Add CPP_lambda macro

src/backports/concepts.h

@@ -17,7 +17,10 @@
 // resolution.
 //
 // `QL_CONCEPT_OR_TYPENAME(arg)`: expands to `arg` in C++20 mode, and to
-// `typename` in C++17 mode. Example usage:
+// `typename` in C++17 mode.
+//
+// `CPP_lambda(capture)(arg)(requires ...)`: Expands lambda to use
+//  `requires` in C++20 mode and `std::enable_if_t` in C++17 mode.
 //
 // Example usages:
 //
@@ -29,15 +32,55 @@
 //
 // `template <QL_CONCEPT_OR_TYPENAME(ql::same_as<int>) T> void f(){...}`
 //
+// `auto myLambda = CPP_lambda(someCapture)(someArg)(requires
+// ranges::same_as<decltype(someArg), int>) {...}`
+//
 // NOTE: The macros are variadic to allow for commas in the argument, like in
 // the second example above.
 
 #ifdef QLEVER_CPP_17
 #define QL_CONCEPT_OR_NOTHING(...)
 #define QL_CONCEPT_OR_TYPENAME(...) typename
+
+#define CPP_lambda CPP_lambda_sfinae
+
+#define CPP_LAMBDA_SFINAE_ARGS(...) \
+    (__VA_ARGS__ CPP_LAMBDA_SFINAE_AUX_
+
+#define CPP_LAMBDA_SFINAE_AUX_WHICH_(FIRST, ...) \
+  CPP_PP_EVAL(CPP_PP_CHECK, CPP_PP_CAT(CPP_LAMBDA_SFINAE_PROBE_CONCEPT_, FIRST))
+
+#define CPP_LAMBDA_SFINAE_AUX_(...)                       \
+  CPP_PP_CAT(CPP_LAMBDA_SFINAE_AUX_,                      \
+             CPP_LAMBDA_SFINAE_AUX_WHICH_(__VA_ARGS__, )) \
+  (__VA_ARGS__)
+
+#define CPP_LAMBDA_SFINAE_AUX_0(...) ,                           \
+    std::enable_if_t<                                            \
+        CPP_PP_CAT(CPP_LAMBDA_SFINAE_AUX_3_, __VA_ARGS__)        \
+        >* = nullptr)
+
+#define CPP_lambda_sfinae(...)     \
+  CPP_PP_IGNORE_CXX2A_COMPAT_BEGIN \
+  [__VA_ARGS__] CPP_LAMBDA_SFINAE_ARGS
+
+#define CPP_LAMBDA_SFINAE_AUX_3_requires
+
 #else
 #define QL_CONCEPT_OR_NOTHING(...) __VA_ARGS__
 #define QL_CONCEPT_OR_TYPENAME(...) __VA_ARGS__
+
+#define CPP_lambda(...) [__VA_ARGS__] CPP_LAMBDA_ARGS
+
+#define CPP_LAMBDA_ARGS(...) (__VA_ARGS__) CPP_LAMBDA_AUX_
+
+#define CPP_LAMBDA_AUX_(...) \
+  CPP_PP_CAT(CPP_LAMBDA_AUX_, CPP_LAMBDA_AUX_WHICH_(__VA_ARGS__, ))(__VA_ARGS__)
+
+#define CPP_LAMBDA_AUX_WHICH_(FIRST, ...) CPP_PP_EVAL(CPP_PP_CHECK, FIRST)
+
+#define CPP_LAMBDA_AUX_0(...) __VA_ARGS__
+
 #endif
 
 // The namespace `ql::concepts` includes concepts that are contained in the

src/engine/sparqlExpressions/SparqlExpressionGenerators.h

@@ -105,20 +105,18 @@ CPP_template(typename Input, typename Transformation = std::identity)(
 
 /// Generate `numItems` many values from the `input` and apply the
 /// `valueGetter` to each of the values.
-inline auto valueGetterGenerator = []<typename ValueGetter, typename Input>(
-                                       size_t numElements,
-                                       EvaluationContext* context,
-                                       Input&& input,
-                                       ValueGetter&& valueGetter) {
-  CPP_assert(SingleExpressionResult<Input>);
-  auto transformation = [context, valueGetter]<typename I>(I&& i)
-      -> CPP_ret(decltype(valueGetter(AD_FWD(i), context)))(
-          requires ranges::invocable<ValueGetter, I&&, EvaluationContext*>) {
+inline auto valueGetterGenerator = CPP_lambda()(
+    size_t numElements, EvaluationContext* context, auto&& input,
+    auto&& valueGetter)(requires SingleExpressionResult<decltype(input)>) {
+  auto transformation = CPP_lambda(context, valueGetter)(auto&& i)(
+      requires ranges::invocable<decltype(valueGetter), decltype(i),
+                                 EvaluationContext*>) {
     context->cancellationHandle_->throwIfCancelled();
     return valueGetter(AD_FWD(i), context);
   };
-  return makeGenerator(std::forward<Input>(input), numElements, context,
-                       transformation);
+
+  return makeGenerator(std::forward<decltype(input)>(input), numElements,
+                       context, transformation);
 };
 
 /// Do the following `numItems` times: Obtain the next elements e_1, ..., e_n