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diff --git a/‎src/compiler/checker.ts
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+1-1Lines changed: 1 addition & 1 deletion b/‎src/compiler/checker.ts
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+1-1Lines changed: 1 addition & 1 deletion
diff --git a/‎src/compiler/commandLineParser.ts
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+8Lines changed: 8 additions & 0 deletions b/‎src/compiler/commandLineParser.ts
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+8Lines changed: 8 additions & 0 deletions
diff --git a/‎src/compiler/diagnosticMessages.json
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+12Lines changed: 12 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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diff --git a/‎src/compiler/moduleNameResolver.ts
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+41-48Lines changed: 41 additions & 48 deletions b/‎src/compiler/moduleNameResolver.ts
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+41-48Lines changed: 41 additions & 48 deletions
diff --git a/‎src/compiler/moduleSpecifiers.ts
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+15Lines changed: 15 additions & 0 deletions b/‎src/compiler/moduleSpecifiers.ts
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diff --git a/‎src/compiler/parser.ts
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+5-1Lines changed: 5 additions & 1 deletion b/‎src/compiler/parser.ts
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+5-1Lines changed: 5 additions & 1 deletion
diff --git a/‎src/compiler/program.ts
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+15-4Lines changed: 15 additions & 4 deletions b/‎src/compiler/program.ts
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+15-4Lines changed: 15 additions & 4 deletions
diff --git a/‎src/compiler/types.ts
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+2-1Lines changed: 2 additions & 1 deletion b/‎src/compiler/types.ts
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+2-1Lines changed: 2 additions & 1 deletion
@@ -4738,7 +4738,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const mode = contextSpecifier && isStringLiteralLike(contextSpecifier) ? getModeForUsageLocation(currentSourceFile, contextSpecifier) : currentSourceFile.impliedNodeFormat;
         const moduleResolutionKind = getEmitModuleResolutionKind(compilerOptions);
         const resolvedModule = getResolvedModule(currentSourceFile, moduleReference, mode);
-        const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
+        const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule, currentSourceFile);
         const sourceFile = resolvedModule
             && (!resolutionDiagnostic || resolutionDiagnostic === Diagnostics.Module_0_was_resolved_to_1_but_jsx_is_not_set)
             && host.getSourceFile(resolvedModule.resolvedFileName);
 
@@ -1206,6 +1206,14 @@ const commandOptionsWithoutBuild: CommandLineOption[] = [
         description: Diagnostics.Enable_importing_json_files,
         defaultValueDescription: false,
     },
+    {
+        name: "allowArbitraryExtensions",
+        type: "boolean",
+        affectsModuleResolution: true,
+        category: Diagnostics.Modules,
+        description: Diagnostics.Enable_importing_files_with_any_extension_provided_a_declaration_file_is_present,
+        defaultValueDescription: false,
+    },
 
     {
         name: "out",
 
@@ -5190,6 +5190,18 @@
         "category": "Message",
         "code": 6261
     },
+    "File name '{0}' has a '{1}' extension - looking up '{2}' instead.": {
+        "category": "Message",
+        "code": 6262
+    },
+    "Module '{0}' was resolved to '{1}', but '--allowArbitraryExtensions' is not set.": {
+        "category": "Error",
+        "code": 6263
+    },
+    "Enable importing files with any extension, provided a declaration file is present.": {
+        "category": "Message",
+        "code": 6264
+    },
 
     "Directory '{0}' has no containing package.json scope. Imports will not resolve.": {
         "category": "Message",
 
