From 04d77d27db8cfddcf8d7e738d4dc835585e5eae2 Mon Sep 17 00:00:00 2001
From: Andreas Fertig <andy@cppinsights.io>
Date: Thu, 26 Jun 2025 09:02:59 +0200
Subject: [PATCH] Switched implementation of P0315Visitor from `std::variant`
 to compile-time based approach.

---
 InsightsHelpers.cpp | 17 ++++++++---------
 InsightsHelpers.h   | 26 ++++++++++++++++----------
 2 files changed, 24 insertions(+), 19 deletions(-)

diff --git a/InsightsHelpers.cpp b/InsightsHelpers.cpp
index 44beb098..dc5ce3bd 100644
--- a/InsightsHelpers.cpp
+++ b/InsightsHelpers.cpp
@@ -1619,18 +1619,17 @@ struct overloaded : Ts...
 };
 template<class... Ts>
 overloaded(Ts...) -> overloaded<Ts...>;
+//-----------------------------------------------------------------------------
 
-bool P0315Visitor::VisitLambdaExpr(const LambdaExpr* expr)
+void P0315Visitor_HandleLambdaExpr(OutputFormatHelper& ofm, const LambdaExpr* expr)
 {
-    mLambdaExpr = expr;
-
-    std::visit(overloaded{
-                   [&](OutputFormatHelper& ofm) { ofm.Append(GetLambdaName(*expr)); },
-                   [&](CodeGenerator& cg) { cg.InsertArg(expr); },
-               },
-               mConsumer);
+    ofm.Append(GetLambdaName(*expr));
+}
+//-----------------------------------------------------------------------------
 
-    return false;
+void P0315Visitor_HandleLambdaExpr(CodeGenerator& cg, const LambdaExpr* expr)
+{
+    cg.InsertArg(expr);
 }
 //-----------------------------------------------------------------------------
 
diff --git a/InsightsHelpers.h b/InsightsHelpers.h
index 7d0ac3f4..75cd2a28 100644
--- a/InsightsHelpers.h
+++ b/InsightsHelpers.h
@@ -17,7 +17,6 @@
 #include <functional>
 #include <optional>
 #include <string>
-#include <variant>
 
 #include "InsightsStrongTypes.h"
 #include "StackList.h"
@@ -355,25 +354,32 @@ is(T) -> is<T>;
 const DeclRefExpr* FindDeclRef(const Stmt* stmt);
 //-----------------------------------------------------------------------------
 
+void P0315Visitor_HandleLambdaExpr(class OutputFormatHelper&, const LambdaExpr*);
+void P0315Visitor_HandleLambdaExpr(class CodeGenerator&, const LambdaExpr*);
+//-----------------------------------------------------------------------------
+
 ///! Find a LambdaExpr inside a Decltype
-class P0315Visitor : public RecursiveASTVisitor<P0315Visitor>
+template<typename T>
+    requires(std::derived_from<T, CodeGenerator> or std::same_as<T, OutputFormatHelper>)
+class P0315Visitor : public RecursiveASTVisitor<P0315Visitor<T>>
 {
-    std::variant<std::reference_wrapper<class OutputFormatHelper>, std::reference_wrapper<class CodeGenerator>>
-                      mConsumer;
+    T&                mConsumer;
     const LambdaExpr* mLambdaExpr{};
 
 public:
-    P0315Visitor(class OutputFormatHelper& ofm)
-    : mConsumer{ofm}
+    constexpr P0315Visitor(T& consumer)
+    : mConsumer{consumer}
     {
     }
 
-    P0315Visitor(class CodeGenerator& cg)
-    : mConsumer{cg}
+    bool VisitLambdaExpr(const LambdaExpr* expr)
     {
-    }
+        mLambdaExpr = expr;
+
+        P0315Visitor_HandleLambdaExpr(mConsumer, expr);
 
-    bool VisitLambdaExpr(const LambdaExpr* expr);
+        return false;
+    }
 
     const LambdaExpr* Get() const { return mLambdaExpr; }
 };