The type is defined as a class in varsupport.h and forward declared as a
class in cffread_abs.h. absfont.h forward declared it as a struct, which
trips -Wmismatched-tags on clang and C4099 on MSVC when both headers are
pulled into the same translation unit. absfont.h is only ever included by
C++ sources, so the class keyword is safe here.

--- afdko/c/shared/include/absfont.h
+++ afdko/c/shared/include/absfont.h
@@ -102,7 +102,7 @@
 };
 
 /* item variation store  */
-struct itemVariationStore;
+class itemVariationStore;
 
 /* Top dictionary data. Comments indicate default/initial values. */
 typedef struct {