From a8ebf4dc0056fbd4d2a9637bad423ed1c3ee0d14 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Mon, 3 Aug 2020 21:54:38 +0000
Subject: [PATCH] Cherry-pick PR #39889 into release-4.0

Component commits:
c3ac7fa2e7 Handle the fact that noResolveResolution resolution is reused Fixes #39795
---
 src/compiler/resolutionCache.ts                |  5 +++--
 .../tsserver/approximateSemanticOnlyServer.ts  | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/src/compiler/resolutionCache.ts b/src/compiler/resolutionCache.ts
index 56ea6a03e8047..d6dfaf40966b5 100644
--- a/src/compiler/resolutionCache.ts
+++ b/src/compiler/resolutionCache.ts
@@ -406,7 +406,7 @@ namespace ts {
                         perDirectoryResolution.set(name, resolution);
                     }
                     resolutionsInFile.set(name, resolution);
-                    watchFailedLookupLocationsOfExternalModuleResolutions(name, resolution, path, getResolutionWithResolvedFileName);
+                    watchFailedLookupLocationsOfExternalModuleResolutions(name, resolution, path, getResolutionWithResolvedFileName, noResolveResolution);
                     if (existingResolution) {
                         stopWatchFailedLookupLocationOfResolution(existingResolution, path, getResolutionWithResolvedFileName);
                     }
@@ -561,6 +561,7 @@ namespace ts {
             resolution: T,
             filePath: Path,
             getResolutionWithResolvedFileName: GetResolutionWithResolvedFileName<T, R>,
+            noResolveResolution: T
         ) {
             if (resolution.refCount) {
                 resolution.refCount++;
@@ -568,7 +569,7 @@ namespace ts {
             }
             else {
                 resolution.refCount = 1;
-                Debug.assert(resolution.files === undefined);
+                Debug.assert(resolution.files === undefined || (resolution === noResolveResolution && length(noResolveResolution.files) === 0));
                 if (isExternalModuleNameRelative(name)) {
                     watchFailedLookupLocationOfResolution(resolution);
                 }
diff --git a/src/testRunner/unittests/tsserver/approximateSemanticOnlyServer.ts b/src/testRunner/unittests/tsserver/approximateSemanticOnlyServer.ts
index 1b517e027b882..3cf036223404b 100644
--- a/src/testRunner/unittests/tsserver/approximateSemanticOnlyServer.ts
+++ b/src/testRunner/unittests/tsserver/approximateSemanticOnlyServer.ts
@@ -213,5 +213,23 @@ function fooB() { }`
             assert.isFalse(project.dirty);
             checkProjectActualFiles(project, [libFile.path, file1.path, file2.path, file3.path, something.path]);
         });
+
+        it("should not crash when external module name resolution is reused", () => {
+            const { session, file1, file2, file3 } = setup();
+            const service = session.getProjectService();
+            openFilesForSession([file1], session);
+            checkNumberOfProjects(service, { inferredProjects: 1 });
+            const project = service.inferredProjects[0];
+            checkProjectActualFiles(project, [libFile.path, file1.path, file2.path]);
+
+            // Close the file that contains non relative external module name and open some file that doesnt have non relative external module import
+            closeFilesForSession([file1], session);
+            openFilesForSession([file3], session);
+            checkProjectActualFiles(project, [libFile.path, file3.path]);
+
+            // Open file with non relative external module name
+            openFilesForSession([file2], session);
+            checkProjectActualFiles(project, [libFile.path, file2.path, file3.path]);
+        });
     });
 }