From 1570033d07eb625dd3c2624c866eeb241f8639ef Mon Sep 17 00:00:00 2001
From: gpicciuca <dangertauros@gmail.com>
Date: Fri, 14 Feb 2025 08:46:53 +0100
Subject: [PATCH] Backport concepts in util/parser module for C++17 (#1799)

Replace all C++20 concept usages in the `util/parser` module using v3-range macros and/or the custom made ones from `backport/concepts`.
---
 src/backports/concepts.h                      | 17 +++++++
 src/backports/cppTemplate2.h                  | 44 ++++++++++++++++
 src/parser/GeoPoint.h                         |  5 +-
 src/parser/Iri.h                              |  5 +-
 src/parser/Literal.h                          |  6 ++-
 src/parser/LiteralOrIri.h                     |  6 +--
 src/parser/ParallelParseBuffer.h              | 18 +++++--
 src/parser/RdfParser.h                        |  5 +-
 src/parser/TokenizerCtre.h                    |  1 +
 src/parser/TripleComponent.h                  | 40 ++++++++++-----
 .../sparqlParser/SparqlQleverVisitor.cpp      | 51 ++++++++++---------
 src/parser/sparqlParser/SparqlQleverVisitor.h | 20 ++++----
 test/backports/ConceptsTest.cpp               | 15 ++++++
 13 files changed, 172 insertions(+), 61 deletions(-)

diff --git a/src/backports/concepts.h b/src/backports/concepts.h
index 17a2f3e5c4..942466f23d 100644
--- a/src/backports/concepts.h
+++ b/src/backports/concepts.h
@@ -21,6 +21,13 @@
 // `QL_CONCEPT_OR_TYPENAME(arg)`: expands to `arg` in C++20 mode, and to
 // `typename` in C++17 mode. Example usage:
 //
+// `CPP_lambda(capture)(arg)(requires ...)`: Expands lambda to use
+//  `requires` in C++20 mode and `std::enable_if_t` in C++17 mode.
+//
+// `CPP_template_lambda(capture)(typenames...)(arg)(requires ...)`: Expands
+// lambda with (C++20) explicit gemplate parameters  to use
+//  `requires` in C++20 mode and `std::enable_if_t` in C++17 mode.
+//
 // Example usages:
 //
 // `QL_CONCEPT_OR_NOTHING(std::view) auto x = someFunction();`
@@ -31,6 +38,12 @@
 //
 // `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>) {...}`
+//
+// `auto myLambda2 = CPP_lambda(someCapture)(typename F)(F arg)(requires
+// ranges::same_as<F, int>) {...}`
+//
 // NOTE: The macros are variadic to allow for commas in the argument, like in
 // the second example above.
 
@@ -48,6 +61,8 @@
 #define CPP_and_2_def CPP_and_2_def_sfinae
 #define CPP_variadic_template CPP_template_NO_DEFAULT_SFINAE
 #define CPP_member_def CPP_member_def_sfinae
+#define CPP_lambda CPP_lambda_sfinae
+#define CPP_template_lambda CPP_template_lambda_sfinae
 #else
 #define QL_CONCEPT_OR_NOTHING(...) __VA_ARGS__
 #define QL_CONCEPT_OR_TYPENAME(...) __VA_ARGS__
@@ -57,6 +72,8 @@
 #define CPP_and_2_def CPP_and
 #define CPP_variadic_template CPP_template
 #define CPP_member_def CPP_member
+#define CPP_lambda CPP_LAMBDA_20
+#define CPP_template_lambda CPP_TEMPLATE_LAMBDA_20
 #endif
 
 // The namespace `ql::concepts` includes concepts that are contained in the
diff --git a/src/backports/cppTemplate2.h b/src/backports/cppTemplate2.h
index a5c89b2c16..171dc5dca4 100644
--- a/src/backports/cppTemplate2.h
+++ b/src/backports/cppTemplate2.h
@@ -58,3 +58,47 @@
 
