X-Git-Url: http://nitlanguage.org

diff --git a/src/modelbuilder_base.nit b/src/modelbuilder_base.nit
index 9995338..93423df 100644
--- a/src/modelbuilder_base.nit
+++ b/src/modelbuilder_base.nit
@@ -81,6 +81,16 @@ class ModelBuilder
 		return res
 	end
 
+	# Like `try_get_mclass_by_name` but display an error message when the class is not found
+	fun get_mclass_by_name(node: ANode, mmodule: MModule, name: String): nullable MClass
+	do
+		var mclass = try_get_mclass_by_name(node, mmodule, name)
+		if mclass == null then
+			error(node, "Type Error: missing primitive class `{name}'.")
+		end
+		return mclass
+	end
+
 	# Return a property named `name` on the type `mtype` visible in the module `mmodule`.
 	# Visibility in modules is correctly handled.
 	# Protected properties are returned (it is up to the caller to check and reject protected properties).
@@ -305,7 +315,9 @@ class ModelBuilder
 		if mtype isa MGenericType then
 			var mclass = mtype.mclass
 			for i in [0..mclass.arity[ do
-				var bound = mclass.intro.bound_mtype.arguments[i]
+				var intro = mclass.try_intro
+				if intro == null then return null # skip error
+				var bound = intro.bound_mtype.arguments[i]
 				var nt = ntype.n_types[i]
 				var mt = resolve_mtype(mmodule, mclassdef, nt)
 				if mt == null then return null # forward error