fix: deep-import of index files breaks ts3.9 compilers (#81)

The `typesVersions` directive applies recursively when the TypeScript
compiler searches for the appropriate object to load... When attempting
to resolve a directory, it will first rewrite that directory according
to the typesVersions directive, and then it will rewrite the `index`
sub-path within there again, resulting in two distinct rewrites
happening within the same resolution. This appears to be a TypeScript
bug as reported in microsoft/TypeScript#43133.

The work-around is to include a second rewrite candidate that explicitly
targets the `*/index.d.ts` sub-path, which removes the need for a
rewrite when searching down the `index` route.

Causes projen/projen#2570



---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

fixtures/@scope/jsii-calc-base/lib/deep/index.ts

@@ -0,0 +1 @@
+export class BarrelImportClass {}

fixtures/@scope/jsii-calc-base/lib/index.ts

@@ -35,3 +35,5 @@ export class StaticConsumer {
     return StaticConsumerBase.consume(...args);
   }
 }
+
+export * as deep from './deep';

fixtures/@scope/jsii-calc-base/package.json

@@ -26,10 +26,20 @@
   "typesVersions": {
     "<=3.9": {
       "*": [
-        ".types-compat/ts3.9/*"
+        ".types-compat/ts3.9/*",
+        ".types-compat/ts3.9/*/index.d.ts"
       ]
     }
   },
+  "exports": {
+    ".": {
+      "types": "./lib/index.d.ts",
+      "import": "./lib/index.js",
+      "require": "./lib/index.js"
+    },
+    "./lib/deep": "./lib/deep.js",
+    "./.warnings.jsii.js": "./.warnings.jsii.js"
+  },
   "dependencies": {
     "@scope/jsii-calc-base-of-base": "^2.1.1"
   },

fixtures/@scope/jsii-calc-lib/lib/index.ts

@@ -1,4 +1,5 @@
 import * as base from '@scope/jsii-calc-base';
+import * as deep from '@scope/jsii-calc-base/lib/deep';
 import { Very } from '@scope/jsii-calc-base-of-base';
 
 /**
@@ -124,6 +125,9 @@ export class BaseFor2647 {
 
   public foo(obj: base.IBaseInterface): void {
     obj.bar();
+
+    // Just so it's used... no other interest here.
+    new deep.BarrelImportClass();
   }
 }
 

fixtures/jsii-calc/package.json

@@ -35,7 +35,8 @@
   "typesVersions": {
     "<=3.9": {
       "*": [
-        ".types-compat/ts3.9/*"
+        ".types-compat/ts3.9/*",
+        ".types-compat/ts3.9/*/index.d.ts"
       ]
     }
   },

src/downlevel-dts.ts

@@ -115,7 +115,8 @@ export function emitDownleveledDeclarations({ packageJson, projectRoot, tsc }: P
     typesVersions ??= {};
     const from = [...(tsc?.outDir != null ? [tsc?.outDir] : []), '*'].join('/');
     const to = [...(tsc?.outDir != null ? [tsc?.outDir] : []), TYPES_COMPAT, versionSuffix, '*'].join('/');
-    typesVersions[`<=${version}`] = { [from]: [to] };
+    // We put 2 candidate redirects (first match wins), so that it works for nested imports, too (see: https://github.com/microsoft/TypeScript/issues/43133)
+    typesVersions[`<=${version}`] = { [from]: [to, `${to}/index.d.ts`] };
   }
 
   // Compare JSON stringifications, as the order of keys is important here...