp:26:7), skytest::param_ref_t<skytest::constexpr_params_instance>>' requested here
  131 | inline constexpr auto param_invocable_v = param_invocable<F, Params>::value;
      |                                           ^
external/skytest+/src/test_param.hpp:252:11: note: in instantiation of variable template specialization 'skytest::detail::param_invocable_v' requested here
  252 |           param_invocable_v<F, params_type> and
      |           ^
external/skytest+/src/test_param.hpp:256:8: note: while substituting prior template arguments into non-type template parameter [with F = (lambda at sel/test/constant_test.cpp:26:7)]
  256 |   auto operator=(const F& func) && -> void
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  257 |   {
      |   ~
  258 |     assign_impl(
      |     ~~~~~~~~~~~~
  259 |         param_sequence_t<params_type>{},
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  260 |         param_bound_closure<F, params_type>{func, params_});
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  261 |   }
      |   ~
sel/test/constant_test.cpp:25:38: note: while substituting deduced template arguments into function template 'operator=' [with F = (lambda at sel/test/constant_test.cpp:26:7), $1 = (no value)]
   25 |       constexpr_params<1, 2.0F, 3.0> =  //
      |                                      ^
sel/test/constant_test.cpp:25:38: error: no viable overloaded '='
   24 |   "constants always define a strong ordering"_ctest *
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   25 |       constexpr_params<1, 2.0F, 3.0> =  //
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
   26 |       []<class T>(T value) {
      |       ~~~~~~~~~~~~~~~~~~~~~~
   27 |         auto a = sel::constant{value};
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   28 |         auto b = sel::constant{T{2} * value};
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   29 |         return expect(
      |         ~~~~~~~~~~~~~~
   30 |             eq(std::strong_ordering::less == (a <=> b)) and     //
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   31 |             eq(std::strong_ordering::greater, (b <=> a)) and  //
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   32 |             eq(std::strong_ordering::equal, (b <=> b))
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   33 |         );
      |         ~~
   34 |       };
      |       ~
external/skytest+/src/test_param.hpp:256:8: note: candidate template ignored: substitution failure [with F = (lambda at sel/test/constant_test.cpp:26:7)]
  256 |   auto operator=(const F& func) && -> void
      |        ^
external/skytest+/src/test_param.hpp:269:8: note: candidate template ignored: substitution failure [with F = (lambda at sel/test/constant_test.cpp:26:7)]
  269 |   auto operator=(const F& func) && -> void
      |        ^
external/skytest+/src/test_param.hpp:174:7: note: candidate function (the implicit copy assignment operator) not viable: no known conversion from '(lambda at sel/test/constant_test.cpp:26:7)' to 'const parameterized_test<param_ref_t<skytest::constexpr_params_instance>, compile_time>' for 1st argument
  174 | class parameterized_test
      |       ^~~~~~~~~~~~~~~~~~
3 errors generated.
```

</details>

<details><summary>after</summary>

```
external/skytest+/src/detail/predicate.hpp:35:18: error: no matching function for call to object of type 'const std::equal_to<void>'
   35 |     auto value = static_cast<const F&>(*this)(std::as_const(args)...);
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
sel/test/constant_test.cpp:30:13: note: in instantiation of function template specialization 'skytest::detail::predicate<std::equal_to<void>, skytest::detail::type_name, skytest::detail::pred_fmt::eq_>::operator()<bool>' requested here
   30 |             eq(std::strong_ordering::less == (a <=> b)) and     //
      |             ^
external/skytest+/src/test_param.hpp:161:26: note: in instantiation of function template specialization 'main()::(anonymous class)::operator()<double>' requested here
  161 |       return [] { return func(get<I>(params)); };
      |                          ^
external/skytest+/src/test_param.hpp:161:17: note: while substituting into a lambda expression here
  161 |       return [] { return func(get<I>(params)); };
      |                 ^
external/skytest+/src/test_param.hpp:231:15: note: in instantiation of function template specialization 'skytest::detail::param_bound_static_closure<f, skytest::constexpr_params_instance>::operator[]<2UL>' requested here
  231 |               g[constant<Is>{}],
      |               ^
external/skytest+/src/test_param.hpp:253:7: note: in instantiation of function template specialization 'skytest::detail::parameterized_test<skytest::param_ref_t<skytest::constexpr_params_instance>, skytest::detail::test_style::compile_time>::assign_impl<0UL, 1UL, 2UL, skytest::detail::param_bound_static_closure<f, skytest::constexpr_params_instance>>' requested here
  253 |       assign_impl(
      |       ^
sel/test/constant_test.cpp:25:38: note: in instantiation of function template specialization 'skytest::detail::parameterized_test<skytest::param_ref_t<skytest::constexpr_params_instance>, skytest::detail::test_style::compile_time>::operator=<(lambda at sel/test/constant_test.cpp:26:7)>' requested here
   25 |       constexpr_params<1, 2.0F, 3.0> =  //
      |                                      ^
external/toolchains_llvm++llvm+llvm_toolchain_llvm/bin/../include/c++/v1/__functional/operations.h:308:60: note: candidate function template not viable: requires 2 arguments, but 1 was provided
  308 |   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI auto operator()(_T1&& __t, _T2&& __u) const
      |                                                            ^          ~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

