diff -up ./gazebo/common/SingletonT.hh.singleton ./gazebo/common/SingletonT.hh
--- ./gazebo/common/SingletonT.hh.singleton	2020-02-23 13:32:20.278829812 -0500
+++ ./gazebo/common/SingletonT.hh	2020-02-23 13:32:29.579857904 -0500
@@ -55,19 +55,7 @@ class SingletonT
 /// \brief Helper to declare typed SingletonT
 // clang doesn't compile if it explicitly specializes a type before
 // the type is defined. (forward declaration is not enough.)
-#ifdef __clang__
 #define GZ_SINGLETON_DECLARE(visibility, n1, n2, singletonType)
-#else
-#define GZ_SINGLETON_DECLARE(visibility, n1, n2, singletonType) \
-namespace n1 \
-{ \
-  namespace n2 \
-  { \
-    class singletonType; \
-  } \
-} \
-template class visibility ::SingletonT<n1::n2::singletonType>;
-#endif
 
 /// \}