Avoid unnecessary type assertions in module resolution code

src/compiler/program.ts

@@ -760,32 +760,32 @@ namespace ts {
         let _compilerOptionsObjectLiteralSyntax: ObjectLiteralExpression | null | undefined;
 
         let moduleResolutionCache: ModuleResolutionCache | undefined;
-        let resolveModuleNamesWorker: (moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference) => ResolvedModuleFull[];
+        let resolveModuleNamesWorker: NonNullable<CompilerHost["resolveModuleNames"]>;
         const hasInvalidatedResolution = host.hasInvalidatedResolution || returnFalse;
         if (host.resolveModuleNames) {
             resolveModuleNamesWorker = (moduleNames, containingFile, reusedNames, redirectedReference) => host.resolveModuleNames!(Debug.assertEachDefined(moduleNames), containingFile, reusedNames, redirectedReference).map(resolved => {
                 // An older host may have omitted extension, in which case we should infer it from the file extension of resolvedFileName.
-                if (!resolved || (resolved as ResolvedModuleFull).extension !== undefined) {
+                if (!resolved || resolved.extension !== undefined) {
                     return resolved as ResolvedModuleFull;
                 }
-                const withExtension = clone(resolved) as ResolvedModuleFull;
+                const withExtension = clone(resolved);
                 withExtension.extension = extensionFromPath(resolved.resolvedFileName);
                 return withExtension;
             });
         }
         else {
             moduleResolutionCache = createModuleResolutionCache(currentDirectory, x => host.getCanonicalFileName(x), options);
-            const loader = (moduleName: string, containingFile: string, redirectedReference: ResolvedProjectReference | undefined) => resolveModuleName(moduleName, containingFile, options, host, moduleResolutionCache, redirectedReference).resolvedModule!; // TODO: GH#18217
-            resolveModuleNamesWorker = (moduleNames, containingFile, _reusedNames, redirectedReference) => loadWithLocalCache<ResolvedModuleFull>(Debug.assertEachDefined(moduleNames), containingFile, redirectedReference, loader);
+            const loader = (moduleName: string, containingFile: string, redirectedReference: ResolvedProjectReference | undefined) => resolveModuleName(moduleName, containingFile, options, host, moduleResolutionCache, redirectedReference).resolvedModule;
+            resolveModuleNamesWorker = (moduleNames, containingFile, _reusedNames, redirectedReference) => loadWithLocalCache(Debug.assertEachDefined(moduleNames), containingFile, redirectedReference, loader);
         }
 
-        let resolveTypeReferenceDirectiveNamesWorker: (typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference) => (ResolvedTypeReferenceDirective | undefined)[];
+        let resolveTypeReferenceDirectiveNamesWorker: NonNullable<CompilerHost["resolveTypeReferenceDirectives"]>;
         if (host.resolveTypeReferenceDirectives) {
             resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile, redirectedReference) => host.resolveTypeReferenceDirectives!(Debug.assertEachDefined(typeDirectiveNames), containingFile, redirectedReference);
         }
         else {
-            const loader = (typesRef: string, containingFile: string, redirectedReference: ResolvedProjectReference | undefined) => resolveTypeReferenceDirective(typesRef, containingFile, options, host, redirectedReference).resolvedTypeReferenceDirective!; // TODO: GH#18217
-            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile, redirectedReference) => loadWithLocalCache<ResolvedTypeReferenceDirective>(Debug.assertEachDefined(typeReferenceDirectiveNames), containingFile, redirectedReference, loader);
+            const loader = (typesRef: string, containingFile: string, redirectedReference: ResolvedProjectReference | undefined) => resolveTypeReferenceDirective(typesRef, containingFile, options, host, redirectedReference).resolvedTypeReferenceDirective;
+            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile, redirectedReference) => loadWithLocalCache(Debug.assertEachDefined(typeReferenceDirectiveNames), containingFile, redirectedReference, loader);
         }
 
         // Map from a stringified PackageId to the source file with that id.
@@ -1059,7 +1059,7 @@ namespace ts {
              * Needs to be reset to undefined before returning,
              * * ResolvedModuleFull instance: can be reused.
              */
-            let result: ResolvedModuleFull[] | undefined;
+            let result: (ResolvedModuleFull | undefined)[] | undefined;
             let reusedNames: string[] | undefined;
             /** A transient placeholder used to mark predicted resolution in the result list. */
             const predictedToResolveToAmbientModuleMarker: ResolvedModuleFull = <any>{};

src/compiler/types.ts

@@ -5121,7 +5121,7 @@ namespace ts {
          * If resolveModuleNames is implemented then implementation for members from ModuleResolutionHost can be just
          * 'throw new Error("NotImplemented")'
          */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /**
          * This method is a companion for 'resolveModuleNames' and is used to resolve 'types' references to actual type declaration files
          */

src/compiler/utilities.ts

@@ -224,9 +224,9 @@ namespace ts {
         return sourceFile && sourceFile.resolvedModules && sourceFile.resolvedModules.get(moduleNameText);
     }
 
