fix TypeExprKinds crashing on a ThisExpression

Author: erik-krogh

javascript/extractor/src/com/semmle/js/extractor/TypeExprKinds.java

@@ -7,6 +7,7 @@
 import com.semmle.js.ast.Literal;
 import com.semmle.js.ast.MemberExpression;
 import com.semmle.js.ast.TemplateElement;
+import com.semmle.js.ast.ThisExpression;
 import com.semmle.js.extractor.ASTExtractor.IdContext;
 import com.semmle.ts.ast.ArrayTypeExpr;
 import com.semmle.ts.ast.ConditionalTypeExpr;
@@ -100,6 +101,11 @@ public Integer visit(KeywordTypeExpr nd, Void c) {
                 return keywordTypeExpr;
               }
 
+              @Override
+              public Integer visit(ThisExpression nd, Void c) {
+                return thisVarTypeAccess;
+              }
+
               @Override
               public Integer visit(ArrayTypeExpr nd, Void c) {
                 return arrayTypeExpr;

javascript/ql/test/library-tests/TypeScript/Types/badTypes.ts

@@ -0,0 +1,6 @@
+// Test case taken from babel: https://github.com/babel/babel/blob/main/packages/babel-parser/test/fixtures/typescript/regression/keyword-qualified-type-2/input.ts
+
+// These are valid TypeScript syntax (the parser doesn't produce any error),
+// but they are always type-checking errors.
+interface A extends this.B {}
+type T = typeof var.bar;