Delay the calculation of common source root if it would be needed when calculation dts files

src/compiler/program.ts

@@ -4441,7 +4441,8 @@ export function createProgram(rootNamesOrOptions: readonly string[] | CreateProg
             options.outDir || // there is --outDir specified
             options.rootDir || // there is --rootDir specified
             options.sourceRoot || // there is --sourceRoot specified
-            options.mapRoot // there is --mapRoot specified
+            options.mapRoot || // there is --mapRoot specified
+            (getEmitDeclarations(options) && options.declarationDir) // there is --declarationDir specified
         ) {
             // Precalculate and cache the common source directory
             const dir = getCommonSourceDirectory();

src/compiler/utilities.ts

@@ -6315,15 +6315,15 @@ export function getOwnEmitOutputFilePath(fileName: string, host: EmitHost, exten
 
 /** @internal */
 export function getDeclarationEmitOutputFilePath(fileName: string, host: EmitHost) {
-    return getDeclarationEmitOutputFilePathWorker(fileName, host.getCompilerOptions(), host.getCurrentDirectory(), host.getCommonSourceDirectory(), f => host.getCanonicalFileName(f));
+    return getDeclarationEmitOutputFilePathWorker(fileName, host.getCompilerOptions(), host);
 }
 
 /** @internal */
-export function getDeclarationEmitOutputFilePathWorker(fileName: string, options: CompilerOptions, currentDirectory: string, commonSourceDirectory: string, getCanonicalFileName: GetCanonicalFileName): string {
+export function getDeclarationEmitOutputFilePathWorker(fileName: string, options: CompilerOptions, host: Pick<EmitHost, "getCommonSourceDirectory" | "getCurrentDirectory" | "getCanonicalFileName">): string {
     const outputDir = options.declarationDir || options.outDir; // Prefer declaration folder if specified
 
     const path = outputDir
-        ? getSourceFilePathInNewDirWorker(fileName, outputDir, currentDirectory, commonSourceDirectory, getCanonicalFileName)
+        ? getSourceFilePathInNewDirWorker(fileName, outputDir, host.getCurrentDirectory(), host.getCommonSourceDirectory(), f => host.getCanonicalFileName(f))
         : fileName;
     const declarationExtension = getDeclarationEmitExtensionForPath(path);
     return removeFileExtension(path) + declarationExtension;

src/server/project.ts

@@ -1699,7 +1699,7 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
                                 !sourceFile ||
                                 sourceFile.resolvedPath !== source ||
                                 !this.isValidGeneratedFileWatcher(
-                                    getDeclarationEmitOutputFilePathWorker(sourceFile.fileName, this.compilerOptions, this.currentDirectory, this.program!.getCommonSourceDirectory(), this.getCanonicalFileName),
+                                    getDeclarationEmitOutputFilePathWorker(sourceFile.fileName, this.compilerOptions, this.program!),
                                     watcher,
                                 )
                             ) {

src/services/sourcemaps.ts

@@ -117,7 +117,7 @@ export function getSourceMapper(host: SourceMapperHost): SourceMapper {
 
         const declarationPath = outPath ?
             removeFileExtension(outPath) + Extension.Dts :
-            getDeclarationEmitOutputFilePathWorker(info.fileName, program.getCompilerOptions(), currentDirectory, program.getCommonSourceDirectory(), getCanonicalFileName);
+            getDeclarationEmitOutputFilePathWorker(info.fileName, program.getCompilerOptions(), program);
         if (declarationPath === undefined) return undefined;
 
         const newLoc = getDocumentPositionMapper(declarationPath, info.fileName).getGeneratedPosition(info);

src/testRunner/unittests/tsserver/projectErrors.ts

@@ -1,5 +1,6 @@
 import * as ts from "../../_namespaces/ts.js";
 import { jsonToReadableText } from "../helpers.js";
+import { libContent } from "../helpers/contents.js";
 import {
     baselineTsserverLogs,
     closeFilesForSession,
@@ -833,3 +834,29 @@ describe("unittests:: tsserver:: projectErrors:: with file rename on wsl2::", ()
         baselineTsserverLogs("projectErrors", "file rename on wsl2", session);
     });
 });
+
+describe("unittests:: tsserver:: projectErrors:: dts errors when files dont belong to common root", () => {
+    [undefined, "decls"].forEach(declarationDir => {
+        it(`dts errors when files dont belong to common root${declarationDir ? " with declarationDir" : ""}`, () => {
+            const host = createServerHost({
+                "/home/src/projects/project/src/file.ts": `import { a } from "../a";`,
+                "/home/src/projects/project/a.ts": `export const a = 10;`,
+                "/home/src/projects/project/src/tsconfig.json": jsonToReadableText({
+                    compilerOptions: {
+                        composite: true,
+                        noEmit: true,
+                        declarationDir,
+                    },
+                }),
+                [libFile.path]: libContent,
+            });
+            const session = new TestSession(host);
+            openFilesForSession(["/home/src/projects/project/src/file.ts"], session);
+            verifyGetErrRequest({
+                session,
+                files: ["/home/src/projects/project/src/file.ts"],
+            });
+            baselineTsserverLogs("projectErrors", `dts errors when files dont belong to common root${declarationDir ? " with declarationDir" : ""}`, session);
+        });
+    });
+});