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diff --git a/‎src/compiler/checker.ts
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+92-13Lines changed: 92 additions & 13 deletions b/‎src/compiler/checker.ts
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+92-13Lines changed: 92 additions & 13 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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+12Lines changed: 12 additions & 0 deletions
diff --git a/‎src/compiler/emitter.ts
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+13Lines changed: 13 additions & 0 deletions b/‎src/compiler/emitter.ts
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+13Lines changed: 13 additions & 0 deletions
@@ -3491,7 +3491,11 @@ namespace ts {
                     }
                     if (getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Node12 || getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeNext) {
                         const isSyncImport = (currentSourceFile.impliedNodeFormat === ModuleKind.CommonJS && !findAncestor(location, isImportCall)) || !!findAncestor(location, isImportEqualsDeclaration);
-                        if (isSyncImport && sourceFile.impliedNodeFormat === ModuleKind.ESNext) {
+                        const overrideClauseHost = findAncestor(location, l => isImportTypeNode(l) || isExportDeclaration(l) || isImportDeclaration(l)) as ImportTypeNode | ImportDeclaration | ExportDeclaration | undefined;
+                        const overrideClause = overrideClauseHost && isImportTypeNode(overrideClauseHost) ? overrideClauseHost.assertions?.assertClause : overrideClauseHost?.assertClause;
+                        // An override clause will take effect for type-only imports and import types, and allows importing the types across formats, regardless of
+                        // normal mode restrictions
+                        if (isSyncImport && sourceFile.impliedNodeFormat === ModuleKind.ESNext && !getResolutionModeOverrideForClause(overrideClause)) {
                             error(errorNode, Diagnostics.Module_0_cannot_be_imported_using_this_construct_The_specifier_only_resolves_to_an_ES_module_which_cannot_be_imported_synchronously_Use_dynamic_import_instead, moduleReference);
                         }
                         if (mode === ModuleKind.ESNext && compilerOptions.resolveJsonModule && resolvedModule.extension === Extension.Json) {
@@ -5985,7 +5989,7 @@ namespace ts {
                 return top;
             }
 
-            function getSpecifierForModuleSymbol(symbol: Symbol, context: NodeBuilderContext) {
+            function getSpecifierForModuleSymbol(symbol: Symbol, context: NodeBuilderContext, overrideImportMode?: SourceFile["impliedNodeFormat"]) {
                 let file = getDeclarationOfKind<SourceFile>(symbol, SyntaxKind.SourceFile);
                 if (!file) {
                     const equivalentFileSymbol = firstDefined(symbol.declarations, d => getFileSymbolIfFileSymbolExportEqualsContainer(d, symbol));
@@ -6018,8 +6022,10 @@ namespace ts {
                     return getSourceFileOfNode(getNonAugmentationDeclaration(symbol)!).fileName; // A resolver may not be provided for baselines and errors - in those cases we use the fileName in full
                 }
                 const contextFile = getSourceFileOfNode(getOriginalNode(context.enclosingDeclaration));
+                const resolutionMode = overrideImportMode || contextFile?.impliedNodeFormat;
+                const cacheKey = getSpecifierCacheKey(contextFile.path, resolutionMode);
                 const links = getSymbolLinks(symbol);
-                let specifier = links.specifierCache && links.specifierCache.get(contextFile.path);
+                let specifier = links.specifierCache && links.specifierCache.get(cacheKey);
                 if (!specifier) {
                     const isBundle = !!outFile(compilerOptions);
                     // For declaration bundles, we need to generate absolute paths relative to the common source dir for imports,
@@ -6034,12 +6040,22 @@ namespace ts {
                         specifierCompilerOptions,
                         contextFile,
                         moduleResolverHost,
-                        { importModuleSpecifierPreference: isBundle ? "non-relative" : "project-relative", importModuleSpecifierEnding: isBundle ? "minimal" : undefined },
+                        {
+                            importModuleSpecifierPreference: isBundle ? "non-relative" : "project-relative",
+                            importModuleSpecifierEnding: isBundle ? "minimal"
+                                : resolutionMode === ModuleKind.ESNext ? "js"
+                                : undefined,
+                        },
+                        { overrideImportMode }
                     ));
                     links.specifierCache ??= new Map();
-                    links.specifierCache.set(contextFile.path, specifier);
+                    links.specifierCache.set(cacheKey, specifier);
                 }
                 return specifier;
+
+                function getSpecifierCacheKey(path: string, mode: SourceFile["impliedNodeFormat"] | undefined) {
+                    return mode === undefined ? path : `${mode}|${path}`;
+                }
             }
 
