Issue an error on cross-file merges we can't emit

src/compiler/checker.ts

@@ -6029,14 +6029,19 @@ namespace ts {
                         serializeAsNamespaceDeclaration(realMembers, localName, modifierFlags, !!(symbol.flags & (SymbolFlags.Function | SymbolFlags.Assignment)));
                     }
                     if (length(mergedMembers)) {
+                        const containingFile = getSourceFileOfNode(context.enclosingDeclaration);
                         const localName = getInternalSymbolName(symbol, symbolName);
                         const nsBody = createModuleBlock([createExportDeclaration(
                             /*decorators*/ undefined,
                             /*modifiers*/ undefined,
-                            createNamedExports(map(filter(mergedMembers, n => n.escapedName !== InternalSymbolName.ExportEquals), s => {
+                            createNamedExports(mapDefined(filter(mergedMembers, n => n.escapedName !== InternalSymbolName.ExportEquals), s => {
                                 const name = unescapeLeadingUnderscores(s.escapedName);
                                 const localName = getInternalSymbolName(s, name);
                                 const aliasDecl = s.declarations && getDeclarationOfAliasSymbol(s);
+                                if (containingFile && (aliasDecl ? containingFile !== getSourceFileOfNode(aliasDecl) : !some(s.declarations, d => getSourceFileOfNode(d) === containingFile))) {
+                                    context.tracker?.reportNonlocalAugmentation?.(containingFile, symbol, s);
+                                    return undefined;
+                                }
                                 const target = aliasDecl && getTargetOfAliasDeclaration(aliasDecl, /*dontRecursivelyResolve*/ true);
                                 includePrivateSymbol(target || s);
                                 const targetName = target ? getInternalSymbolName(target, unescapeLeadingUnderscores(target.escapedName)) : localName;

src/compiler/diagnosticMessages.json

@@ -4388,6 +4388,14 @@
         "category": "Error",
         "code": 6231
     },
+    "Declaration augments declaration in another file. This cannot be serialized.": {
+        "category": "Error",
+        "code": 6232
+    },
+    "This is the declaration being augmented. Consider moving the augmenting declaration into the same file.": {
+        "category": "Error",
+        "code": 6233
+    },
 
     "Projects to reference": {
         "category": "Message",

src/compiler/transformers/declarations.ts

@@ -76,7 +76,8 @@ namespace ts {
             reportLikelyUnsafeImportRequiredError,
             moduleResolverHost: host,
             trackReferencedAmbientModule,
-            trackExternalModuleSymbolOfImportTypeNode
+            trackExternalModuleSymbolOfImportTypeNode,
+            reportNonlocalAugmentation
         };
         let errorNameNode: DeclarationName | undefined;
 
@@ -190,6 +191,17 @@ namespace ts {
             }
         }
 
+        function reportNonlocalAugmentation(containingFile: SourceFile, parentSymbol: Symbol, symbol: Symbol) {
+            const primaryDeclaration = find(parentSymbol.declarations, d => getSourceFileOfNode(d) === containingFile)!;
+            const augmentingDeclarations = filter(symbol.declarations, d => getSourceFileOfNode(d) !== containingFile);
+            for (const augmentations of augmentingDeclarations) {
+                context.addDiagnostic(addRelatedInfo(
+                    createDiagnosticForNode(augmentations, Diagnostics.Declaration_augments_declaration_in_another_file_This_cannot_be_serialized),
+                    createDiagnosticForNode(primaryDeclaration, Diagnostics.This_is_the_declaration_being_augmented_Consider_moving_the_augmenting_declaration_into_the_same_file)
+                ));
+            }
+        }
+
         function transformDeclarationsForJS(sourceFile: SourceFile, bundled?: boolean) {
             const oldDiag = getSymbolAccessibilityDiagnostic;
             getSymbolAccessibilityDiagnostic = (s) => ({

src/compiler/types.ts

@@ -6468,6 +6468,7 @@ namespace ts {
         moduleResolverHost?: ModuleSpecifierResolutionHost & { getCommonSourceDirectory(): string };
         trackReferencedAmbientModule?(decl: ModuleDeclaration, symbol: Symbol): void;
         trackExternalModuleSymbolOfImportTypeNode?(symbol: Symbol): void;
+        reportNonlocalAugmentation?(containingFile: SourceFile, parentSymbol: Symbol, augmentingSymbol: Symbol): void;
     }
 