 #define CPP_member_def_sfinae \
   template <bool (&CPP_true_fn)(::concepts::detail::xNil)>
+
+#define CPP_LAMBDA_20(...) [__VA_ARGS__] CPP_LAMBDA_ARGS
+
+#define CPP_TEMPLATE_LAMBDA_20(...) [__VA_ARGS__] CPP_TEMPLATE_LAMBDA_ARGS
+
+// The internals of the `CPP_lambda` template
+#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_TEMPLATE_LAMBDA_ARGS_sfinae(...) \
+  <__VA_ARGS__> CPP_LAMBDA_SFINAE_ARGS
+
+#define CPP_template_lambda_sfinae(...) \
+  [__VA_ARGS__] CPP_TEMPLATE_LAMBDA_ARGS_sfinae
+
+#define CPP_LAMBDA_SFINAE_AUX_3_requires
+
+#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__
+
+#define CPP_TEMPLATE_LAMBDA_ARGS(...) <__VA_ARGS__> CPP_LAMBDA_ARGS
diff --git a/src/parser/GeoPoint.h b/src/parser/GeoPoint.h
index 9a4a1a6eb1..d3cc9d1e82 100644
--- a/src/parser/GeoPoint.h
+++ b/src/parser/GeoPoint.h
@@ -40,8 +40,9 @@ class GeoPoint {
  public:
   using T = uint64_t;
 
-  template <typename H>
-  friend H AbslHashValue(H h, const std::same_as<GeoPoint> auto& g) {
+  CPP_template(typename H,
+               typename G)(requires std::same_as<G, GeoPoint>) friend H
+      AbslHashValue(H h, const G& g) {
     return H::combine(std::move(h), g.lat_, g.lng_);
   }
 
diff --git a/src/parser/Iri.h b/src/parser/Iri.h
index f302ae26cc..dc20e479fa 100644
--- a/src/parser/Iri.h
+++ b/src/parser/Iri.h
@@ -7,6 +7,7 @@
 
 #include <string_view>
 
+#include "backports/concepts.h"
 #include "parser/NormalizedString.h"
 
 namespace ad_utility::triple_component {
@@ -30,8 +31,8 @@ class Iri {
  public:
   // A default constructed IRI is empty.
   Iri() = default;
-  template <typename H>
-  friend H AbslHashValue(H h, const std::same_as<Iri> auto& iri) {
+  CPP_template(typename H, typename I)(requires std::same_as<I, Iri>) friend H
+      AbslHashValue(H h, const I& iri) {
     return H::combine(std::move(h), iri.iri_);
   }
   bool operator==(const Iri&) const = default;
diff --git a/src/parser/Literal.h b/src/parser/Literal.h
index 5367c261ae..a02c4cb100 100644
--- a/src/parser/Literal.h
+++ b/src/parser/Literal.h
@@ -7,6 +7,7 @@
 #include <optional>
 #include <variant>
 
+#include "backports/concepts.h"
 #include "parser/Iri.h"
 #include "parser/NormalizedString.h"
 
@@ -41,8 +42,9 @@ class Literal {
   }
 
  public:
-  template <typename H>
-  friend H AbslHashValue(H h, const std::same_as<Literal> auto& literal) {
+  CPP_template(typename H,
+               typename L)(requires std::same_as<L, Literal>) friend H
+      AbslHashValue(H h, const L& literal) {
     return H::combine(std::move(h), literal.content_);
   }
   bool operator==(const Literal&) const = default;
diff --git a/src/parser/LiteralOrIri.h b/src/parser/LiteralOrIri.h
index 04cc4c32af..18be5d79a5 100644
--- a/src/parser/LiteralOrIri.h
+++ b/src/parser/LiteralOrIri.h
@@ -50,9 +50,9 @@ class alignas(16) LiteralOrIri {
       return LiteralOrIri{Iri::fromStringRepresentation(std::move(internal))};
     }
   }
-  template <typename H>
-  friend H AbslHashValue(H h,
-                         const std::same_as<LiteralOrIri> auto& literalOrIri) {
+  CPP_template(typename H,
+               typename L)(requires std::same_as<L, LiteralOrIri>) friend H
+      AbslHashValue(H h, const L& literalOrIri) {
     return H::combine(std::move(h), literalOrIri.data_);
   }
   bool operator==(const LiteralOrIri&) const = default;
diff --git a/src/parser/ParallelParseBuffer.h b/src/parser/ParallelParseBuffer.h
index 6ad203a499..53e1b33452 100644
--- a/src/parser/ParallelParseBuffer.h
+++ b/src/parser/ParallelParseBuffer.h
@@ -15,6 +15,16 @@ using std::array;
 using std::string;
 using std::vector;
 
