Don't allow ".d.ts" extension in an import either.

src/compiler/checker.ts

@@ -1366,10 +1366,10 @@ namespace ts {
 
             if (moduleNotFoundError) {
                 // report errors only if it was requested
-                const nonDtsExtension = tryExtractTypeScriptExtensionNonDts(moduleName);
-                if (nonDtsExtension) {
-                    const diag = Diagnostics.An_import_path_should_not_end_with_a_0_extension_Consider_importing_1_instead;
-                    error(moduleReferenceLiteral, diag, nonDtsExtension, removeExtension(moduleName, nonDtsExtension));
+                const tsExtension = tryExtractTypeScriptExtension(moduleName);
+                if (tsExtension) {
+                    const diag = Diagnostics.An_import_path_cannot_end_with_a_0_extension_Consider_importing_1_instead;
+                    error(moduleReferenceLiteral, diag, tsExtension, removeExtension(moduleName, tsExtension));
                 }
                 else {
                     error(moduleReferenceLiteral, moduleNotFoundError, moduleName);

src/compiler/core.ts

@@ -1198,8 +1198,7 @@ namespace ts {
     /**
      *  List of supported extensions in order of file resolution precedence.
      */
-    export const supportedTypeScriptExtensionsNonDts = [".ts", ".tsx"];
-    export const supportedTypeScriptExtensions = supportedTypeScriptExtensionsNonDts.concat([".d.ts"]);
+    export const supportedTypeScriptExtensions = [".ts", ".tsx", ".d.ts"];
     export const supportedJavascriptExtensions = [".js", ".jsx"];
     const allSupportedExtensions  = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
 

src/compiler/diagnosticMessages.json

@@ -1951,7 +1951,7 @@
         "category": "Error",
         "code": 2690
     },
-    "An import path should not end with a '{0}' extension. Consider importing '{1}' instead.": {
+    "An import path cannot end with a '{0}' extension. Consider importing '{1}' instead.": {
         "category": "Error",
         "code": 2691
     },

src/compiler/program.ts

@@ -669,17 +669,7 @@ namespace ts {
      * in cases when we know upfront that all load attempts will fail (because containing folder does not exists) however we still need to record all failed lookup locations.
      */
     function loadModuleFromFile(candidate: string, extensions: string[], failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState): string | undefined {
-        // If the candidate already has an extension load that or quit.
-        if (hasTypeScriptFileExtension(candidate)) {
-            // Don't allow `.ts` to appear at the end
-            if (!fileExtensionIs(candidate, ".d.ts")) {
-                return undefined;
-            }
-            return tryFile(candidate, failedLookupLocation, onlyRecordFailures, state);
-        }
-
-        // Next, try adding an extension.
-        // We don't allow an import of "foo.ts" to be matched by "foo.ts.ts", but we do allow "foo.js" to be matched by "foo.js.ts".
+        // First, try adding an extension. An import of "foo" could be matched by a file "foo.ts", or "foo.js" by "foo.js.ts"
         const resolvedByAddingExtension = tryAddingExtensions(candidate, extensions, failedLookupLocation, onlyRecordFailures, state);
         if (resolvedByAddingExtension) {
             return resolvedByAddingExtension;
@@ -736,7 +726,9 @@ namespace ts {
             }
             const typesFile = tryReadTypesSection(packageJsonPath, candidate, state);
             if (typesFile) {
-                const result = loadModuleFromFile(typesFile, extensions, failedLookupLocation, !directoryProbablyExists(getDirectoryPath(typesFile), state.host), state);
+                const onlyRecordFailures = !directoryProbablyExists(getDirectoryPath(typesFile), state.host);
+                // The package.json "typings" property must specify the file with extension, so just try that exact filename.
+                const result = tryFile(typesFile, failedLookupLocation, onlyRecordFailures, state);
                 if (result) {
                     return result;
                 }

src/compiler/utilities.ts

@@ -2727,9 +2727,11 @@ namespace ts {
     }
 
     /** Return ".ts" or ".tsx" if that is the extension. */
-    export function tryExtractTypeScriptExtensionNonDts(fileName: string): string | undefined {
-        return find(supportedTypeScriptExtensionsNonDts, extension => fileExtensionIs(fileName, extension));
+    export function tryExtractTypeScriptExtension(fileName: string): string | undefined {
+        return find(supportedTypescriptExtensionsWithDtsFirst, extension => fileExtensionIs(fileName, extension));
     }
+    // Must have '.d.ts' first because if '.ts' goes first, that will be detected as the extension instead of '.d.ts'.
+    const supportedTypescriptExtensionsWithDtsFirst = supportedTypeScriptExtensions.slice().reverse();
 
     /**
      * Replace each instance of non-ascii characters by one, two, three, or four escape sequences

src/harness/compilerRunner.ts

@@ -52,7 +52,7 @@ class CompilerBaselineRunner extends RunnerBase {
     private makeUnitName(name: string, root: string) {
         const path = ts.toPath(name, root, (fileName) => Harness.Compiler.getCanonicalFileName(fileName));
         const pathStart = ts.toPath(Harness.IO.getCurrentDirectory(), "", (fileName) => Harness.Compiler.getCanonicalFileName(fileName));
-        return path.replace(pathStart, "/");
+        return pathStart === "" ? path : path.replace(pathStart, "/");
     };
 
     public checkTestCodeOutput(fileName: string) {