             function symbolToEntityNameNode(symbol: Symbol): EntityName {
@@ -6055,13 +6071,53 @@ namespace ts {
                     // module is root, must use `ImportTypeNode`
                     const nonRootParts = chain.length > 1 ? createAccessFromSymbolChain(chain, chain.length - 1, 1) : undefined;
                     const typeParameterNodes = overrideTypeArguments || lookupTypeParameterNodes(chain, 0, context);
-                    const specifier = getSpecifierForModuleSymbol(chain[0], context);
+                    const contextFile = getSourceFileOfNode(getOriginalNode(context.enclosingDeclaration));
+                    const targetFile = getSourceFileOfModule(chain[0]);
+                    let specifier: string | undefined;
+                    let assertion: ImportTypeAssertionContainer | undefined;
+                    if (getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Node12 || getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeNext) {
+                        // An `import` type directed at an esm format file is only going to resolve in esm mode - set the esm mode assertion
+                        if (targetFile?.impliedNodeFormat === ModuleKind.ESNext && targetFile.impliedNodeFormat !== contextFile?.impliedNodeFormat) {
+                            specifier = getSpecifierForModuleSymbol(chain[0], context, ModuleKind.ESNext);
+                            assertion = factory.createImportTypeAssertionContainer(factory.createAssertClause(factory.createNodeArray([
+                                factory.createAssertEntry(
+                                    factory.createStringLiteral("resolution-mode"),
+                                    factory.createStringLiteral("import")
+                                )
+                            ])));
+                        }
+                    }
+                    if (!specifier) {
+                        specifier = getSpecifierForModuleSymbol(chain[0], context);
+                    }
                     if (!(context.flags & NodeBuilderFlags.AllowNodeModulesRelativePaths) && getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Classic && specifier.indexOf("/node_modules/") >= 0) {
-                        // If ultimately we can only name the symbol with a reference that dives into a `node_modules` folder, we should error
-                        // since declaration files with these kinds of references are liable to fail when published :(
-                        context.encounteredError = true;
-                        if (context.tracker.reportLikelyUnsafeImportRequiredError) {
-                            context.tracker.reportLikelyUnsafeImportRequiredError(specifier);
+                        const oldSpecifier = specifier;
+                        if (getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Node12 || getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.NodeNext) {
+                            // We might be able to write a portable import type using a mode override; try specifier generation again, but with a different mode set
+                            const swappedMode = contextFile?.impliedNodeFormat === ModuleKind.ESNext ? ModuleKind.CommonJS : ModuleKind.ESNext;
+                            specifier = getSpecifierForModuleSymbol(chain[0], context, swappedMode);
+
+                            if (specifier.indexOf("/node_modules/") >= 0) {
+                                // Still unreachable :(
+                                specifier = oldSpecifier;
+                            }
+                            else {
+                                assertion = factory.createImportTypeAssertionContainer(factory.createAssertClause(factory.createNodeArray([
+                                    factory.createAssertEntry(
+                                        factory.createStringLiteral("resolution-mode"),
+                                        factory.createStringLiteral(swappedMode === ModuleKind.ESNext ? "import" : "require")
+                                    )
+                                ])));
+                            }
+                        }
+
+                        if (!assertion) {
+                            // If ultimately we can only name the symbol with a reference that dives into a `node_modules` folder, we should error
+                            // since declaration files with these kinds of references are liable to fail when published :(
+                            context.encounteredError = true;
+                            if (context.tracker.reportLikelyUnsafeImportRequiredError) {
+                                context.tracker.reportLikelyUnsafeImportRequiredError(oldSpecifier);
+                            }
                         }
                     }
                     const lit = factory.createLiteralTypeNode(factory.createStringLiteral(specifier));
@@ -6072,12 +6128,12 @@ namespace ts {
                             const lastId = isIdentifier(nonRootParts) ? nonRootParts : nonRootParts.right;
                             lastId.typeArguments = undefined;
                         }
-                        return factory.createImportTypeNode(lit, nonRootParts as EntityName, typeParameterNodes as readonly TypeNode[], isTypeOf);
+                        return factory.createImportTypeNode(lit, assertion, nonRootParts as EntityName, typeParameterNodes as readonly TypeNode[], isTypeOf);
                     }
                     else {
                         const splitNode = getTopmostIndexedAccessType(nonRootParts);
                         const qualifier = (splitNode.objectType as TypeReferenceNode).typeName;
-                        return factory.createIndexedAccessTypeNode(factory.createImportTypeNode(lit, qualifier, typeParameterNodes as readonly TypeNode[], isTypeOf), splitNode.indexType);
+                        return factory.createIndexedAccessTypeNode(factory.createImportTypeNode(lit, assertion, qualifier, typeParameterNodes as readonly TypeNode[], isTypeOf), splitNode.indexType);
                     }
                 }
 
