Add tests for "implicit this"

ql/test/queries/style/ImplicitThis/Bad.qll

@@ -0,0 +1,11 @@
+import ql
+
+class Foo extends string {
+  Foo() { this = "hello" }
+
+  string getBar() { result = "bar" }
+
+  string getBarWithThis() { result = this.getBar() }
+
+  string getBarWithoutThis() { result = getBar() }
+}

ql/test/queries/style/ImplicitThis/Good.qll

@@ -0,0 +1,16 @@
+import ql
+
+class Foo extends string {
+  Foo() { this = "hello" }
+
+  string getBar() { result = "bar" }
+
+  string getBarWithThis() { result = this.getBar() }
+
+  /* Okay because not a member predicate. */
+  string getBaz() { result = Baz::baz() }
+}
+
+module Baz {
+  string baz() { result = "baz" }
+}

ql/test/queries/style/ImplicitThis/ImplicitThis.expected

@@ -0,0 +1 @@
+| Bad.qll:10:41:10:48 | PredicateCall | Use of implicit `this`. |

ql/test/queries/style/ImplicitThis/ImplicitThis.qlref

@@ -0,0 +1 @@
+queries/style/ImplicitThis.ql

ql/test/queries/style/ImplicitThis/Okay.qll

@@ -0,0 +1,13 @@
+import ql
+
+class Foo extends string {
+  Foo() { this = "hello" }
+
+  string getBar() { result = "bar" }
+
+  /* Okay, because we don't write `this.some_method` anywhere */
+  string getBarWithoutThis() { result = getBar() }
+
+  /* Okay, because this is the only way to cast `this`. */
+  string useThisWithInlineCast() { result = this.(string).toUpperCase() }
+}