C++: Fix GVN for ImplicitThisFieldAccess

Author: calumgrant

cpp/ql/lib/semmle/code/cpp/valuenumbering/HashCons.qll

@@ -372,7 +372,8 @@ private predicate analyzablePointerFieldAccess(PointerFieldAccess access) {
 private predicate mk_PointerFieldAccess(HashCons qualifier, Field target, PointerFieldAccess access) {
   analyzablePointerFieldAccess(access) and
   target = access.getTarget() and
-  qualifier = hashCons(access.getQualifier().getFullyConverted())
+  qualifier = hashCons(access.getQualifier().getFullyConverted()) and
+  not access instanceof ImplicitThisFieldAccess
 }
 
 private predicate analyzableImplicitThisFieldAccess(ImplicitThisFieldAccess access) {

cpp/ql/test/examples/expressions/PrintAST.expected

@@ -1298,7 +1298,7 @@ union_etc.cpp:
 #    6|       getExpr(): [AssignExpr] ... = ...
 #    6|           Type = [IntType] int
 #    6|           ValueCategory = lvalue
-#    6|         getLValue(): [PointerFieldAccess] x
+#    6|         getLValue(): [ImplicitThisFieldAccess,PointerFieldAccess] x
 #    6|             Type = [IntType] int
 #    6|             ValueCategory = lvalue
 #    6|           getQualifier(): [ThisExpr] this
@@ -1488,7 +1488,7 @@ union_etc.cpp:
 #   33|       getExpr(): [AssignExpr] ... = ...
 #   33|           Type = [IntType] int
 #   33|           ValueCategory = lvalue
-#   33|         getLValue(): [PointerFieldAccess] q
+#   33|         getLValue(): [ImplicitThisFieldAccess,PointerFieldAccess] q
 #   33|             Type = [IntType] int
 #   33|             ValueCategory = lvalue
 #   33|           getQualifier(): [ThisExpr] this