From aebf74be971c11b8d8c1cc1edcbda82de0d0e984 Mon Sep 17 00:00:00 2001
From: Jose Luis Blanco-Claraco <joseluisblancoc@gmail.com>
Date: Thu, 9 Nov 2023 00:13:46 +0100
Subject: [PATCH] FIX: MCP_SAVE() with class enums

---
 doc/source/doxygen-docs/changelog.md          |  2 ++
 .../containers/include/mrpt/containers/yaml.h | 22 ++++++++++++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/doc/source/doxygen-docs/changelog.md b/doc/source/doxygen-docs/changelog.md
index 10873745bb..4735991392 100644
--- a/doc/source/doxygen-docs/changelog.md
+++ b/doc/source/doxygen-docs/changelog.md
@@ -16,6 +16,8 @@
     - mrpt::maps::TSetOfMetricMapInitializers::loadFromConfigFile() now throws if it finds a `*_count` entry with an unknown map class name.
   - \ref mrpt_math_grp
     - New template mrpt::math::confidenceIntervalsFromHistogram()
+- BUG FIXES:
+  - Fix compilation errors if using the `MCP_SAVE()` macro with class enum types.
 
 
 # Version 2.11.2: Released Oct 25th, 2023
diff --git a/libs/containers/include/mrpt/containers/yaml.h b/libs/containers/include/mrpt/containers/yaml.h
index 6ee0b8c371..c376ae8b67 100644
--- a/libs/containers/include/mrpt/containers/yaml.h
+++ b/libs/containers/include/mrpt/containers/yaml.h
@@ -912,6 +912,22 @@ std::ostream& operator<<(std::ostream& o, const yaml& p);
 	MCP_LOAD_OPT(Yaml__, Var__);                                               \
 	Var__ = mrpt::DEG2RAD(Var__)
 
+namespace internal
+{
+// We need to implement this as a template for the "if constexpr()" false branch
+// not to be evaluated for enums, which would lead to build errors.
+template <typename T, typename YAML_T>
+void impl_mcp_save(YAML_T& y, const T& var, const char* varName)
+{
+	using enum_t = std::remove_cv_t<T>;
+
+	if constexpr (std::is_enum_v<enum_t>)
+		y[varName] = mrpt::typemeta::TEnumType<enum_t>::value2name(var);
+	else
+		y[varName] = var;
+}
+}  // namespace internal
+
 /** Macro to store a variable into a mrpt::containers::yaml (initials MCP)
  * dictionary, using as "key" the name of the variable.
  *
@@ -931,11 +947,7 @@ std::ostream& operator<<(std::ostream& o, const yaml& p);
  * values. Note that this requires enums to implement mrpt::typemeta::TEnumType.
  */
 #define MCP_SAVE(Yaml__, Var__)                                                \
-	if constexpr (std::is_enum_v<decltype(Var__)>)                             \
-		Yaml__[#Var__] = mrpt::typemeta::TEnumType<                            \
-			std::remove_cv_t<decltype(Var__)>>::value2name(Var__);             \
-	else                                                                       \
-		Yaml__[#Var__] = Var__;
+	mrpt::containers::internal::impl_mcp_save(Yaml__, Var__, #Var__);
 
 #define MCP_SAVE_DEG(Yaml__, Var__) Yaml__[#Var__] = mrpt::RAD2DEG(Var__);