+namespace ad_utility::detail {
+
+template <typename Parser>
+CPP_requires(ParserGetBatchRequires, requires(Parser& p)(p.getBatch()));
+
+template <typename Parser>
+CPP_concept ParserGetBatch = CPP_requires_ref(ParserGetBatchRequires, Parser);
+
+}  // namespace ad_utility::detail
+
 /**
  * A wrapper to make the different Parsers interfaces compatible with the
  * parallel pipeline
@@ -57,11 +67,9 @@ class ParserBatcher {
     }
   }
 
-  // The second requires evaluates to `true` only if the `Parser` type has a
-  // getBatch() member function. The first requires enables this function only
-  // if the second "requires" evaluates to true
-  std::optional<std::vector<TurtleTriple>> getBatch()
-      requires requires(Parser& p) { p.getBatch(); } {
+  CPP_member auto getBatch()
+      -> CPP_ret(std::optional<std::vector<TurtleTriple>>)(
+          requires ad_utility::detail::ParserGetBatch<Parser>) {
     if (m_numTriplesAlreadyParsed >= m_maxNumTriples) {
       return std::nullopt;
     }
diff --git a/src/parser/RdfParser.h b/src/parser/RdfParser.h
index 063cfc35e3..1fb36871c9 100644
--- a/src/parser/RdfParser.h
+++ b/src/parser/RdfParser.h
@@ -459,8 +459,9 @@ class NQuadParser : public TurtleParser<Tokenizer_T> {
  * Parses turtle from std::string. Used to perform unit tests for
  * the different parser rules
  */
-template <std::derived_from<RdfParserBase> Parser>
-class RdfStringParser : public Parser {
+CPP_template(typename Parser)(
+    requires std::derived_from<Parser, RdfParserBase>) class RdfStringParser
+    : public Parser {
  public:
   using Parser::getLine;
   using Parser::prefixMap_;
diff --git a/src/parser/TokenizerCtre.h b/src/parser/TokenizerCtre.h
index 1801b448f1..28c2e48731 100644
--- a/src/parser/TokenizerCtre.h
+++ b/src/parser/TokenizerCtre.h
@@ -336,6 +336,7 @@ class TokenizerCtre : public SkipWhitespaceAndCommentsMixin<TokenizerCtre> {
    * <true, matchContent> on success and <false, emptyStringView> on failure
    */
   struct Matcher {
+    // TODO<C++17, joka921>: Template-value feature not available in C++17
     template <auto& regex>
     static std::pair<bool, std::string_view> process(
         std::string_view data) noexcept {
diff --git a/src/parser/TripleComponent.h b/src/parser/TripleComponent.h
index 84abe54f7e..2b6ced96d9 100644
--- a/src/parser/TripleComponent.h
+++ b/src/parser/TripleComponent.h
@@ -22,6 +22,17 @@
 #include "util/Exception.h"
 #include "util/Forward.h"
 
