Initial import/export/self-name support

src/compiler/checker.ts

@@ -303,7 +303,7 @@ namespace ts {
             host.getSourceFiles().forEach(sf => {
                 if (!sf.resolvedModules) return;
 
-                forEachEntry(sf.resolvedModules, r => {
+                sf.resolvedModules.forEach(r => {
                     if (r && r.packageId) map.set(r.packageId.name, r.extension === Extension.Dts || !!map.get(r.packageId.name));
                 });
             });
@@ -1832,7 +1832,7 @@ namespace ts {
                         isInExternalModule = true;
                         // falls through
                     case SyntaxKind.ModuleDeclaration:
-                        const moduleExports = getSymbolOfNode(location as SourceFile | ModuleDeclaration).exports || emptySymbols;
+                        const moduleExports = getSymbolOfNode(location as SourceFile | ModuleDeclaration)?.exports || emptySymbols;
                         if (location.kind === SyntaxKind.SourceFile || (isModuleDeclaration(location) && location.flags & NodeFlags.Ambient && !isGlobalScopeAugmentation(location))) {
 
                             // It's an external module. First see if the module has an export default and if the local
@@ -1875,7 +1875,7 @@ namespace ts {
                         }
                         break;
                     case SyntaxKind.EnumDeclaration:
-                        if (result = lookup(getSymbolOfNode(location)!.exports!, name, meaning & SymbolFlags.EnumMember)) {
+                        if (result = lookup(getSymbolOfNode(location)?.exports || emptySymbols, name, meaning & SymbolFlags.EnumMember)) {
                             break loop;
                         }
                         break;
@@ -3377,7 +3377,15 @@ namespace ts {
                 return ambientModule;
             }
             const currentSourceFile = getSourceFileOfNode(location);
-            const resolvedModule = getResolvedModule(currentSourceFile, moduleReference)!; // TODO: GH#18217
+            const contextSpecifier = isStringLiteralLike(location)
+                ? location
+                :   (isImportCall(location) ? location : findAncestor(location, isImportCall))?.arguments[0] ||
+                    (isImportDeclaration(location) ? location : findAncestor(location, isImportDeclaration))?.moduleSpecifier ||
+                    (isExternalModuleImportEqualsDeclaration(location) ? location : findAncestor(location, isExternalModuleImportEqualsDeclaration))?.moduleReference.expression ||
+                    (isExportDeclaration(location) ? location : findAncestor(location, isExportDeclaration))?.moduleSpecifier ||
+                    (isModuleDeclaration(location) ? location : location.parent && isModuleDeclaration(location.parent) && location.parent.name === location ? location.parent : undefined)?.name ||
+                    (isLiteralImportTypeNode(location) ? location : undefined)?.argument.literal;
+            const resolvedModule = getResolvedModule(currentSourceFile, moduleReference, contextSpecifier && isStringLiteralLike(contextSpecifier) ? getModeForUsageLocation(currentSourceFile, contextSpecifier) : undefined)!; // TODO: GH#18217
             const resolutionDiagnostic = resolvedModule && getResolutionDiagnostic(compilerOptions, resolvedModule);
             const sourceFile = resolvedModule && !resolutionDiagnostic && host.getSourceFile(resolvedModule.resolvedFileName);
             if (sourceFile) {
@@ -3453,7 +3461,7 @@ namespace ts {
                          * @see https://github.com/microsoft/TypeScript/issues/42151
                          */
                         if (moduleKind >= ModuleKind.ES2015) {
-                            replacedImportSource += ".js";
+                            replacedImportSource += tsExtension === Extension.Mts ? ".mjs" : tsExtension === Extension.Cts ? ".cjs" : ".js";
                         }
                         error(errorNode, diag, tsExtension, replacedImportSource);
                     }
@@ -4603,6 +4611,7 @@ namespace ts {
                         isSourceOfProjectReferenceRedirect: fileName => host.isSourceOfProjectReferenceRedirect(fileName),
                         fileExists: fileName => host.fileExists(fileName),
                         getFileIncludeReasons: () => host.getFileIncludeReasons(),
+                        readFile: host.readFile ? (fileName => host.readFile!(fileName)) : undefined,
                     } : undefined },
                     encounteredError: false,
                     reportedDiagnostic: false,

src/compiler/diagnosticMessages.json

@@ -4895,6 +4895,35 @@
         "code": 6258
     },
 
+    "Directory '{0}' has no containing package.json scope. Imports will not resolve.": {
+        "category": "Message",
+        "code": 6270
+    },
+    "Import specifier '{0}' does not exist in package.json scope at path '{1}'.": {
+        "category": "Message",
+        "code": 6271
+    },
+    "Invalid import specifier '{0}' has no possible resolutions.": {
+        "category": "Message",
+        "code": 6272
+    },
+    "package.json scope '{0}' has no imports defined.": {
+        "category": "Message",
+        "code": 6273
+    },
+    "package.json scope '{0}' explicitly maps specifier '{1}' to null.": {
+        "category": "Message",
+        "code": 6274
+    },
+    "package.json scope '{0}' has invalid type for target of specifier '{1}'": {
+        "category": "Message",
+        "code": 6275
+    },
+    "Export specifier '{0}' does not exist in package.json scope at path '{1}'.": {
+        "category": "Message",
+        "code": 6276
+    },
+
     "Enable project compilation": {
         "category": "Message",
         "code": 6302