@@ -52,7 +52,6 @@ import {
     getRelativePathFromDirectory,
     getResolveJsonModule,
     getRootLength,
-    hasJSFileExtension,
     hasProperty,
     hasTrailingDirectorySeparator,
     hostGetCanonicalFileName,
@@ -99,7 +98,6 @@ import {
     startsWith,
     stringContains,
     supportedDeclarationExtensions,
-    supportedTSExtensionsFlat,
     supportedTSImplementationExtensions,
     toPath,
     tryExtractTSExtension,
@@ -151,7 +149,7 @@ function removeIgnoredPackageId(r: Resolved | undefined): PathAndExtension | und
 /** Result of trying to resolve a module. */
 interface Resolved {
     path: string;
-    extension: Extension;
+    extension: string;
     packageId: PackageId | undefined;
     /**
      * When the resolved is not created from cache, the value is
@@ -170,7 +168,7 @@ interface Resolved {
 interface PathAndExtension {
     path: string;
     // (Use a different name than `extension` to make sure Resolved isn't assignable to PathAndExtension.)
-    ext: Extension;
+    ext: string;
     resolvedUsingTsExtension: boolean | undefined;
 }
 
@@ -1856,21 +1854,21 @@ function loadModuleFromFile(extensions: Extensions, candidate: string, onlyRecor
 }
 
 function loadModuleFromFileNoImplicitExtensions(extensions: Extensions, candidate: string, onlyRecordFailures: boolean, state: ModuleResolutionState): PathAndExtension | undefined {
-    // If that didn't work, try stripping a ".js" or ".jsx" extension and replacing it with a TypeScript one;
-    // e.g. "./foo.js" can be matched by "./foo.ts" or "./foo.d.ts"
-    if (hasJSFileExtension(candidate) ||
-        extensions & Extensions.Json && fileExtensionIs(candidate, Extension.Json) ||
-        extensions & (Extensions.TypeScript | Extensions.Declaration)
-            && moduleResolutionSupportsResolvingTsExtensions(state.compilerOptions)
-            && fileExtensionIsOneOf(candidate, supportedTSExtensionsFlat)
-    ) {
-        const extensionless = removeFileExtension(candidate);
-        const extension = candidate.substring(extensionless.length);
-        if (state.traceEnabled) {
-            trace(state.host, Diagnostics.File_name_0_has_a_1_extension_stripping_it, candidate, extension);
-        }
-        return tryAddingExtensions(extensionless, extensions, extension, onlyRecordFailures, state);
+    const filename = getBaseFileName(candidate);
+    if (filename.indexOf(".") === -1) {
+        return undefined; // extensionless import, no lookups performed, since we don't support extensionless files
+    }
+    let extensionless = removeFileExtension(candidate);
+    if (extensionless === candidate) {
+        // Once TS native extensions are handled, handle arbitrary extensions for declaration file mapping
+        extensionless = candidate.substring(0, candidate.lastIndexOf("."));
+    }
+
+    const extension = candidate.substring(extensionless.length);
+    if (state.traceEnabled) {
+        trace(state.host, Diagnostics.File_name_0_has_a_1_extension_stripping_it, candidate, extension);
     }
+    return tryAddingExtensions(extensionless, extensions, extension, onlyRecordFailures, state);
 }
 
