fix(jsii): consider interfaces from erased base classes

packages/jsii-calc/lib/erasures.ts

@@ -37,3 +37,12 @@ interface IJSII417PrivateBase extends IJSII417PublicBaseOfBase {
 export interface IJSII417Derived extends IJSII417PrivateBase {
   baz(): void;
 }
+
+//
+// Interfaces should be copied from erased classes to public classes
+// https://github.com/awslabs/jsii/issues/487
+//
+// tslint:disable-next-line:no-empty-interface
+export interface IJsii487External { }
+class Jsii487Internal implements IJsii487External { }
+export class Jsii487Derived extends Jsii487Internal { }

packages/jsii-calc/test/assembly.jsii

@@ -3035,6 +3035,16 @@
         }
       ]
     },
+    "jsii-calc.IJsii487External": {
+      "assembly": "jsii-calc",
+      "fqn": "jsii-calc.IJsii487External",
+      "kind": "interface",
+      "locationInModule": {
+        "filename": "lib/erasures.ts",
+        "line": 46
+      },
+      "name": "IJsii487External"
+    },
     "jsii-calc.IMutableObjectLiteral": {
       "assembly": "jsii-calc",
       "fqn": "jsii-calc.IMutableObjectLiteral",
@@ -4038,6 +4048,20 @@
         }
       ]
     },
+    "jsii-calc.Jsii487Derived": {
+      "assembly": "jsii-calc",
+      "fqn": "jsii-calc.Jsii487Derived",
+      "initializer": {},
+      "interfaces": [
+        "jsii-calc.IJsii487External"
+      ],
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/erasures.ts",
+        "line": 48
+      },
+      "name": "Jsii487Derived"
+    },
     "jsii-calc.JsiiAgent": {
       "assembly": "jsii-calc",
       "docs": {
@@ -6744,5 +6768,5 @@
     }
   },
   "version": "0.10.3",
-  "fingerprint": "HkyspjelXmNd6gQibMCkuk+WfsM6Q9e3uZLl2ee3kbY="
+  "fingerprint": "CSE0eCLDxK7s+qnmR58ZJo0mcEROi4f8Sxlx2pHaBDs="
 }

packages/jsii/lib/assembler.ts

@@ -450,10 +450,11 @@ export class Assembler implements Emitter {
         continue;
       }
 
-      /*
-       * Crawl up the inheritance tree if the current base type is not exported, so we identify the type(s) to be
-       * erased, and identify the closest exported base class, should there be one.
-       */
+      //
+      // base classes ("extends foo")
+
+      // Crawl up the inheritance tree if the current base type is not exported, so we identify the type(s) to be
+      // erased, and identify the closest exported base class, should there be one.
       // tslint:disable-next-line: no-bitwise
       while (base && this._isPrivateOrInternal(base.symbol)) {
         LOG.debug(`Base class of ${colors.green(jsiiType.fqn)} named ${colors.green(base.symbol.name)} is not exported, erasing it...`);
@@ -482,15 +483,29 @@ export class Assembler implements Emitter {
       });
       jsiiType.base = ref.fqn;
     }
-    for (const clause of (type.symbol.valueDeclaration as ts.ClassDeclaration).heritageClauses || []) {
-      if (clause.token === ts.SyntaxKind.ExtendsKeyword) {
-        // Handled by `getBaseTypes`
-        continue;
-      } else if (clause.token !== ts.SyntaxKind.ImplementsKeyword) {
-        this._diagnostic(clause, ts.DiagnosticCategory.Error, `Ignoring ${ts.SyntaxKind[clause.token]} heritage clause`);
-        continue;
+
+    //
+    // base interfaces ("implements foo")
+
+    // collect all "implements" declarations from the current type and all
+    // erased base types (because otherwise we lose them, see jsii#487)
+    const implementsClauses = new Array<ts.HeritageClause>();
+    for (const heritage of [ type, ...erasedBases ].map(t => (t.symbol.valueDeclaration as ts.ClassDeclaration).heritageClauses || [])) {
+      for (const clause of heritage) {
+        if (clause.token === ts.SyntaxKind.ExtendsKeyword) {
+          // Handled by `getBaseTypes`
+          continue;
+        } else if (clause.token !== ts.SyntaxKind.ImplementsKeyword) {
+          this._diagnostic(clause, ts.DiagnosticCategory.Error, `Ignoring ${ts.SyntaxKind[clause.token]} heritage clause`);
+          continue;
+        }
+
+        implementsClauses.push(clause);
       }
+    }
 
+    // process all "implements" clauses
+    for (const clause of implementsClauses) {
       const { interfaces } = await this._processBaseInterfaces(fqn, clause.types.map(t => this._typeChecker.getTypeFromTypeNode(t)));
       jsiiType.interfaces = apply(interfaces, arr => arr.map(i => i.fqn));
       if (interfaces) {
@@ -512,7 +527,8 @@ export class Assembler implements Emitter {
 
     const allDeclarations: Array<{ decl: ts.Declaration, type: ts.InterfaceType | ts.BaseType }>
       = type.symbol.declarations.map(decl => ({ decl, type }));
-    // Considering erased bases' declarations, too, so they are "blended in"
+
+      // Considering erased bases' declarations, too, so they are "blended in"
     for (const base of erasedBases) {
       allDeclarations.push(...base.symbol.declarations.map(decl => ({ decl, type: base })));
     }