Keep declaration files out of the module order

A specifier that resolves ONLY to a declaration file has no runtime module
to link: Node throws ERR_MODULE_NOT_FOUND at it. Preflight already refused
the import form (SC1010), but the re-export arm and both require arms took
the declaration file as a module-order edge. Once a declaration file is a
program module, its own imports are lowered as code — which is how a
package's own `.d.mts` files emit SC2013 for every specifier inside them
(the pi/typebox shape: 133 errors, all at import statements inside
`node_modules/typebox/build/**/*.d.mts`).

Refuse the re-export form with the same SC1010 the import form uses, and
skip declaration files as edges in both require arms. `splitFiles` gains
the matching invariant: a declaration file is never lowered as a runtime
module even if one reaches the order by another path.

Test: tests/diagnostics/declaration-only-module (SC1010 snapshot) and the
recorded preflight/order baselines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit 534ec68eb13f15812376aa5126f013da693a48d6)

packages/compiler/src/frontend/lowering/lower-modules.ts

@@ -328,7 +328,15 @@ function appendEntryModules(
  * (tests, coverage on broken files) may arrive with an empty order — fall
  * back to the entry alone. */
 export function splitFiles(lowerer: Lowerer): FileParts[] {
-  const files = lowerer.moduleOrder.length > 0 ? lowerer.moduleOrder : [lowerer.entry];
+  // A declaration file is never a runtime module: it carries no
+  // implementation, so it has no %init to lower and its imports never load
+  // anything. Preflight keeps declarations out of the module order (a
+  // specifier that resolves only to declarations is refused at its own
+  // statement); this filter is the invariant that keeps a stray declaration
+  // from being lowered as code.
+  const files = (lowerer.moduleOrder.length > 0 ? lowerer.moduleOrder : [lowerer.entry]).filter(
+    (sf) => !sf.isDeclarationFile,
+  );
   return files.map((sf) => {
     const fp: FileParts = { sf, fnDecls: [], classDecls: [], topStmts: [] };
     const rebindable = collectRebindableFunctions(lowerer, sf);

packages/compiler/src/frontend/program.ts

@@ -6060,6 +6060,21 @@ function preflight7(load: LoadResult): {
             continue;
           }
         }
+        // A re-export whose specifier resolves ONLY to a declaration file
+        // has no runtime module to link (Node throws
+        // ERR_MODULE_NOT_FOUND), and a declaration file is not a program
+        // module: an edge to it would lower the declarations' own imports
+        // as code.
+        if (reDep !== null && reDep.isDeclarationFile) {
+          diags.push(
+            unsupportedDiag(
+              "SC1010",
+              locOf7(stmt),
+              `the '${fromSpec}' re-export (it resolves only to type declarations — there is no runtime implementation to compile)`,
+            ),
+          );
+          continue;
+        }
         if (reDep && !reDep.fileName.endsWith(".json")) {
           deps.push({ dep: reDep, stmt });
           depPositions.get(sf)!.push({ dep: reDep, pos: ts.tokenStartOfNode(stmt) });
@@ -6250,6 +6265,22 @@ function preflight7(load: LoadResult): {
       const clause = stmt.importClause;
       const dep = isRelative ? resolveImport7(program, sf, spec) : (npmStaticDep ?? projDep);
       const isJson = dep !== null && dep.fileName.endsWith(".json");
+      // A specifier that resolves ONLY to a declaration file has no runtime
+      // module to link: Node throws ERR_MODULE_NOT_FOUND at this import.
+      // Declarations are not program modules — taking one as a module-order
+      // edge would lower its imports as if it were code (SC2013 for every
+      // specifier inside a package's `.d.ts`). Type-only imports were
+      // already erased above; this is the value form.
+      if (dep !== null && dep.isDeclarationFile && !isJson) {
+        diags.push(
+          unsupportedDiag(
+            "SC1010",
+            locOf7(stmt),
+            `the '${spec}' import (it resolves only to type declarations — there is no runtime implementation to compile)`,
+          ),
+        );
+        continue;
+      }
       // SELF-imports: the bindings alias this module's OWN declarations,
       // so a TOP-LEVEL read lexically above the aliased declaration is
       // Node's TDZ ReferenceError at runtime (exactly what tsc's TS2448
@@ -6532,6 +6563,10 @@ function preflight7(load: LoadResult): {
             }
           }
           if (earlyRequireFence(req, bindingKind, loc)) continue;
+          // A declaration file is never a runtime module (see the import
+          // arm): the require throws MODULE_NOT_FOUND in Node, so it is not
+          // a module-order edge.
+          if (dep !== null && dep.isDeclarationFile) continue;
           if (dep) deps.push({ dep });
         }
       }
@@ -6610,7 +6645,8 @@ function preflight7(load: LoadResult): {
             lazy ? { dep, lazy: true, call } : { dep, call };
           const projectDep = resolveImport7(program, sf, spec, "require");
           if (projectDep !== null) {
-            if (!projectDep.fileName.endsWith(".json")) deps.push(edgeTo(projectDep));
+            if (!projectDep.fileName.endsWith(".json") && !projectDep.isDeclarationFile)
+              deps.push(edgeTo(projectDep));
             continue;
           }
           if (isRelativeSpecifier(spec) || spec.startsWith("#")) continue;