 /**
@@ -1909,47 +1907,46 @@ function tryAddingExtensions(candidate: string, extensions: Extensions, original
         case Extension.Mjs:
         case Extension.Mts:
         case Extension.Dmts:
-            return extensions & Extensions.TypeScript && tryExtension(Extension.Mts)
-                || extensions & Extensions.Declaration && tryExtension(Extension.Dmts)
+            return extensions & Extensions.TypeScript && tryExtension(Extension.Mts, originalExtension === Extension.Mts || originalExtension === Extension.Dmts)
+                || extensions & Extensions.Declaration && tryExtension(Extension.Dmts, originalExtension === Extension.Mts || originalExtension === Extension.Dmts)
                 || extensions & Extensions.JavaScript && tryExtension(Extension.Mjs)
                 || undefined;
         case Extension.Cjs:
         case Extension.Cts:
         case Extension.Dcts:
-            return extensions & Extensions.TypeScript && tryExtension(Extension.Cts)
-                || extensions & Extensions.Declaration && tryExtension(Extension.Dcts)
+            return extensions & Extensions.TypeScript && tryExtension(Extension.Cts, originalExtension === Extension.Cts || originalExtension === Extension.Dcts)
+                || extensions & Extensions.Declaration && tryExtension(Extension.Dcts, originalExtension === Extension.Cts || originalExtension === Extension.Dcts)
                 || extensions & Extensions.JavaScript && tryExtension(Extension.Cjs)
                 || undefined;
         case Extension.Json:
-            const originalCandidate = candidate;
-            if (extensions & Extensions.Declaration) {
-                candidate += Extension.Json;
-                const result = tryExtension(Extension.Dts);
-                if (result) return result;
-            }
-            if (extensions & Extensions.Json) {
-                candidate = originalCandidate;
-                const result = tryExtension(Extension.Json);
-                if (result) return result;
-            }
-            return undefined;
-        case Extension.Ts:
+            return extensions & Extensions.Declaration && tryExtension(".d.json.ts")
+                || extensions & Extensions.Json && tryExtension(Extension.Json)
+                || undefined;
         case Extension.Tsx:
+        case Extension.Jsx:
+            // basically idendical to the ts/js case below, but prefers matching tsx and jsx files exactly before falling back to the ts or js file path
+            // (historically, we disallow having both a a.ts and a.tsx file in the same compilation, since their outputs clash)
+            // TODO: We should probably error if `"./a.tsx"` resolved to `"./a.ts"`, right?
+            return extensions & Extensions.TypeScript && (tryExtension(Extension.Tsx, originalExtension === Extension.Tsx) || tryExtension(Extension.Ts, originalExtension === Extension.Tsx))
+                || extensions & Extensions.Declaration && tryExtension(Extension.Dts, originalExtension === Extension.Tsx)
+                || extensions & Extensions.JavaScript && (tryExtension(Extension.Jsx) || tryExtension(Extension.Js))
+                || undefined;
+        case Extension.Ts:
         case Extension.Dts:
-            if (moduleResolutionSupportsResolvingTsExtensions(state.compilerOptions) && extensionIsOk(extensions, originalExtension)) {
-                return tryExtension(originalExtension, /*resolvedUsingTsExtension*/ true);
-            }
-            // falls through
-        default:
-            return extensions & Extensions.TypeScript && (tryExtension(Extension.Ts) || tryExtension(Extension.Tsx))
-                || extensions & Extensions.Declaration && tryExtension(Extension.Dts)
+        case Extension.Js:
+        case "":
+            return extensions & Extensions.TypeScript && (tryExtension(Extension.Ts, originalExtension === Extension.Ts || originalExtension === Extension.Dts) || tryExtension(Extension.Tsx, originalExtension === Extension.Ts || originalExtension === Extension.Dts))
+                || extensions & Extensions.Declaration && tryExtension(Extension.Dts, originalExtension === Extension.Ts || originalExtension === Extension.Dts)
                 || extensions & Extensions.JavaScript && (tryExtension(Extension.Js) || tryExtension(Extension.Jsx))
                 || state.isConfigLookup && tryExtension(Extension.Json)
                 || undefined;
+        default:
+            return extensions & Extensions.Declaration && !isDeclarationFileName(candidate + originalExtension) && tryExtension(`.d${originalExtension}.ts`)
+                || undefined;
 
     }
 
-    function tryExtension(ext: Extension, resolvedUsingTsExtension?: boolean): PathAndExtension | undefined {
+    function tryExtension(ext: string, resolvedUsingTsExtension?: boolean): PathAndExtension | undefined {
         const path = tryFile(candidate + ext, onlyRecordFailures, state);
         return path === undefined ? undefined : { path, ext, resolvedUsingTsExtension };
     }
@@ -2984,14 +2981,10 @@ export function classicNameResolver(moduleName: string, containingFile: string,
     }
 }
 
-export function moduleResolutionSupportsResolvingTsExtensions(compilerOptions: CompilerOptions) {
-    return getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Bundler;
-}
-
 // Program errors validate that `noEmit` or `emitDeclarationOnly` is also set,
 // so this function doesn't check them to avoid propagating errors.
 export function shouldAllowImportingTsExtension(compilerOptions: CompilerOptions, fromFileName?: string) {
-    return moduleResolutionSupportsResolvingTsExtensions(compilerOptions) && (
+    return getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Bundler && (
         !!compilerOptions.allowImportingTsExtensions ||
         fromFileName && isDeclarationFileName(fromFileName));
 }
 
