Index: src/tests/polymorphism.c
===================================================================
--- src/tests/polymorphism.c	(revision a7caf3d39000047777d818c855ecc4954932e20b)
+++ src/tests/polymorphism.c	(revision 1f37045134019dcc85ce9bbfe573c87275cfb714)
@@ -89,6 +89,8 @@
 		// ensure that the size of aggregates with polymorphic members
 		// matches the size of the aggregates in a monomorphic context
-		assertf( struct_size(x, y) == sizeof(S), "struct size differs in polymorphic context." );
-		assertf( union_size(x, y) == sizeof(U), "union size differs in polymorphic context." );
+		size_t ssz = struct_size(x, y);
+		size_t usz = union_size(x, y);
+		assertf( ssz == sizeof(S), "struct size differs in polymorphic context: %zd / %zd", ssz, sizeof(S));
+		assertf( usz == sizeof(U), "union size differs in polymorphic context: %zd / %zd", usz, sizeof(U));
 
 		y_type ?=?(y_type & this, zero_t) {