packages/compiler/test/ts7/baselines/order-parity/tests/diagnostics/declaration-only-module/main.ts.json

@@ -0,0 +1,18 @@
+{
+ "order": [
+  "<repo>/tests/diagnostics/declaration-only-module/main.ts"
+ ],
+ "diags": [
+  {
+   "code": "SC1010",
+   "message": "the './types.js' import (it resolves only to type declarations — there is no runtime implementation to compile) is not supported yet",
+   "loc": {
+    "file": "<repo>/tests/diagnostics/declaration-only-module/main.ts",
+    "start": 246,
+    "end": 281
+   },
+   "milestone": "M4",
+   "hint": "relative imports (./file, ../dir/file), package.json-mediated project imports (#alias via the imports field, self-name references via exports), statically compiled npm packages, and the built-in fs, fs/promises, path, os, url, crypto, zlib, child_process, net, http, tls, https, http2, dgram, dns, dns/promises, util, util/types, string_decoder, querystring, readline, events, stream, stream/promises, stream/consumers, stream/web, buffer, assert, assert/strict, worker_threads, cluster, tty, async_hooks, timers, timers/promises, diagnostics_channel, perf_hooks, module, v8, and console modules (bare or node:-prefixed) and node:test (node:-prefixed only, like in Node) are supported"
+  }
+ ]
+}

tests/diagnostics/declaration-only-module/main.ts

@@ -0,0 +1,7 @@
+// A value import whose specifier resolves ONLY to a declaration file has no
+// runtime module to link — Node throws ERR_MODULE_NOT_FOUND here, and a
+// declaration file is not a program module (its own imports must never be
+// lowered as code).
+import { value } from "./types.js";
+
+console.log(value);

tests/diagnostics/declaration-only-module/types.d.ts

@@ -0,0 +1,6 @@
+// A module that exists only as declarations: it has no runtime
+// implementation for either side to link.
+export interface Shape {
+  kind: string;
+}
+export declare const value: number;

tests/harness/__snapshots__/declaration-only-module/main.ts.txt

@@ -0,0 +1,8 @@
+declaration-only-module/main.ts:5:1 - error SC1010: the './types.js' import (it resolves only to type declarations — there is no runtime implementation to compile) is not supported yet
+
+  4 | // lowered as code).
+  5 | import { value } from "./types.js";
+    | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  6 | 
+
+  hint: relative imports (./file, ../dir/file), package.json-mediated project imports (#alias via the imports field, self-name references via exports), statically compiled npm packages, and the built-in fs, fs/promises, path, os, url, crypto, zlib, child_process, net, http, tls, https, http2, dgram, dns, dns/promises, util, util/types, string_decoder, querystring, readline, events, stream, stream/promises, stream/consumers, stream/web, buffer, assert, assert/strict, worker_threads, cluster, tty, async_hooks, timers, timers/promises, diagnostics_channel, perf_hooks, module, v8, and console modules (bare or node:-prefixed) and node:test (node:-prefixed only, like in Node) are supported