@@ -35293,6 +35349,16 @@ namespace ts {
 
         function checkImportType(node: ImportTypeNode) {
             checkSourceElement(node.argument);
+
+            if (node.assertions) {
+                const override = getResolutionModeOverrideForClause(node.assertions.assertClause, grammarErrorOnNode);
+                if (override) {
+                    if (getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Node12 && getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeNext) {
+                        grammarErrorOnNode(node.assertions.assertClause, Diagnostics.Resolution_modes_are_only_supported_when_moduleResolution_is_node12_or_nodenext);
+                    }
+                }
+            }
+
             getTypeFromTypeNode(node);
         }
 
@@ -40133,6 +40199,15 @@ namespace ts {
 
         function checkAssertClause(declaration: ImportDeclaration | ExportDeclaration) {
             if (declaration.assertClause) {
+                const validForTypeAssertions = isExclusivelyTypeOnlyImportOrExport(declaration);
+                const override = getResolutionModeOverrideForClause(declaration.assertClause, validForTypeAssertions ? grammarErrorOnNode : undefined);
+                if (validForTypeAssertions && override) {
+                    if (getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Node12 && getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeNext) {
+                        return grammarErrorOnNode(declaration.assertClause, Diagnostics.Resolution_modes_are_only_supported_when_moduleResolution_is_node12_or_nodenext);
+                    }
+                    return; // Other grammar checks do not apply to type-only imports with resolution mode assertions
+                }
+
                 const mode = (moduleKind === ModuleKind.NodeNext) && declaration.moduleSpecifier && getUsageModeForExpression(declaration.moduleSpecifier);
                 if (mode !== ModuleKind.ESNext && moduleKind !== ModuleKind.ESNext) {
                     return grammarErrorOnNode(declaration.assertClause,
@@ -40144,6 +40219,10 @@ namespace ts {
                 if (isImportDeclaration(declaration) ? declaration.importClause?.isTypeOnly : declaration.isTypeOnly) {
                     return grammarErrorOnNode(declaration.assertClause, Diagnostics.Import_assertions_cannot_be_used_with_type_only_imports_or_exports);
                 }
+
+                if (override) {
+                    return grammarErrorOnNode(declaration.assertClause, Diagnostics.resolution_mode_can_only_be_set_for_type_only_imports);
+                }
             }
         }
 
 
@@ -1416,6 +1416,18 @@
         "category": "Error",
         "code": 1453
     },
+    "`resolution-mode` can only be set for type-only imports.": {
+        "category": "Error",
+        "code": 1454
+    },
+    "`resolution-mode` is the only valid key for type import assertions.": {
+        "category": "Error",
+        "code": 1455
+    },
+    "Type import assertions should have exactly one key - `resolution-mode` - with value `import` or `require`.": {
+        "category": "Error",
+        "code": 1456
+    },
 
     "The 'import.meta' meta-property is not allowed in files which will build into CommonJS output.": {
         "category": "Error",
 
@@ -2389,6 +2389,19 @@ namespace ts {
             writeKeyword("import");
             writePunctuation("(");
             emit(node.argument);
+            if (node.assertions) {
+                writePunctuation(",");
+                writeSpace();
+                writePunctuation("{");
+                writeSpace();
+                writeKeyword("assert");
+                writePunctuation(":");
+                writeSpace();
+                const elements = node.assertions.assertClause.elements;
+                emitList(node.assertions.assertClause, elements, ListFormat.ImportClauseEntries);
+                writeSpace();
+                writePunctuation("}");
+            }
             writePunctuation(")");
             if (node.qualifier) {
                 writePunctuation(".");