Fix bad join order in Shadowing::shadows

Fixes the bad join order in `Shadowing::shadows`:
Tuple counts for Shadowing::shadows#f4fb89a3#ffff@c4b8a90j:
           182915  ~0%    {2} r1 = Variable::LocalVariableDecl::getCallable#dispred#f0820431#ff AND NOT Shadowing::shadows#f4fb89a3#ffff#antijoin_rhs(Lhs.0, Lhs.1)
           182915  ~0%    {3} r2 = JOIN r1 WITH localvars ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Rhs.2
           182915  ~3%    {4} r3 = JOIN r2 WITH Member::Member::getDeclaringType#dispred#f0820431#bf ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0, Lhs.2
           182833  ~0%    {4} r4 = JOIN r3 WITH classes ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.0
           182833  ~3%    {5} r5 = JOIN r4 WITH Element::Element::getName#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.0, Lhs.1, Lhs.3, Rhs.1
        183352620  ~5%    {5} r6 = JOIN r5 WITH Member::Field::getType#dispred#f0820431#bf_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.3, Lhs.1, Lhs.2, Lhs.4
            40529  ~0%    {5} r7 = JOIN r6 WITH Member::Field::getDeclaringType#dispred#f0820431#fb ON FIRST 2 OUTPUT Lhs.0, Lhs.4, Lhs.2, Lhs.3, Lhs.1
              678  ~4%    {4} r8 = JOIN r7 WITH Element::Element::getName#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.2, Lhs.3, Lhs.4, Lhs.0
              670  ~4%    {4} r9 = r8 AND NOT Member::Field::isStatic#dispred#f0820431#b(Lhs.3)
              670  ~3%    {4} r10 = SCAN r9 OUTPUT In.0, In.2, In.3, In.1
                          return r10

After the fix:
Tuple counts for Shadowing::shadows#f4fb89a3#ffff@95ca976v:
        182915  ~0%    {2} r1 = Variable::LocalVariableDecl::getCallable#dispred#f0820431#ff AND NOT Shadowing::shadows#f4fb89a3#ffff#antijoin_rhs(Lhs.0, Lhs.1)
        182915  ~0%    {3} r2 = JOIN r1 WITH localvars ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Rhs.2
        182915  ~0%    {4} r3 = JOIN r2 WITH Member::Member::getDeclaringType#dispred#f0820431#bf ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Lhs.2, Rhs.1
        182915  ~7%    {5} r4 = JOIN r3 WITH Element::Element::getName#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.3, Rhs.1, Lhs.2, Lhs.0, Lhs.1
           678  ~4%    {4} r5 = JOIN r4 WITH Shadowing::getField#f4fb89a3#ffff ON FIRST 3 OUTPUT Lhs.3, Lhs.4, Lhs.0, Rhs.3
           670  ~4%    {4} r6 = r5 AND NOT Member::Field::isStatic#dispred#f0820431#b(Lhs.3)
           670  ~3%    {4} r7 = SCAN r6 OUTPUT In.0, In.2, In.3, In.1
                       return r7

Author: tamasvajk

java/ql/src/Violations of Best Practice/Naming Conventions/Shadowing.qll

@@ -20,13 +20,21 @@ predicate setterFor(Method m, Field f) {
 predicate shadows(LocalVariableDecl d, Class c, Field f, Callable method) {
   d.getCallable() = method and
   method.getDeclaringType() = c and
-  c.getAField() = f and
-  f.getName() = d.getName() and
-  f.getType() = d.getType() and
-  not d.getCallable().isStatic() and
+  f = getField(c, d.getName(), d.getType()) and
+  not method.isStatic() and
   not f.isStatic()
 }
 
+/**
+ * Gets the field with the given name and type from the given class, if any.
+ */
+pragma[nomagic]
+private Field getField(Class c, string name, Type t) {
+  result.getDeclaringType() = c and
+  result.getName() = name and
+  result.getType() = t
+}
+
 predicate thisAccess(LocalVariableDecl d, Field f) {
   shadows(d, _, f, _) and
   exists(VarAccess va | va.getVariable().(Field).getSourceDeclaration() = f |