+namespace ad_utility::detail {
+
+template <typename T, typename U>
+CPP_requires(MoveAssignableWithRequires, requires(T t, U&& u)(t = u));
+
+template <typename T, typename U>
+CPP_concept MoveAssignableWith =
+    CPP_requires_ref(MoveAssignableWithRequires, T, U);
+
+}  // namespace ad_utility::detail
+
 /// A wrapper around a `std::variant` that can hold the different types that the
 /// subject, predicate, or object of a triple can have in the Turtle Parser.
 /// Those currently are `double` (xsd:double and xsd:decimal), `int64_t`
@@ -56,10 +67,11 @@ class TripleComponent {
   TripleComponent() = default;
   /// Construct from anything that is able to construct the underlying
   /// `Variant`.
-  template <typename FirstArg, typename... Args>
-  requires(!std::same_as<std::remove_cvref_t<FirstArg>, TripleComponent> &&
-           std::is_constructible_v<Variant, FirstArg &&, Args && ...>)
-  TripleComponent(FirstArg&& firstArg, Args&&... args)
+  CPP_template(typename FirstArg, typename... Args)(
+      requires CPP_NOT(
+          std::same_as<std::remove_cvref_t<FirstArg>, TripleComponent>) &&
+      std::is_constructible_v<Variant, FirstArg&&, Args&&...>)
+      TripleComponent(FirstArg&& firstArg, Args&&... args)
       : _variant(AD_FWD(firstArg), AD_FWD(args)...) {
     if (isString()) {
       // Storing variables and literals as strings is deprecated. The following
@@ -83,9 +95,9 @@ class TripleComponent {
 
   /// Assignment for types that can be directly assigned to the underlying
   /// variant.
-  template <typename T>
-  requires requires(Variant v, T&& t) { _variant = t; }
-  TripleComponent& operator=(T&& value) {
+  CPP_template(typename T)(requires std::is_assignable_v<Variant, T&&>)
+      TripleComponent&
+      operator=(T&& value) {
     _variant = AD_FWD(value);
     checkThatStringIsValid();
     return *this;
@@ -103,10 +115,11 @@ class TripleComponent {
   TripleComponent& operator=(TripleComponent&&) = default;
 
   /// Make a `TripleComponent` directly comparable to the underlying types.
-  template <typename T>
-  requires requires(T&& t) { _variant == t; }
-  bool operator==(const T& other) const {
-    return _variant == other;
+  CPP_template(typename T)(
+      requires ad_utility::SameAsAnyTypeIn<T, Variant>) bool
+  operator==(const T& other) const {
+    auto ptr = std::get_if<T>(&_variant);
+    return ptr && *ptr == other;
   }
 
   /// Equality comparison between two `TripleComponent`s.
@@ -117,8 +130,9 @@ class TripleComponent {
   /// overload would also be eligible for the contained types that are
   /// implicitly convertible to `TripleComponent` which would lead to strange
   /// bugs.
-  template <typename H>
-  friend H AbslHashValue(H h, const std::same_as<TripleComponent> auto& tc) {
+  CPP_template(typename H,
+               typename TC)(requires std::same_as<TC, TripleComponent>) friend H
+      AbslHashValue(H h, const TC& tc) {
     return H::combine(std::move(h), tc._variant);
   }
 
diff --git a/src/parser/sparqlParser/SparqlQleverVisitor.cpp b/src/parser/sparqlParser/SparqlQleverVisitor.cpp
index f3d051145d..85ced1ed73 100644
--- a/src/parser/sparqlParser/SparqlQleverVisitor.cpp
+++ b/src/parser/sparqlParser/SparqlQleverVisitor.cpp
@@ -7,7 +7,6 @@
 
 #include "parser/sparqlParser/SparqlQleverVisitor.h"
 
