model: make `MPropDef::is_intro` more robust

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/src/model/model.nit b/src/model/model.nit
index 8d6bf7a..941bae7 100644 (file)
--- a/src/model/model.nit
+++ b/src/model/model.nit
@@ -2301,7 +2301,7 @@ abstract class MPropDef
        redef var to_s: String is noinit
 
        # Is self the definition that introduce the property?
-       fun is_intro: Bool do return mproperty.intro == self
+       fun is_intro: Bool do return isset mproperty._intro and mproperty.intro == self
 
        # Return the next definition in linearization of `mtype`.
        #