Fix(jsii): improve diagnostics when dereference fails (#222)

In some cases, when a type could not be referenced, the diagnostic 
message doesn't indicate that (the undefined return value will still 
cause an error, but we also want to know that we couldn't deref the 
type).

Fixes #221

Author: Elad Ben-Israel

packages/jsii/lib/assembler.ts

@@ -162,14 +162,21 @@ export class Assembler implements Emitter {
      *
      * @returns the de-referenced type, if it was found, otherwise ``undefined``.
      */
-    private _dereference(ref: spec.NamedTypeReference): spec.Type | undefined {
+    private _dereference(ref: spec.NamedTypeReference, referencingNode: ts.Node): spec.Type | undefined {
         const [assm, ] = ref.fqn.split('.');
+        let type;
         if (assm === this.projectInfo.name) {
-            return this._types[ref.fqn];
+            type = this._types[ref.fqn];
         } else {
             const assembly = this.projectInfo.transitiveDependencies.find(dep => dep.name === assm);
-            return assembly && assembly.types && assembly.types[ref.fqn];
+            type = assembly && assembly.types && assembly.types[ref.fqn];
         }
+
+        if (!type) {
+            this._diagnostic(referencingNode, ts.DiagnosticCategory.Error, `Unable to resolve referenced type '${ref.fqn}'. Missing export?`);
+        }
+
+        return type;
     }
 
     private _diagnostic(node: ts.Node | null, category: ts.DiagnosticCategory, messageText: string) {
@@ -206,7 +213,7 @@ export class Assembler implements Emitter {
             return `unknown.${typeName}`;
         }
         const fqn = `${pkg.name}.${typeName}`;
-        if (pkg.name !== this.projectInfo.name && !this._dereference({ fqn })) {
+        if (pkg.name !== this.projectInfo.name && !this._dereference({ fqn }, type.symbol.valueDeclaration)) {
             this._diagnostic(type.symbol.valueDeclaration,
                              ts.DiagnosticCategory.Error,
                              `Use of foreign type not present in the ${pkg.name}'s assembly: ${fqn}`);
@@ -314,7 +321,8 @@ export class Assembler implements Emitter {
                 continue;
             }
             this._defer(() => {
-                if (!spec.isClassType(this._dereference(ref))) {
+                const deref = this._dereference(ref, base.symbol.valueDeclaration);
+                if (deref && !spec.isClassType(deref)) {
                     this._diagnostic(base.symbol.valueDeclaration,
                                     ts.DiagnosticCategory.Error,
                                     `Base type of ${jsiiType.fqn} is not a class (${spec.describeTypeReference(ref)})`);
@@ -340,7 +348,8 @@ export class Assembler implements Emitter {
                     continue;
                 }
                 this._defer(() => {
-                    if (!spec.isInterfaceType(this._dereference(typeRef))) {
+                    const deref = this._dereference(typeRef, expression);
+                    if (deref && !spec.isInterfaceType(deref)) {
                         this._diagnostic(expression,
                                         ts.DiagnosticCategory.Error,
                                         `Implements clause of ${jsiiType.fqn} uses ${spec.describeTypeReference(typeRef)} as an interface`);
@@ -393,17 +402,15 @@ export class Assembler implements Emitter {
             }
         } else if (jsiiType.base) {
             this._defer(() => {
-                const baseType = this._dereference(jsiiType.base!);
-                if (!baseType) {
-                    this._diagnostic(type.symbol.valueDeclaration,
-                                    ts.DiagnosticCategory.Error,
-                                    `Unable to resolve type ${jsiiType.base!.fqn} (base type of ${jsiiType.fqn})`);
-                } else if (spec.isClassType(baseType)) {
-                    jsiiType.initializer = baseType.initializer;
-                } else {
-                    this._diagnostic(type.symbol.valueDeclaration,
-                        ts.DiagnosticCategory.Error,
-                        `Base type of ${jsiiType.fqn} (${jsiiType.base!.fqn}) is not a class`);
+                const baseType = this._dereference(jsiiType.base!, type.symbol.valueDeclaration);
+                if (baseType) {
+                    if (spec.isClassType(baseType)) {
+                        jsiiType.initializer = baseType.initializer;
+                    } else {
+                        this._diagnostic(type.symbol.valueDeclaration,
+                            ts.DiagnosticCategory.Error,
+                            `Base type of ${jsiiType.fqn} (${jsiiType.base!.fqn}) is not a class`);
+                    }
                 }
             });
         } else {
@@ -499,17 +506,13 @@ export class Assembler implements Emitter {
                 continue;
             }
             this._defer(() => {
-                if (!spec.isInterfaceType(this._dereference(ref))) {
-                    const baseType = this._dereference(ref);
-                    if (baseType) {
-                        this._diagnostic(base.symbol.valueDeclaration,
-                                        ts.DiagnosticCategory.Error,
-                                        `Base type of ${jsiiType.fqn} is not an interface (${baseType.kind} ${spec.describeTypeReference(ref)})`);
-                    } else {
-                        this._diagnostic(base.symbol.valueDeclaration,
-                            ts.DiagnosticCategory.Error,
-                            `Base type of ${jsiiType.fqn} could not be resolved (${spec.describeTypeReference(ref)})`);
-                    }
+                const baseType = this._dereference(ref, base.symbol.valueDeclaration);
+                if (baseType && !spec.isInterfaceType(baseType)) {
+                    // tslint:disable:max-line-length
+                    this._diagnostic(base.symbol.valueDeclaration,
+                                    ts.DiagnosticCategory.Error,
+                                    `Base type of ${jsiiType.fqn} is not an interface (${baseType.kind} ${spec.describeTypeReference(ref)})`);
+                    // tslint:enable:max-line-length
                 }
             });
             if (jsiiType.interfaces) {

packages/jsii/test/negatives/neg.deref-error.ts

@@ -0,0 +1,9 @@
+///!MATCH_ERROR: Unable to resolve referenced type 'Base'. Missing export?
+
+class Base {
+
+}
+
+export class Derived extends Base {
+
+}