Fix --strictNullChecks error with getDefaultProjectForFile

src/server/editorServices.ts

@@ -693,18 +693,26 @@ namespace ts.server {
         }
 
         getDefaultProjectForFile(fileName: NormalizedPath, ensureProject: boolean): Project | undefined {
-            let scriptInfo = this.getScriptInfoForNormalizedPath(fileName);
-            if (ensureProject && (!scriptInfo || scriptInfo.isOrphan())) {
-                this.ensureProjectStructuresUptoDate();
-                scriptInfo = this.getScriptInfoForNormalizedPath(fileName);
-                if (!scriptInfo) {
-                    return Errors.ThrowNoProject();
-                }
-                return scriptInfo.getDefaultProject();
-            }
+            return ensureProject ? this.ensureDefaultProjectForFile(fileName) : this.tryGetDefaultProjectForFile(fileName);
+        }
+
+        /* @internal */
+        tryGetDefaultProjectForFile(fileName: NormalizedPath): Project | undefined {
+            const scriptInfo = this.getScriptInfoForNormalizedPath(fileName);
             return scriptInfo && !scriptInfo.isOrphan() ? scriptInfo.getDefaultProject() : undefined;
         }
 
+        /* @internal */
+        ensureDefaultProjectForFile(fileName: NormalizedPath): Project {
+            return this.tryGetDefaultProjectForFile(fileName) || this.doEnsureDefaultProjectForFile(fileName);
+        }
+
+        private doEnsureDefaultProjectForFile(fileName: NormalizedPath): Project {
+            this.ensureProjectStructuresUptoDate();
+            const scriptInfo = this.getScriptInfoForNormalizedPath(fileName);
+            return scriptInfo ? scriptInfo.getDefaultProject() : Errors.ThrowNoProject();
+        }
+
         getScriptInfoEnsuringProjectsUptoDate(uncheckedFileName: string) {
             this.ensureProjectStructuresUptoDate();
             return this.getScriptInfo(uncheckedFileName);

src/server/session.ts

@@ -1150,7 +1150,7 @@ namespace ts.server {
             // Since this is syntactic operation, there should always be project for the file
             // we wouldnt have to ensure project but rather throw if we dont get project
             const file = toNormalizedPath(args.file);
-            const project = this.getProject(args.projectFileName) || this.projectService.getDefaultProjectForFile(file, /*ensureProject*/ false);
+            const project = this.getProject(args.projectFileName) || this.projectService.tryGetDefaultProjectForFile(file);
             if (!project) {
                 return Errors.ThrowNoProject();
             }
@@ -1162,7 +1162,7 @@ namespace ts.server {
 
         private getFileAndProjectWorker(uncheckedFileName: string, projectFileName: string | undefined): { file: NormalizedPath, project: Project } {
             const file = toNormalizedPath(uncheckedFileName);
-            const project = this.getProject(projectFileName) || this.projectService.getDefaultProjectForFile(file, /*ensureProject*/ true)!; // TODO: GH#18217
+            const project = this.getProject(projectFileName) || this.projectService.ensureDefaultProjectForFile(file);
             return { file, project };
         }
 
@@ -1454,7 +1454,7 @@ namespace ts.server {
         private createCheckList(fileNames: string[], defaultProject?: Project): PendingErrorCheck[] {
             return mapDefined<string, PendingErrorCheck>(fileNames, uncheckedFileName => {
                 const fileName = toNormalizedPath(uncheckedFileName);
-                const project = defaultProject || this.projectService.getDefaultProjectForFile(fileName, /*ensureProject*/ false);
+                const project = defaultProject || this.projectService.tryGetDefaultProjectForFile(fileName);
                 return project && { fileName, project };
             });
         }
@@ -1852,7 +1852,7 @@ namespace ts.server {
             const lowPriorityFiles: NormalizedPath[] = [];
             const veryLowPriorityFiles: NormalizedPath[] = [];
             const normalizedFileName = toNormalizedPath(fileName);
-            const project = this.projectService.getDefaultProjectForFile(normalizedFileName, /*ensureProject*/ true)!;
+            const project = this.projectService.ensureDefaultProjectForFile(normalizedFileName);
             for (const fileNameInProject of fileNamesInProject) {
                 if (this.getCanonicalFileName(fileNameInProject) === this.getCanonicalFileName(fileName)) {
                     highPriorityFiles.push(fileNameInProject);

src/testRunner/unittests/tsserverProjectSystem.ts

@@ -3097,7 +3097,7 @@ namespace ts.projectSystem {
             checkProjectRootFiles(project, [file.path]);
             checkProjectActualFiles(project, [file.path, libFile.path]);
 
-            assert.strictEqual(projectService.getDefaultProjectForFile(server.toNormalizedPath(file.path), /*ensureProject*/ true), project);
+            assert.strictEqual(projectService.ensureDefaultProjectForFile(server.toNormalizedPath(file.path)), project);
             const indexOfX = file.content.indexOf("x");
             assert.deepEqual(project.getLanguageService(/*ensureSynchronized*/ true).getQuickInfoAtPosition(file.path, indexOfX), {
                 kind: ScriptElementKind.variableElement,

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -13801,6 +13801,9 @@ declare namespace ts.server {
         setCompilerOptionsForInferredProjects(projectCompilerOptions: protocol.ExternalProjectCompilerOptions, projectRootPath?: string): void;
         findProject(projectName: string): Project | undefined;
         getDefaultProjectForFile(fileName: NormalizedPath, ensureProject: boolean): Project | undefined;
+        tryGetDefaultProjectForFile(fileName: NormalizedPath): Project | undefined;
+        ensureDefaultProjectForFile(fileName: NormalizedPath): Project;
+        private doEnsureDefaultProjectForFile;
         getScriptInfoEnsuringProjectsUptoDate(uncheckedFileName: string): ScriptInfo | undefined;
         private ensureProjectStructuresUptoDate;
         getFormatCodeOptions(file: NormalizedPath): FormatCodeSettings;