-    export function setResolvedModule(sourceFile: SourceFile, moduleNameText: string, resolvedModule: ResolvedModuleFull): void {
+    export function setResolvedModule(sourceFile: SourceFile, moduleNameText: string, resolvedModule: ResolvedModuleFull | undefined): void {
         if (!sourceFile.resolvedModules) {
-            sourceFile.resolvedModules = createMap<ResolvedModuleFull>();
+            sourceFile.resolvedModules = createMap<ResolvedModuleFull | undefined>();
         }
 
         sourceFile.resolvedModules.set(moduleNameText, resolvedModule);

src/compiler/watch.ts

@@ -497,7 +497,7 @@ namespace ts {
         getEnvironmentVariable?(name: string): string | undefined;
 
         /** If provided, used to resolve the module names, otherwise typescript's default module resolution */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /** If provided, used to resolve type reference directives, otherwise typescript's default resolution */
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
     }

src/services/types.ts

@@ -211,7 +211,7 @@ namespace ts {
          *
          * If this is implemented, `getResolvedModuleWithFailedLookupLocationsFromCache` should be too.
          */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         getResolvedModuleWithFailedLookupLocationsFromCache?(modulename: string, containingFile: string): ResolvedModuleWithFailedLookupLocations | undefined;
         resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
         /* @internal */ hasInvalidatedResolution?: HasInvalidatedResolution;

src/testRunner/unittests/tscWatch/watchApi.ts

@@ -30,7 +30,7 @@ namespace ts.tscWatch {
                     resolvedFileName: resolvedModule.resolvedFileName,
                     isExternalLibraryImport: resolvedModule.isExternalLibraryImport,
                     originalFileName: resolvedModule.originalPath,
-                };
+                } as ResolvedModule as ResolvedModuleFull;
             });
             const watch = createWatchProgram(compilerHost);
             const program = watch.getCurrentProgram().getProgram();

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

@@ -2754,7 +2754,7 @@ declare namespace ts {
         useCaseSensitiveFileNames(): boolean;
         getNewLine(): string;
         readDirectory?(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /**
          * This method is a companion for 'resolveModuleNames' and is used to resolve 'types' references to actual type declaration files
          */
@@ -4492,7 +4492,7 @@ declare namespace ts {
         /** If provided is used to get the environment variable */
         getEnvironmentVariable?(name: string): string | undefined;
         /** If provided, used to resolve the module names, otherwise typescript's default module resolution */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /** If provided, used to resolve type reference directives, otherwise typescript's default resolution */
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
     }
@@ -4762,7 +4762,7 @@ declare namespace ts {
         realpath?(path: string): string;
         fileExists?(path: string): boolean;
         getTypeRootsVersion?(): number;
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         getResolvedModuleWithFailedLookupLocationsFromCache?(modulename: string, containingFile: string): ResolvedModuleWithFailedLookupLocations | undefined;
         resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
         getDirectories?(directoryName: string): string[];

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

@@ -2754,7 +2754,7 @@ declare namespace ts {
         useCaseSensitiveFileNames(): boolean;
         getNewLine(): string;
         readDirectory?(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /**
          * This method is a companion for 'resolveModuleNames' and is used to resolve 'types' references to actual type declaration files
          */
@@ -4492,7 +4492,7 @@ declare namespace ts {
         /** If provided is used to get the environment variable */
         getEnvironmentVariable?(name: string): string | undefined;
         /** If provided, used to resolve the module names, otherwise typescript's default module resolution */
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         /** If provided, used to resolve type reference directives, otherwise typescript's default resolution */
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
     }
@@ -4762,7 +4762,7 @@ declare namespace ts {
         realpath?(path: string): string;
         fileExists?(path: string): boolean;
         getTypeRootsVersion?(): number;
-        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModule | undefined)[];
+        resolveModuleNames?(moduleNames: string[], containingFile: string, reusedNames?: string[], redirectedReference?: ResolvedProjectReference): (ResolvedModuleFull | undefined)[];
         getResolvedModuleWithFailedLookupLocationsFromCache?(modulename: string, containingFile: string): ResolvedModuleWithFailedLookupLocations | undefined;
         resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string, redirectedReference?: ResolvedProjectReference): (ResolvedTypeReferenceDirective | undefined)[];
         getDirectories?(directoryName: string): string[];