     export interface TextSpan {

tests/baselines/reference/jsDeclarationsCrossfileMerge.errors.txt

@@ -0,0 +1,17 @@
+tests/cases/conformance/jsdoc/declarations/index.js(4,1): error TS6232: Declaration augments declaration in another file. This cannot be serialized.
+
+
+==== tests/cases/conformance/jsdoc/declarations/index.js (1 errors) ====
+    const m = require("./exporter");
+
+    module.exports = m.default;
+    module.exports.memberName = "thing";
+    ~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS6232: Declaration augments declaration in another file. This cannot be serialized.
+!!! related TS6233 /.src/tests/cases/conformance/jsdoc/declarations/exporter.js:1:10: This is the declaration being augmented. Consider moving the augmenting declaration into the same file.
+
+==== tests/cases/conformance/jsdoc/declarations/exporter.js (0 errors) ====
+    function validate() {}
+
+    export default validate;
+

tests/baselines/reference/jsDeclarationsCrossfileMerge.js

@@ -0,0 +1,28 @@
+//// [tests/cases/conformance/jsdoc/declarations/jsDeclarationsCrossfileMerge.ts] ////
+
+//// [index.js]
+const m = require("./exporter");
+
+module.exports = m.default;
+module.exports.memberName = "thing";
+
+//// [exporter.js]
+function validate() {}
+
+export default validate;
+
+
+//// [exporter.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+function validate() { }
+exports.default = validate;
+//// [index.js]
+var m = require("./exporter");
+module.exports = m.default;
+module.exports.memberName = "thing";
+
+
+//// [index.d.ts]
+declare const _exports: typeof import("./exporter").default;
+export = _exports;

tests/baselines/reference/jsDeclarationsCrossfileMerge.symbols

@@ -0,0 +1,28 @@
+=== tests/cases/conformance/jsdoc/declarations/index.js ===
+const m = require("./exporter");
+>m : Symbol(m, Decl(index.js, 0, 5))
+>require : Symbol(require)
+>"./exporter" : Symbol("tests/cases/conformance/jsdoc/declarations/exporter", Decl(exporter.js, 0, 0))
+
+module.exports = m.default;
+>module.exports : Symbol("tests/cases/conformance/jsdoc/declarations/index", Decl(index.js, 0, 0))
+>module : Symbol(export=, Decl(index.js, 0, 32))
+>exports : Symbol(export=, Decl(index.js, 0, 32))
+>m.default : Symbol(default, Decl(exporter.js, 0, 22))
+>m : Symbol(m, Decl(index.js, 0, 5))
+>default : Symbol(default, Decl(exporter.js, 0, 22))
+
+module.exports.memberName = "thing";
+>module.exports.memberName : Symbol(memberName)
+>module.exports : Symbol(memberName, Decl(index.js, 2, 27))
+>module : Symbol(module, Decl(index.js, 0, 32))
+>exports : Symbol("tests/cases/conformance/jsdoc/declarations/index", Decl(index.js, 0, 0))
+>memberName : Symbol(memberName, Decl(index.js, 2, 27))
+
+=== tests/cases/conformance/jsdoc/declarations/exporter.js ===
+function validate() {}
+>validate : Symbol(validate, Decl(exporter.js, 0, 0))
+
+export default validate;
+>validate : Symbol(validate, Decl(exporter.js, 0, 0))
+

tests/baselines/reference/jsDeclarationsCrossfileMerge.types

@@ -0,0 +1,32 @@
+=== tests/cases/conformance/jsdoc/declarations/index.js ===
+const m = require("./exporter");
+>m : typeof import("tests/cases/conformance/jsdoc/declarations/exporter")
+>require("./exporter") : typeof import("tests/cases/conformance/jsdoc/declarations/exporter")
+>require : any
+>"./exporter" : "./exporter"
+
+module.exports = m.default;
+>module.exports = m.default : typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default
+>module.exports : typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default
+>module : { "\"tests/cases/conformance/jsdoc/declarations/index\"": typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default; }
+>exports : typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default
+>m.default : { (): void; memberName: string; }
+>m : typeof import("tests/cases/conformance/jsdoc/declarations/exporter")
+>default : { (): void; memberName: string; }
+
+module.exports.memberName = "thing";
+>module.exports.memberName = "thing" : "thing"
+>module.exports.memberName : string
+>module.exports : typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default
+>module : { "\"tests/cases/conformance/jsdoc/declarations/index\"": typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default; }
+>exports : typeof import("tests/cases/conformance/jsdoc/declarations/exporter").default
+>memberName : string
+>"thing" : "thing"
+
+=== tests/cases/conformance/jsdoc/declarations/exporter.js ===
+function validate() {}
+>validate : typeof validate
+
+export default validate;
+>validate : typeof validate
+

tests/cases/conformance/jsdoc/declarations/jsDeclarationsCrossfileMerge.ts

@@ -0,0 +1,16 @@
+// @allowJs: true
+// @checkJs: true
+// @target: es5
+// @lib: es6
+// @outDir: ./out
+// @declaration: true
+// @filename: index.js
+const m = require("./exporter");
+
+module.exports = m.default;
+module.exports.memberName = "thing";
+
+// @filename: exporter.js
+function validate() {}
+
+export default validate;