reduce use of SFINAE to shorten error messages

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_});
+    }
   }
 };