fix: diagnose when accessing setter only property

src/resolver.ts

@@ -1363,7 +1363,18 @@ export class Resolver extends DiagnosticEmitter {
       }
       case ElementKind.Property: { // someInstance.prop
         let propertyInstance = <Property>target;
-        let getterInstance = assert(propertyInstance.getterInstance); // must have a getter
+        let getterInstance = propertyInstance.getterInstance;
+        if (!getterInstance) {
+          // In TS, getters without setters return `undefined`. Since AS doesn't have
+          // undefined, we instead diagnose it at compile time, but this isn't
+          // compatible with TS.
+          let setterInstance = assert(propertyInstance.setterInstance);
+          this.errorRelated(
+            DiagnosticCode.Property_0_only_has_a_setter_and_is_missing_a_getter,
+            targetNode.range, setterInstance.declaration.range, propertyInstance.name
+          );
+          return null;
+        }
         let type = getterInstance.signature.returnType;
         let classReference = type.getClassOrWrapper(this.program);
         if (!classReference) {

tests/compiler/getter-setter-errors.json

@@ -2,6 +2,7 @@
   "asc_flags": [
   ],
   "stderr": [
+    "AS229: Property 'm' only has a setter and is missing a getter.",
     "TS2808: Get accessor 'm2' must be at least as accessible as the setter.",
     "EOF"
   ]

tests/compiler/getter-setter-errors.ts

@@ -1,3 +1,8 @@
+class UseNonExistedGetter {
+  set m(v: string) {}
+}
+new UseNonExistedGetter().m.toString();
+
 class GetSetWithoutDifferenceVisibility {
   public get m1(): i32 {
     return 1;
@@ -9,7 +14,6 @@ class GetSetWithoutDifferenceVisibility {
   }
   public set m2(v: i32) {}
 }
-
 new GetSetWithoutDifferenceVisibility().m1; // m1 is valid
 new GetSetWithoutDifferenceVisibility().m2; // m2 is invalid