Further relax the loop detection conditions

Mismatches are now allowed up to expressions not involving any of the bound
variables of the quantifier under inspection.

1 files changed, 18 insertions, 5 deletions

diff --git a/Source/Dafny/Triggers/TriggerExtensions.cs b/Source/Dafny/Triggers/TriggerExtensions.cs
index dccd996d..d4a822a8 100644
--- a/Source/Dafny/Triggers/TriggerExtensions.cs
+++ b/Source/Dafny/Triggers/TriggerExtensions.cs
@@ -31,9 +31,15 @@ namespace Microsoft.Dafny.Triggers {
       return ExprExtensions.ExpressionEq(t1.Expr, t2.Expr);

     }

 

-    internal bool CouldCauseLoops(List<TriggerTerm> terms) {

+    /// <summary>

+    ///  This method checks whether this match could actually cause a loop, given a set of terms participating in a trigger; 

+    ///  to compute an answer, we match the Expr of this match against the Exprs of each of these term, allowing for harmless 

+    ///  variations. If any of these tests does match, this term likely won't cause a loop.

+    ///  The boundVars list is useful to determine that forall x :: P(x) == P(y+z) does not loop.

+    /// </summary>

+    internal bool CouldCauseLoops(List<TriggerTerm> terms, ISet<BoundVar> boundVars) {

       var expr = Expr;

-      return !terms.Any(term => term.Expr.ExpressionEqModuloVariableNamesAndConstants(expr));

+      return !terms.Any(term => term.Expr.ExpressionEqModuloExpressionsNotInvolvingBoundVariables(expr, boundVars));

     }

   }

 

@@ -79,15 +85,22 @@ namespace Microsoft.Dafny.Triggers {
       return ShallowEq_Top(expr1, expr2) && TriggerUtils.SameLists(expr1.SubExpressions, expr2.SubExpressions, (e1, e2) => ExpressionEq(e1, e2));

     }

 

-    internal static bool ExpressionEqModuloVariableNamesAndConstants(this Expression expr1, Expression expr2) {

+    internal static bool ExpressionEqModuloExpressionsNotInvolvingBoundVariables(this Expression expr1, Expression expr2, ISet<BoundVar> boundVars) {

       expr1 = expr1.Resolved;

       expr2 = expr2.Resolved;

 

       if (expr1 is IdentifierExpr) {

-        return expr2 is IdentifierExpr || expr2 is LiteralExpr;

+        if (expr2 is IdentifierExpr) {

+          return true;

+        } else {

+          var freeInE2 = Translator.ComputeFreeVariables(expr2);

+          freeInE2.IntersectWith(boundVars);

+          return !freeInE2.Any();

+        }

       }

 

-      return ShallowEq_Top(expr1, expr2) && TriggerUtils.SameLists(expr1.SubExpressions, expr2.SubExpressions, (e1, e2) => ExpressionEqModuloVariableNamesAndConstants(e1, e2));

+      return ShallowEq_Top(expr1, expr2) && TriggerUtils.SameLists(expr1.SubExpressions, 

+        expr2.SubExpressions, (e1, e2) => ExpressionEqModuloExpressionsNotInvolvingBoundVariables(e1, e2, boundVars));

     }

 

     internal static bool MatchesTrigger(this Expression expr, Expression trigger, ISet<BoundVar> holes, Dictionary<IVariable, Expression> bindings) {