Error instead of crashing on reference typed fields

Since reference types can't be written to linear memory, they don't have
proper byte sizes, which triggers an assert. Instead, the compiler
should signal its lack of support for this feature.

Fixes AssemblyScript#2726.

Author: CountBleck

src/resolver.ts

@@ -3407,6 +3407,14 @@ export class Resolver extends DiagnosticEmitter {
               if (boundInstance) {
                 let fieldType = boundInstance.type;
                 if (fieldType == Type.void) break; // failed to resolve earlier
+                if (fieldType.isExternalReference) {
+                  this.error(
+                    DiagnosticCode.Not_implemented_0,
+                    assert(boundPrototype.typeNode).range,
+                    "Reference typed fields"
+                  );
+                  break;
+                }
                 let needsLayout = true;
                 if (base) {
                   let existingMember = base.getMember(boundPrototype.name);

tests/compiler/field-reference-types.json

@@ -0,0 +1,7 @@
+{
+  "stderr": [
+    "Not implemented: Reference typed fields",
+    "Not implemented: Reference typed fields",
+    "EOF"
+  ]
+}

tests/compiler/field-reference-types.ts

@@ -0,0 +1,12 @@
+class Foo {
+  bar: externref = null;
+}
+
+class Baz<T> {
+  qux: T;
+}
+
+new Foo();
+new Baz<externref>();
+
+ERROR("EOF");