-#include <absl/strings/str_join.h>
 #include <absl/strings/str_split.h>
 
 #include <string>
@@ -2308,20 +2307,22 @@ ExpressionPtr Visitor::visit([[maybe_unused]] Parser::BuiltInCallContext* ctx) {
   using namespace sparqlExpression;
   // Create the expression using the matching factory function from
   // `NaryExpression.h`.
-  auto createUnary = [&argList]<typename Function>(Function function)
-      requires std::is_invocable_r_v<ExpressionPtr, Function, ExpressionPtr> {
+  auto createUnary = CPP_template_lambda(&argList)(typename F)(F function)(
+      requires std::is_invocable_r_v<ExpressionPtr, F, ExpressionPtr>) {
     AD_CORRECTNESS_CHECK(argList.size() == 1, argList.size());
     return function(std::move(argList[0]));
   };
-  auto createBinary = [&argList]<typename Function>(Function function)
-      requires std::is_invocable_r_v<ExpressionPtr, Function, ExpressionPtr,
-                                     ExpressionPtr> {
+
+  auto createBinary = CPP_template_lambda(&argList)(typename F)(F function)(
+      requires std::is_invocable_r_v<ExpressionPtr, F, ExpressionPtr,
+                                     ExpressionPtr>) {
     AD_CORRECTNESS_CHECK(argList.size() == 2);
     return function(std::move(argList[0]), std::move(argList[1]));
   };
-  auto createTernary = [&argList]<typename Function>(Function function)
-      requires std::is_invocable_r_v<ExpressionPtr, Function, ExpressionPtr,
-                                     ExpressionPtr, ExpressionPtr> {
+
+  auto createTernary = CPP_template_lambda(&argList)(typename F)(F function)(
+      requires std::is_invocable_r_v<ExpressionPtr, F, ExpressionPtr,
+                                     ExpressionPtr, ExpressionPtr>) {
     AD_CORRECTNESS_CHECK(argList.size() == 3);
     return function(std::move(argList[0]), std::move(argList[1]),
                     std::move(argList[2]));
@@ -2475,9 +2476,11 @@ SparqlExpression::Ptr Visitor::visit(Parser::StrReplaceExpressionContext* ctx) {
     reportError(
         ctx,
         "REPLACE expressions with four arguments (including regex flags) are "
-        "currently not supported by QLever. You can however incorporate flags "
+        "currently not supported by QLever. You can however incorporate "
+        "flags "
         "directly into a regex by prepending `(?<flags>)` to your regex. For "
-        "example `(?i)[ei]` will match the regex `[ei]` in a case-insensitive "
+        "example `(?i)[ei]` will match the regex `[ei]` in a "
+        "case-insensitive "
         "way.");
   }
   return sparqlExpression::makeReplaceExpression(std::move(children.at(0)),
@@ -2535,8 +2538,8 @@ ExpressionPtr Visitor::visit(Parser::AggregateContext* ctx) {
     std::string separator;
     if (ctx->string()) {
       // TODO: The string rule also allow triple quoted strings with different
-      //  escaping rules. These are currently not handled. They should be parsed
-      //  into a typesafe format with a unique representation.
+      //  escaping rules. These are currently not handled. They should be
+      //  parsed into a typesafe format with a unique representation.
       separator = visit(ctx->string()).get();
       // If there was a separator, we have to strip the quotation marks
       AD_CONTRACT_CHECK(separator.size() >= 2);
@@ -2652,19 +2655,20 @@ GraphTerm Visitor::visit(Parser::BlankNodeContext* ctx) {
 }
 
 // ____________________________________________________________________________________
-template <typename Ctx>
-void Visitor::visitVector(const std::vector<Ctx*>& childContexts)
-    requires voidWhenVisited<Visitor, Ctx> {
+CPP_template_def(typename Ctx)(
+    requires Visitor::voidWhenVisited<Visitor, Ctx>) void Visitor::
+    visitVector(const vector<Ctx*>& childContexts) {
   for (const auto& child : childContexts) {
     visit(child);
   }
 }
 
 // ____________________________________________________________________________________
-template <typename Ctx>
-[[nodiscard]] auto Visitor::visitVector(const std::vector<Ctx*>& childContexts)
-    -> std::vector<decltype(visit(childContexts[0]))>
-    requires(!voidWhenVisited<Visitor, Ctx>) {
+CPP_template_def(typename Ctx)(
+    requires CPP_NOT(Visitor::voidWhenVisited<Visitor, Ctx>))
+    [[nodiscard]] auto Visitor::visitVector(
+        const std::vector<Ctx*>& childContexts)
+        -> std::vector<decltype(visit(childContexts[0]))> {
   std::vector<decltype(visit(childContexts[0]))> children;
   for (const auto& child : childContexts) {
     children.emplace_back(visit(child));
@@ -2681,7 +2685,8 @@ Out Visitor::visitAlternative(Contexts*... ctxs) {
     (..., visitIf(ctxs));
   } else {
     std::optional<Out> out;
-    // Visit the one `context` which is not null and write the result to `out`.
+    // Visit the one `context` which is not null and write the result to
+    // `out`.
     (..., visitIf<std::optional<Out>, Out>(&out, ctxs));
     return std::move(out.value());
   }
@@ -2706,8 +2711,8 @@ void Visitor::visitIf(Target* target, Ctx* ctx) {
 }
 
