Merge branch 'cxx20.GeneratorObjects-20210106' into 'master'

GeneratorObjects: Fix compilation with c++20.

See merge request atlas/athena!39549

Generators/GeneratorObjects/GeneratorObjects/HepMcParticleLink.h

 /*
-  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef GENERATOROBJECTS_HEPMCPARTICLELINK_H
@@ -665,6 +665,16 @@ public:
 };
 
 
+/**
+ * @brief Comparison with GenParticle*.
+ * Needed with c++20 to break an ambiguity between the built-in GenParticle*
+ * equality and operator== defined above, arising due to the conversions
+ * back and forth between HepMcParticleLink and GenParticle*.
+ */
+bool operator== (const HepMC::GenParticle* a,
+                 const HepMcParticleLink& b);
+
+
 /**
  * @brief Output operator.
  * @param os Stream to which to output.

Generators/GeneratorObjects/GeneratorObjects/HepMcParticleLink.icc

 /*
- * Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration.
+ * Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration.
  */
 /**
  * @file GeneratorObjects/HepMcParticleLink.icc
@@ -600,3 +600,17 @@ HepMcParticleLink::barcode_type HepMcParticleLink::compress() const
   return ( ((m_extBarcode.barcode()&0xFFFF) << 16) |
            eventIndex() );
 }
+
+
+/**
+ * @brief Comparison with GenParticle*.
+ * Needed with c++20 to break an ambiguity between the built-in GenParticle*
+ * equality and operator== defined above, arising due to the conversions
+ * back and forth between HepMcParticleLink and GenParticle*.
+ */
+inline
+bool operator== (const HepMC::GenParticle* a,
+                 const HepMcParticleLink& b)
+{
+  return a == b.cptr();
+}