Nullness library: recognise switches with null checks

java/ql/lib/semmle/code/java/Expr.qll

@@ -1542,6 +1542,11 @@ class SwitchExpr extends Expr, StmtParent, @switchexpr {
     exists(YieldStmt yield | yield.getTarget() = this and result = yield.getValue())
   }
 
+  /** Holds if this switch has a case handling a null literal. */
+  predicate hasNullCase() {
+    this.getAConstCase().getValue(_) instanceof NullLiteral
+  }
+
   /** Gets a printable representation of this expression. */
   override string toString() { result = "switch (...)" }
 

java/ql/lib/semmle/code/java/Statement.qll

@@ -409,6 +409,11 @@ class SwitchStmt extends Stmt, @switchstmt {
   /** Gets the expression of this `switch` statement. */
   Expr getExpr() { result.getParent() = this }
 
+  /** Holds if this switch has a case handling a null literal. */
+  predicate hasNullCase() {
+    this.getAConstCase().getValue(_) instanceof NullLiteral
+  }
+
   override string pp() { result = "switch (...)" }
 
   override string toString() { result = "switch (...)" }

java/ql/lib/semmle/code/java/dataflow/Nullness.qll

@@ -100,9 +100,9 @@ predicate dereference(Expr e) {
   or
   exists(SynchronizedStmt synch | synch.getExpr() = e)
   or
-  exists(SwitchStmt switch | switch.getExpr() = e)
+  exists(SwitchStmt switch | switch.getExpr() = e and not switch.hasNullCase())
   or
-  exists(SwitchExpr switch | switch.getExpr() = e)
+  exists(SwitchExpr switch | switch.getExpr() = e and not switch.hasNullCase())
   or
   exists(FieldAccess fa, Field f | fa.getQualifier() = e and fa.getField() = f and not f.isStatic())
   or