In the definition of SOFT-LIST-OF in types.lisp, the DEFTYPE form uses SUBTYPEP to see if NULL is a subtype of the element type. It takes NIL being returned as meaning NIL is not in the type. However, if SUBTYPEP returned NIL as its second return value, we cannot conclude this, and the generated type may be wrong.
In the definition of
SOFT-LIST-OFin types.lisp, theDEFTYPEform usesSUBTYPEPto see ifNULLis a subtype of the element type. It takesNILbeing returned as meaningNILis not in the type. However, ifSUBTYPEPreturnedNILas its second return value, we cannot conclude this, and the generated type may be wrong.