Fix check for when combined domain is empty

Fixes #765

changes.rst

@@ -24,10 +24,11 @@ Bug fixes:
 -  Fix definition of ``array_intersect`` so that it can be used in non-positive
    contexts.
 -  Fix standards definitions of ``increasing`` variants to correct be ignored
-   when the argument array is empty (:bugref:`762`).\
+   when the argument array is empty (:bugref:`762`).
 -  Fix a problem where exceptions thrown for undefined expressions were not
    caught during the aggregation of linear expressions, breaking relational
    semantics (:bugref:`760`).
+-  Fix crash when calculating computed domain of a declaration (:bugref:`765`).
 
 Changes:
 ^^^^^^^^

lib/flatten.cpp

@@ -2850,7 +2850,7 @@ KeepAlive compute_combined_domain(EnvI& env, TypeInst* ti, Expression* cur) {
         Ranges::Inter<IntVal, IntSetRanges, IntSetRanges> i(dr, ibr);
         combinedDomain = IntSetVal::ai(i);
         if (combinedDomain->card() == 0 && !Expression::type(cur).isSet() &&
-            (!Expression::type(cur).isOpt() && !Expression::type(ident->decl()).isOpt())) {
+            !Expression::type(cur).isOpt()) {
           env.fail();
         }
       }

tests/spec/unit/regression/github_765.mzn

@@ -0,0 +1,8 @@
+/***
+!Test
+expected: !Result
+  status: UNSATISFIABLE
+solvers: [gecode]
+***/
+var 1..1: x;
+var 1..1: y :: output = if false then x else 0 endif;