add a CopyStep type-tracking step, for loadStoreSteps that loads and stores the same property

javascript/ql/src/semmle/javascript/Collections.qll

@@ -87,10 +87,10 @@ module CollectionsTypeTracking {
       summary = StoreStep(field) and
       step.store(pred, result, field)
       or
+      summary = CopyStep(field) and
+      step.loadStore(pred, result, field)
+      or
       exists(string toField | summary = LoadStoreStep(field, toField) |
-        field = toField and
-        step.loadStore(pred, result, field)
-        or
         step.loadStore(pred, result, field, toField)
       )
     )

javascript/ql/src/semmle/javascript/Promises.qll

@@ -176,7 +176,7 @@ module PromiseTypeTracking {
       summary = StoreStep(field) and
       step.store(pred, result, field)
       or
-      summary = LoadStoreStep(field, field) and
+      summary = CopyStep(field) and
       step.loadStore(pred, result, field)
     )
   }

javascript/ql/src/semmle/javascript/dataflow/TypeTracking.qll

@@ -57,6 +57,8 @@ class TypeTracker extends TTypeTracker {
       result = MkTypeTracker(hasCall, toProp)
     )
     or
+    step = CopyStep(prop) and result = this
+    or
     step = CallStep() and result = MkTypeTracker(true, prop)
     or
     step = ReturnStep() and hasCall = false and result = this
@@ -219,6 +221,8 @@ class TypeBackTracker extends TTypeBackTracker {
       result = MkTypeBackTracker(hasReturn, fromProp)
     )
     or
+    step = CopyStep(prop) and result = this
+    or
     step = CallStep() and hasReturn = false and result = this
     or
     step = ReturnStep() and result = MkTypeBackTracker(true, prop)

javascript/ql/src/semmle/javascript/dataflow/internal/StepSummary.qll

@@ -40,9 +40,8 @@ newtype TStepSummary =
   ReturnStep() or
   StoreStep(PropertyName prop) or
   LoadStep(PropertyName prop) or
+  CopyStep(PropertyName prop) or
   LoadStoreStep(PropertyName fromProp, PropertyName toProp) {
-    fromProp = toProp
-    or
     exists(TypeTrackingPseudoProperty prop | fromProp = prop and toProp = prop.getLoadStoreToProp())
   }
 
@@ -64,6 +63,8 @@ class StepSummary extends TStepSummary {
     or
     exists(string prop | this = LoadStep(prop) | result = "load " + prop)
     or
+    exists(string prop | this = CopyStep(prop) | result = "copy " + prop)
+    or
     exists(string fromProp, string toProp | this = LoadStoreStep(fromProp, toProp) |
       result = "copy " + fromProp + " to " + toProp
     )