 // _____________________________________________________________________________
-template <typename Ctx>
-void Visitor::visitIf(Ctx* ctx) requires voidWhenVisited<Visitor, Ctx> {
+CPP_template_def(typename Ctx)(requires Visitor::voidWhenVisited<
+                               Visitor, Ctx>) void Visitor::visitIf(Ctx* ctx) {
   if (ctx) {
     visit(ctx);
   }
diff --git a/src/parser/sparqlParser/SparqlQleverVisitor.h b/src/parser/sparqlParser/SparqlQleverVisitor.h
index b3d629145a..c1d891aa8e 100644
--- a/src/parser/sparqlParser/SparqlQleverVisitor.h
+++ b/src/parser/sparqlParser/SparqlQleverVisitor.h
@@ -534,16 +534,18 @@ class SparqlQleverVisitor {
         std::array<ExpressionPtr, sizeof...(children)>{std::move(children)...});
   }
 
-  template <typename Ctx>
-  void visitVector(const std::vector<Ctx*>& childContexts)
-      requires voidWhenVisited<SparqlQleverVisitor, Ctx>;
+  CPP_template(typename Ctx)(
+      requires SparqlQleverVisitor::voidWhenVisited<
+          SparqlQleverVisitor, Ctx>) void visitVector(const std::vector<Ctx*>&
+                                                          childContexts);
 
   // Call `visit` for each of the `childContexts` and return the results of
   // those calls as a `vector`.
-  template <typename Ctx>
-  auto visitVector(const std::vector<Ctx*>& childContexts)
-      -> std::vector<decltype(visit(childContexts[0]))>
-      requires(!voidWhenVisited<SparqlQleverVisitor, Ctx>);
+  CPP_template(typename Ctx)(requires CPP_NOT(
+      SparqlQleverVisitor::voidWhenVisited<
+          SparqlQleverVisitor, Ctx>)) auto visitVector(const std::vector<Ctx*>&
+                                                           childContexts)
+      -> std::vector<decltype(visit(childContexts[0]))>;
 
   // Check that exactly one of the `ctxs` is not `null`, visit that context,
   // cast the result to `Out` and return it. Requires that for all of the
@@ -566,8 +568,8 @@ class SparqlQleverVisitor {
   template <typename Target, typename Intermediate = Target, typename Ctx>
   void visitIf(Target* target, Ctx* ctx);
 
-  template <typename Ctx>
-  void visitIf(Ctx* ctx) requires voidWhenVisited<SparqlQleverVisitor, Ctx>;
+  CPP_template(typename Ctx)(requires SparqlQleverVisitor::voidWhenVisited<
+                             SparqlQleverVisitor, Ctx>) void visitIf(Ctx* ctx);
 
  public:
   [[noreturn]] static void reportError(const antlr4::ParserRuleContext* ctx,
diff --git a/test/backports/ConceptsTest.cpp b/test/backports/ConceptsTest.cpp
index 5452e3a046..8814a941b9 100644
--- a/test/backports/ConceptsTest.cpp
+++ b/test/backports/ConceptsTest.cpp
@@ -39,3 +39,18 @@ CPP_template(typename T)(requires(Something<T>)) struct C {
 // the `CPP_and...` macros won't work here.
 CPP_variadic_template(typename... Ts)(
     requires(Something<Ts...>&& Something<Ts...>)) void f(Ts...) {}
+
+TEST(ConceptBackports, lambdas) {
+  int i = 3;
+  auto f = CPP_lambda(i)(auto t)(requires std::is_same_v<decltype(t), int>) {
+    return i + t;
+  };
+  auto g = CPP_template_lambda(&i)(typename T)(T t)(
+      requires std::is_same_v<decltype(t), int>) {
+    return (i++) + t;
+  };
+
+  EXPECT_EQ(g(4), 7);
+  EXPECT_EQ(f(5), 8);
+  EXPECT_EQ(i, 4);
+}