Fix: Ability to use .Key as a constant name in datatypes and classes

Source/DafnyCore/Verifier/Translator.cs

@@ -22,6 +22,7 @@
 using static Microsoft.Dafny.Util;
 using Core;
 using Microsoft.BaseTypes;
+using Microsoft.Dafny.Compilers;
 using Microsoft.Dafny.Triggers;
 using Action = System.Action;
 using PODesc = Microsoft.Dafny.ProofObligationDescription;
@@ -6686,10 +6687,11 @@ Bpl.Function GetReadonlyField(Field f) {
           return predef.ArrayLength;
         } else if (f.EnclosingClass is ValuetypeDecl { Name: "real" } && f.Name == "Floor") {
           return predef.RealFloor;
-        } else if (f is SpecialField && !(f is DatatypeDestructor)) {
+        } else if (f is SpecialField && !(f is DatatypeDestructor || f.EnclosingClass is IndDatatypeDecl or ClassLikeDecl)) {
           if (f.Name is "Keys" or "Values" or "Items") {
-            Contract.Assert(f.Type is SetType);
-            var setType = (SetType)f.Type;
+            var fType = f.Type.NormalizeExpand();
+            Contract.Assert(fType is SetType);
+            var setType = (SetType)fType;
             return f.Name switch {
               "Keys" => setType.Finite ? predef.MapDomain : predef.IMapDomain,
               "Values" => setType.Finite ? predef.MapValues : predef.IMapValues,

Test/git-issues/git-issue-4394.dfy

@@ -0,0 +1,16 @@
+// RUN: %baredafny verify %args "%s" > "%t"
+// RUN: %diff "%s.expect" "%t"
+
+datatype T = T(
+    f:map<nat,nat>
+) {
+    const Keys := f.Keys // no problem if another name is used for the constant
+}
+
+class C {
+  const Keys: set<nat>
+
+  constructor(f:map<nat,nat>) {
+    this.Keys := f.Keys;
+  }
+}

Test/git-issues/git-issue-4394.dfy.expect

@@ -0,0 +1,2 @@
+
+Dafny program verifier finished with 0 verified, 0 errors

docs/dev/news/4394.fix

@@ -0,0 +1 @@
+Ability to use .Key as a constant name in datatypes and classes