Isolate test failure from #57570

src/compiler/resolutionCache.ts

@@ -739,6 +739,19 @@ export function createResolutionCache(resolutionHost: ResolutionCacheHost, rootD
                 const expected = isExternalOrCommonJsModule(newFile) ? newFile.packageJsonLocations?.length ?? 0 : 0;
                 const existing = impliedFormatPackageJsons.get(newFile.path) ?? emptyArray;
                 for (let i = existing.length; i < expected; i++) {
+                    // Somehow, this loop is incorrect. Before the debug assertion happens in the failing test,
+                    // a referenced config file is updated to include "declarationDir": "decls", which causes
+                    // a file from that project to be renamed from `.../logic/index.d.ts` to
+                    // `.../logic/decls/index.d.ts` between `oldProgram` and `newProgram` here. The new
+                    // version of the source file has an additional package.json lookup location
+                    // (`.../logic/decls/package.json`) compared to the old. `existing.length` is 5 and
+                    // `expected` is 6, so the loop runs once to create a file watcher for the last element
+                    // of `newFile.packageJsonLocations`, which is `/package.json`, incrementing its
+                    // `files` count. It seems like the correct behavior would have been to create a file
+                    // watcher for the *new* package.json location, which happens to be the *first* element
+                    // of `newFile.packageJsonLocations`, but I'm not sure how this code is supposed to know
+                    // which locations have been added and/or removed - relying on length/order doesn't appear
+                    // to be reliable.
                     createFileWatcherOfAffectingLocation(newFile.packageJsonLocations![i], /*forResolution*/ false);
                 }
                 if (existing.length > expected) {

src/testRunner/unittests/helpers/sampleProjectReferences.ts

@@ -35,6 +35,8 @@ export function getFsContentsForSampleProjectReferences(): FsContents {
         [libFile.path]: libFile.content,
         "/user/username/projects/sample1/core/tsconfig.json": jsonToReadableText({
             compilerOptions: {
+                module: "nodenext",
+                target: "es5",
                 composite: true,
                 declaration: true,
                 declarationMap: true,
@@ -64,6 +66,8 @@ export function getFsContentsForSampleProjectReferences(): FsContents {
             ],
             files: ["index.ts"],
             compilerOptions: {
+                module: "nodenext",
+                target: "es5",
                 composite: true,
                 declaration: true,
                 forceConsistentCasingInFileNames: true,

src/testRunner/unittests/tscWatch/projectsWithReferences.ts

@@ -63,7 +63,7 @@ describe("unittests:: tsc-watch:: projects with references: invoking when refere
                     sys.writeFile(
                         "/user/username/projects/sample1/logic/tsconfig.json",
                         jsonToReadableText({
-                            compilerOptions: { composite: true, declaration: true, declarationDir: "decls" },
+                            compilerOptions: { module: "nodenext", target: "es5", composite: true, declaration: true, declarationDir: "decls" },
                             references: [{ path: "../core" }],
                         }),
                     );