@@ -31,6 +31,7 @@ import {
     flatten,
     forEach,
     forEachAncestorDirectory,
+    getBaseFileName,
     GetCanonicalFileName,
     getDirectoryPath,
     getEmitModuleResolutionKind,
@@ -85,6 +86,7 @@ import {
     pathIsBareSpecifier,
     pathIsRelative,
     PropertyAccessExpression,
+    removeExtension,
     removeFileExtension,
     removeSuffix,
     ResolutionMode,
@@ -1036,6 +1038,10 @@ function processEnding(fileName: string, allowedEndings: readonly ModuleSpecifie
     if (fileExtensionIsOneOf(fileName, [Extension.Dmts, Extension.Mts, Extension.Dcts, Extension.Cts])) {
         return noExtension + getJSExtensionForFile(fileName, options);
     }
+    else if (!fileExtensionIsOneOf(fileName, [Extension.Dts]) && fileExtensionIsOneOf(fileName, [Extension.Ts]) && stringContains(fileName, ".d.")) {
+        // `foo.d.json.ts` and the like - remap back to `foo.json`
+        return tryGetRealFileNameForNonJsDeclarationFileName(fileName)!;
+    }
 
     switch (allowedEndings[0]) {
         case ModuleSpecifierEnding.Minimal:
@@ -1066,6 +1072,15 @@ function processEnding(fileName: string, allowedEndings: readonly ModuleSpecifie
     }
 }
 
+/** @internal */
+export function tryGetRealFileNameForNonJsDeclarationFileName(fileName: string) {
+    const baseName = getBaseFileName(fileName);
+    if (!endsWith(fileName, Extension.Ts) || !stringContains(baseName, ".d.") || fileExtensionIsOneOf(baseName, [Extension.Dts])) return undefined;
+    const noExtension = removeExtension(fileName, Extension.Ts);
+    const ext = noExtension.substring(noExtension.lastIndexOf("."));
+    return noExtension.substring(0, noExtension.indexOf(".d.")) + ext;
+}
+
 function getJSExtensionForFile(fileName: string, options: CompilerOptions): Extension {
     return tryGetJSExtensionForFile(fileName, options) ?? Debug.fail(`Extension ${extensionFromPath(fileName)} is unsupported:: FileName:: ${fileName}`);
 }
 
@@ -84,7 +84,9 @@ import {
     Expression,
     ExpressionStatement,
     ExpressionWithTypeArguments,
+    Extension,
     ExternalModuleReference,
+    fileExtensionIs,
     fileExtensionIsOneOf,
     findIndex,
     forEach,
@@ -98,6 +100,7 @@ import {
     FunctionOrConstructorTypeNode,
     FunctionTypeNode,
     GetAccessorDeclaration,
+    getBaseFileName,
     getBinaryOperatorPrecedence,
     getFullWidth,
     getJSDocCommentRanges,
@@ -328,6 +331,7 @@ import {
     SpreadElement,
     startsWith,
     Statement,
+    stringContains,
     StringLiteral,
     supportedDeclarationExtensions,
     SwitchStatement,
@@ -10114,7 +10118,7 @@ namespace IncrementalParser {
 
 /** @internal */
 export function isDeclarationFileName(fileName: string): boolean {
-    return fileExtensionIsOneOf(fileName, supportedDeclarationExtensions);
+    return fileExtensionIsOneOf(fileName, supportedDeclarationExtensions) || (fileExtensionIs(fileName, Extension.Ts) && stringContains(getBaseFileName(fileName), ".d."));
 }
 
 function parseResolutionMode(mode: string | undefined, pos: number, end: number, reportDiagnostic: PragmaDiagnosticReporter): ResolutionMode {
 
