Substitution can fail in some invalid cases.

Arguably, this code should not be run in invalid cases,
but that seems difficult to enforce given the numnber of paths
through the type-checker.  It's better to simply be more
conservative here.

Author: rjmccall

lib/Sema/CSApply.cpp

@@ -179,6 +179,9 @@ Type Solution::computeSubstitutions(
       currentReplacement = req.getFirstType().subst(currentModule,
                                                     typeSubstitutions,
                                                     None);
+      if (!currentReplacement)
+        currentReplacement = tc.Context.TheErrorType;
+
       break;
     }
   }