From d85f072dddc5b55388ed6ddc0bebd89f2a9e72e4 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 14 Nov 2025 01:06:49 +0000
Subject: [PATCH] Fix: Only allow synthetic defaults for declaration files in
 export specifiers
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Corrected the fix to only allow synthetic defaults when re-exporting
from declaration files (not TypeScript source files). This aligns with
the issue description which states that declaration files may correspond
to CommonJS modules at runtime where a default import can reference the
whole module.exports symbol.

Only tests targeting .d.ts files should pass without errors:
- reexportMissingDefault4.ts (targets b.d.ts) ✓
- reexportMissingDefault5.ts (targets b.d.ts) ✓

Tests targeting .ts files correctly maintain their errors:
- reexportMissingDefault.ts, 1, 2, 3, 6, 7 (all target b.ts)

Co-authored-by: andrewbranch <3277153+andrewbranch@users.noreply.github.com>
---
 src/compiler/checker.ts                              |  9 ++++-----
 .../reference/reexportMissingDefault.errors.txt      | 11 +++++++++++
 .../baselines/reference/reexportMissingDefault.types |  5 +++--
 .../reference/reexportMissingDefault1.errors.txt     | 12 ++++++++++++
 .../reference/reexportMissingDefault1.types          |  5 +++--
 .../reference/reexportMissingDefault2.errors.txt     | 11 +++++++++++
 .../reference/reexportMissingDefault2.types          |  5 +++--
 .../reference/reexportMissingDefault3.errors.txt     | 11 +++++++++++
 .../reference/reexportMissingDefault3.symbols        |  1 -
 .../reference/reexportMissingDefault3.types          |  9 +++++----
 10 files changed, 63 insertions(+), 16 deletions(-)
 create mode 100644 tests/baselines/reference/reexportMissingDefault.errors.txt
 create mode 100644 tests/baselines/reference/reexportMissingDefault1.errors.txt
 create mode 100644 tests/baselines/reference/reexportMissingDefault2.errors.txt
 create mode 100644 tests/baselines/reference/reexportMissingDefault3.errors.txt

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 85be34b8d29..8f420077f27 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3870,11 +3870,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
         const hasDefaultOnly = isOnlyImportableAsDefault(specifier, moduleSymbol);
         let hasSyntheticDefault = canHaveSyntheticDefault(file, moduleSymbol, dontResolveAlias, specifier);
-        // For export specifiers re-exporting 'default', when allowSyntheticDefaultImports is enabled,
-        // allow synthetic default even from TypeScript source files (not just declaration files).
-        // This is consistent with the intent of allowSyntheticDefaultImports to allow treating modules
-        // without a default export as if they had one.
-        if (!hasSyntheticDefault && isExportSpecifier(node) && allowSyntheticDefaultImports) {
+        // For export specifiers re-exporting 'default' from declaration files, when allowSyntheticDefaultImports
+        // is enabled, allow synthetic default. Declaration files may correspond to CommonJS modules at runtime,
+        // where a default import is allowed to reference the whole module.exports symbol.
+        if (!hasSyntheticDefault && isExportSpecifier(node) && allowSyntheticDefaultImports && (!file || file.isDeclarationFile)) {
             hasSyntheticDefault = true;
         }
         if (!exportDefaultSymbol && !hasSyntheticDefault && !hasDefaultOnly) {
diff --git a/tests/baselines/reference/reexportMissingDefault.errors.txt b/tests/baselines/reference/reexportMissingDefault.errors.txt
new file mode 100644
index 00000000000..91637fa1f74
--- /dev/null
+++ b/tests/baselines/reference/reexportMissingDefault.errors.txt
@@ -0,0 +1,11 @@
+a.ts(2,10): error TS2305: Module '"./b"' has no exported member 'default'.
+
+
+==== b.ts (0 errors) ====
+    export const b = null;
+    
+==== a.ts (1 errors) ====
+    export { b } from "./b";
+    export { default } from "./b";
+             ~~~~~~~
+!!! error TS2305: Module '"./b"' has no exported member 'default'.
\ No newline at end of file
diff --git a/tests/baselines/reference/reexportMissingDefault.types b/tests/baselines/reference/reexportMissingDefault.types
index 3ef3ab2eeef..afba6a1c2f7 100644
--- a/tests/baselines/reference/reexportMissingDefault.types
+++ b/tests/baselines/reference/reexportMissingDefault.types
@@ -3,6 +3,7 @@
 === b.ts ===
 export const b = null;
 >b : any
+>  : ^^^
 
 === a.ts ===
 export { b } from "./b";
@@ -10,6 +11,6 @@ export { b } from "./b";
 >  : ^^^
 
 export { default } from "./b";
->default : typeof import("b")
->        : ^^^^^^^^^^^^^^^^^^
+>default : any
+>        : ^^^
 
diff --git a/tests/baselines/reference/reexportMissingDefault1.errors.txt b/tests/baselines/reference/reexportMissingDefault1.errors.txt
new file mode 100644
index 00000000000..dfee891aa42
--- /dev/null
+++ b/tests/baselines/reference/reexportMissingDefault1.errors.txt
@@ -0,0 +1,12 @@
+a.ts(2,10): error TS2305: Module '"./b"' has no exported member 'default'.
+
+
+==== b.ts (0 errors) ====
+    export const b = null;
+    
+==== a.ts (1 errors) ====
+    export { b } from "./b";
+    export { default } from "./b";
+             ~~~~~~~
+!!! error TS2305: Module '"./b"' has no exported member 'default'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/reexportMissingDefault1.types b/tests/baselines/reference/reexportMissingDefault1.types
index 7527bc6cad3..3b058c8337b 100644
--- a/tests/baselines/reference/reexportMissingDefault1.types
+++ b/tests/baselines/reference/reexportMissingDefault1.types
@@ -3,6 +3,7 @@
 === b.ts ===
 export const b = null;
 >b : any
+>  : ^^^
 