</details>

Change-Id: Ibc7ab9d85dc732ae7b895093147584211ef713ac

Author: oliverlee

README.md

@@ -94,9 +94,9 @@ results in the follow build error (snippet):
 external/llvm_toolchain_llvm/bin/../include/c++/v1/__functional/operations.h:374:37: note: read of non-const variable 'n' is not allowed in a constant expression
   374 |     return std::forward<_T1>(__t) < std::forward<_T2>(__u);
       |                                     ^
-./src/detail/predicate.hpp:38:24: note: in call to 'static_cast<const std::less<void> &>(*this).operator()<const int &, const int &>(0, n)'
-   38 |     const auto value = static_cast<const F&>(*this)(std::as_const(args)...);
-      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+./src/detail/predicate.hpp:35:18: note: in call to 'static_cast<const std::less<void> &>(*this).operator()<const int &, const int &>(0, n)'
+   35 |     auto value = static_cast<const F&>(*this)(std::as_const(args)...);
+      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 example/ctest_fail.cpp:10:47: note: (skipping 2 calls in backtrace; use -fconstexpr-backtrace-limit=0 to see all)
    10 |   "read non-const"_ctest = [] { return expect(lt(0, n)); };
       |                                               ^

example/ctest_fail.log

@@ -10,9 +10,9 @@
 external/llvm_toolchain_llvm/bin/../include/c++/v1/__functional/operations.h:374:37: note: read of non-const variable 'n' is not allowed in a constant expression
   374 |     return std::forward<_T1>(__t) < std::forward<_T2>(__u);
       |                                     ^
-./src/detail/predicate.hpp:38:24: note: in call to 'static_cast<const std::less<void> &>(*this).operator()<const int &, const int &>(0, n)'
-   38 |     const auto value = static_cast<const F&>(*this)(std::as_const(args)...);
-      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+./src/detail/predicate.hpp:35:18: note: in call to 'static_cast<const std::less<void> &>(*this).operator()<const int &, const int &>(0, n)'
+   35 |     auto value = static_cast<const F&>(*this)(std::as_const(args)...);
+      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 example/ctest_fail.cpp:10:47: note: (skipping 2 calls in backtrace; use -fconstexpr-backtrace-limit=0 to see all)
    10 |   "read non-const"_ctest = [] { return expect(lt(0, n)); };
       |                                               ^

src/detail/predicate.hpp

@@ -29,15 +29,14 @@ struct predicate : F
   constexpr predicate(T&& f) : F{std::forward<T>(f)}
   {}
 
-  template <
-      class... Ts,
-      class =
-          std::enable_if_t<std::is_invocable_r_v<bool, const F&, const Ts&...>>>
+  template <class... Ts>
   constexpr auto operator()(Ts&&... args) const
   {
-    const auto value = static_cast<const F&>(*this)(std::as_const(args)...);
+    auto value = static_cast<const F&>(*this)(std::as_const(args)...);
+
     return relation<predicate, std::decay_t<Ts>...>{
-        std::tuple<std::decay_t<Ts>...>{std::forward<Ts>(args)...}, value};
+        std::tuple<std::decay_t<Ts>...>{std::forward<Ts>(args)...},
+        std::move(value)};
   }
 };
 

src/test_param.hpp

@@ -152,21 +152,14 @@ struct param_bound_closure
 template <const auto& func, const auto& params>
 struct param_bound_static_closure
 {
-  template <
-      std::size_t I,
-      class P = remove_cvref_t<decltype(params)>,
-      std::enable_if_t<not is_range_v<P>, bool> = true>
-  constexpr auto operator[](constant<I>) const
-  {
-    return [] { return func(get<I>(params)); };
-  }
-  template <
-      std::size_t I,
-      class P = remove_cvref_t<decltype(params)>,
-      std::enable_if_t<is_range_v<P>, bool> = true>
+  template <std::size_t I>
   constexpr auto operator[](constant<I>) const
   {
-    return [] { return func(params.begin()[I]); };
+    if constexpr (is_range_v<decltype(params)>) {
+      return [] { return func(params.begin()[I]); };
+    } else {
+      return [] { return func(get<I>(params)); };
+    }
   }
 };
 
@@ -246,34 +239,25 @@ class parameterized_test
       : params_{params}, basename_{basename}
   {}
 
-  template <
-      class F,
-      std::enable_if_t<
-          param_invocable_v<F, params_type> and
-              not(is_static_closure_constructible_v<F> and
-                  has_static_value_v<params_type>),
-          bool> = true>
+  template <class F>
   auto operator=(const F& func) && -> void
   {
-    assign_impl(
-        param_sequence_t<params_type>{},
-        param_bound_closure<F, params_type>{func, params_});
-  }
-  template <
-      class F,
-      std::enable_if_t<
-          param_invocable_v<F, params_type> and
-              is_static_closure_constructible_v<F> and
-              has_static_value_v<params_type>,
-          bool> = true>
-  auto operator=(const F& func) && -> void
-  {
-    static const auto f = func;
-    static constexpr const auto& p = params_type::value;
-
-    assign_impl(
-        std::make_index_sequence<static_size_v<params_type>>{},
-        param_bound_static_closure<f, p>{});
+    constexpr auto is_constexpr_invocable =
+        is_static_closure_constructible_v<F> and
+        has_static_value_v<params_type>;
+
+    if constexpr (is_constexpr_invocable) {
+      static const auto f = func;
+      static constexpr const auto& p = params_type::value;
+
+      assign_impl(
+          std::make_index_sequence<static_size_v<params_type>>{},
+          param_bound_static_closure<f, p>{});
+    } else {
+      assign_impl(
+          param_sequence_t<params_type>{},
+          param_bound_closure<F, params_type>{func, params_});
+    }
   }
 };