Dataflow: Add Unit type for all languages.

Author: aschackmull

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowPrivate.qll

@@ -280,6 +280,15 @@ predicate isUnreachableInCall(Node n, DataFlowCall call) { none() } // stub impl
 
 int accessPathLimit() { result = 5 }
 
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}
+
 /**
  * Holds if `n` does not require a `PostUpdateNode` as it either cannot be
  * modified or its modification cannot be observed, for example if it is a

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -510,6 +510,15 @@ predicate isUnreachableInCall(Node n, DataFlowCall call) { none() } // stub impl
 
 int accessPathLimit() { result = 5 }
 
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}
+
 /**
  * Holds if `n` does not require a `PostUpdateNode` as it either cannot be
  * modified or its modification cannot be observed, for example if it is a

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -1932,6 +1932,15 @@ private predicate viableConstantBooleanParamArg(
 
 int accessPathLimit() { result = 5 }
 
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}
+
 /**
  * Holds if `n` does not require a `PostUpdateNode` as it either cannot be
  * modified or its modification cannot be observed, for example if it is a

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -1150,5 +1150,14 @@ predicate isImmutableOrUnobservable(Node n) { none() }
 
 int accessPathLimit() { result = 5 }
 
+/** The unit type. */
+private newtype TUnit = TMkUnit()
+
+/** The trivial type with a single element. */
+class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
+  string toString() { result = "unit" }
+}
+
 /** Holds if `n` should be hidden from path explanations. */
 predicate nodeIsHidden(Node n) { none() }