@@ -218,7 +218,6 @@ import {
     moduleResolutionIsEqualTo,
     ModuleResolutionKind,
     moduleResolutionSupportsPackageJsonExportsAndImports,
-    moduleResolutionSupportsResolvingTsExtensions,
     Mutable,
     Node,
     NodeArray,
@@ -3845,7 +3844,7 @@ export function createProgram(rootNamesOrOptions: readonly string[] | CreateProg
                 // Don't add the file if it has a bad extension (e.g. 'tsx' if we don't have '--allowJs')
                 // This may still end up being an untyped module -- the file won't be included but imports will be allowed.
                 const shouldAddFile = resolvedFileName
-                    && !getResolutionDiagnostic(optionsForFile, resolution)
+                    && !getResolutionDiagnostic(optionsForFile, resolution, file)
                     && !optionsForFile.noResolve
                     && index < file.imports.length
                     && !elideImport
@@ -4213,7 +4212,7 @@ export function createProgram(rootNamesOrOptions: readonly string[] | CreateProg
             createOptionValueDiagnostic("importsNotUsedAsValues", Diagnostics.Option_preserveValueImports_can_only_be_used_when_module_is_set_to_es2015_or_later);
         }
 
-        if (options.allowImportingTsExtensions && !(moduleResolutionSupportsResolvingTsExtensions(options) && (options.noEmit || options.emitDeclarationOnly))) {
+        if (options.allowImportingTsExtensions && !(options.noEmit || options.emitDeclarationOnly)) {
             createOptionValueDiagnostic("allowImportingTsExtensions", Diagnostics.Option_allowImportingTsExtensions_can_only_be_used_when_moduleResolution_is_set_to_bundler_and_either_noEmit_or_emitDeclarationOnly_is_set);
         }
 
@@ -4947,20 +4946,28 @@ export function resolveProjectReferencePath(hostOrRef: ResolveProjectReferencePa
  *
  * @internal
  */
-export function getResolutionDiagnostic(options: CompilerOptions, { extension }: ResolvedModuleFull): DiagnosticMessage | undefined {
+export function getResolutionDiagnostic(options: CompilerOptions, { extension }: ResolvedModuleFull, { isDeclarationFile }: { isDeclarationFile: SourceFile["isDeclarationFile"] }): DiagnosticMessage | undefined {
     switch (extension) {
         case Extension.Ts:
         case Extension.Dts:
+        case Extension.Mts:
+        case Extension.Dmts:
+        case Extension.Cts:
+        case Extension.Dcts:
             // These are always allowed.
             return undefined;
         case Extension.Tsx:
             return needJsx();
         case Extension.Jsx:
             return needJsx() || needAllowJs();
         case Extension.Js:
+        case Extension.Mjs:
+        case Extension.Cjs:
             return needAllowJs();
         case Extension.Json:
             return needResolveJsonModule();
+        default:
+            return needAllowArbitraryExtensions();
     }
 
     function needJsx() {
@@ -4972,6 +4979,10 @@ export function getResolutionDiagnostic(options: CompilerOptions, { extension }:
     function needResolveJsonModule() {
         return getResolveJsonModule(options) ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_resolveJsonModule_is_not_used;
     }
+    function needAllowArbitraryExtensions() {
+        // But don't report the allowArbitraryExtensions error from declaration files (no reason to report it, since the import doesn't have a runtime component)
+        return isDeclarationFile || options.allowArbitraryExtensions ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_allowArbitraryExtensions_is_not_set;
+    }
 }
 
 function getModuleNames({ imports, moduleAugmentations }: SourceFile): StringLiteralLike[] {
 
@@ -6974,6 +6974,7 @@ export interface CompilerOptions {
     allowImportingTsExtensions?: boolean;
     allowJs?: boolean;
     /** @internal */ allowNonTsExtensions?: boolean;
+    allowArbitraryExtensions?: boolean;
     allowSyntheticDefaultImports?: boolean;
     allowUmdGlobalAccess?: boolean;
     allowUnreachableCode?: boolean;
@@ -7557,7 +7558,7 @@ export interface ResolvedModuleFull extends ResolvedModule {
      * Extension of resolvedFileName. This must match what's at the end of resolvedFileName.
      * This is optional for backwards-compatibility, but will be added if not provided.
      */
-    extension: Extension;
+    extension: string;
     packageId?: PackageId;
 }