Derive null to a nullable type in reference contexts (AssemblyScript#724)

Author: jtenner

src/compiler.ts

@@ -7140,8 +7140,11 @@ export class Compiler extends DiagnosticEmitter {
     switch (expression.kind) {
       case NodeKind.NULL: {
         let options = this.options;
-        if (!contextualType.classReference) {
+        let classReference = contextualType.classReference;
+        if (!classReference) {
           this.currentType = options.usizeType;
+        } else {
+          this.currentType = classReference.type.asNullable();
         }
         return options.isWasm64
           ? module.i64(0)

tests/compiler/nullable.json

@@ -0,0 +1,9 @@
+{
+  "asc_flags": [
+    "--runtime none"
+  ],
+  "stderr": [
+    "TS2531: Object is possibly 'null'.",
+    "EOF"
+  ]
+}

tests/compiler/nullable.ts

@@ -0,0 +1,7 @@
+class Example {}
+
+function notNullable(a: Example): void {}
+
+notNullable(null);
+
+ERROR("EOF")