From cdbd140e7a9e496e876eac0bbc57a2bb9ca5ab0f Mon Sep 17 00:00:00 2001
From: puerling <t.puerling@fz-juelich.de>
Date: Tue, 6 Aug 2024 15:46:18 +0200
Subject: [PATCH] fix: calling `hasInteraction()` though an instance is not
 `constexpr`

* clang identified the call to `hasInteraction()` as not being a
  constant expression.
* `Hamiltonian::hasInteraction()` being a `constexpr` static member
  function might not be a constant expression when invoked through an
  instance of a `Hamiltonian` object.
  Accessing it through the type by using `decltype` is guaranteed to
  work.
---
 core/include/engine/common/Hamiltonian.hpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/core/include/engine/common/Hamiltonian.hpp b/core/include/engine/common/Hamiltonian.hpp
index fe648c826..452efc8ed 100644
--- a/core/include/engine/common/Hamiltonian.hpp
+++ b/core/include/engine/common/Hamiltonian.hpp
@@ -626,7 +626,8 @@ class HamiltonianVariant : public TypeTraits
     template<class T>
     [[nodiscard]] bool hasInteraction()
     {
-        return std::visit( []( auto & h ) { return h.template hasInteraction<T>(); }, hamiltonian );
+        return std::visit(
+            []( auto & h ) { return std::decay_t<decltype( h )>::template hasInteraction<T>(); }, hamiltonian );
     };
 
     template<class T>
@@ -641,7 +642,7 @@ class HamiltonianVariant : public TypeTraits
         return std::visit(
             []( const auto & h ) -> const typename T::Data *
             {
-                if constexpr( h.template hasInteraction<T>() )
+                if constexpr( std::decay_t<decltype( h )>::template hasInteraction<T>() )
                     return &h.template data<T>();
                 else
                     return nullptr;
@@ -655,7 +656,7 @@ class HamiltonianVariant : public TypeTraits
         return std::visit(
             []( const auto & h ) -> const typename T::Cache *
             {
-                if constexpr( h.template hasInteraction<T>() )
+                if constexpr( std::decay_t<decltype( h )>::template hasInteraction<T>() )
                     return &h.template cache<T>();
                 else
                     return nullptr;
@@ -670,7 +671,7 @@ class HamiltonianVariant : public TypeTraits
             [this,
              data = typename T::Data( std::forward<Args>( args )... )]( auto & h ) mutable -> std::optional<std::string>
             {
-                if constexpr( h.template hasInteraction<T>() )
+                if constexpr( std::decay_t<decltype( h )>::template hasInteraction<T>() )
                     return h.template set_data<T>( std::move( data ) );
                 else
                     return fmt::format(