 === a.ts ===
 export { b } from "./b";
@@ -10,6 +11,6 @@ export { b } from "./b";
 >  : ^^^
 
 export { default } from "./b";
->default : typeof import("b")
->        : ^^^^^^^^^^^^^^^^^^
+>default : any
+>        : ^^^
 
diff --git a/tests/baselines/reference/reexportMissingDefault2.errors.txt b/tests/baselines/reference/reexportMissingDefault2.errors.txt
new file mode 100644
index 00000000000..91637fa1f74
--- /dev/null
+++ b/tests/baselines/reference/reexportMissingDefault2.errors.txt
@@ -0,0 +1,11 @@
+a.ts(2,10): error TS2305: Module '"./b"' has no exported member 'default'.
+
+
+==== b.ts (0 errors) ====
+    export const b = null;
+    
+==== a.ts (1 errors) ====
+    export { b } from "./b";
+    export { default } from "./b";
+             ~~~~~~~
+!!! error TS2305: Module '"./b"' has no exported member 'default'.
\ No newline at end of file
diff --git a/tests/baselines/reference/reexportMissingDefault2.types b/tests/baselines/reference/reexportMissingDefault2.types
index f003a83ba96..56de966eefd 100644
--- a/tests/baselines/reference/reexportMissingDefault2.types
+++ b/tests/baselines/reference/reexportMissingDefault2.types
@@ -3,6 +3,7 @@
 === b.ts ===
 export const b = null;
 >b : any
+>  : ^^^
 
 === a.ts ===
 export { b } from "./b";
@@ -10,6 +11,6 @@ export { b } from "./b";
 >  : ^^^
 
 export { default } from "./b";
->default : typeof import("b")
->        : ^^^^^^^^^^^^^^^^^^
+>default : any
+>        : ^^^
 
diff --git a/tests/baselines/reference/reexportMissingDefault3.errors.txt b/tests/baselines/reference/reexportMissingDefault3.errors.txt
new file mode 100644
index 00000000000..b7965fa1eb3
--- /dev/null
+++ b/tests/baselines/reference/reexportMissingDefault3.errors.txt
@@ -0,0 +1,11 @@
+a.ts(2,10): error TS2305: Module '"./b"' has no exported member 'default'.
+
+
+==== b.ts (0 errors) ====
+    export const b = null;
+    
+==== a.ts (1 errors) ====
+    export { b } from "./b";
+    export { default as a } from "./b";
+             ~~~~~~~
+!!! error TS2305: Module '"./b"' has no exported member 'default'.
\ No newline at end of file
diff --git a/tests/baselines/reference/reexportMissingDefault3.symbols b/tests/baselines/reference/reexportMissingDefault3.symbols
index 2c6fdef6f2d..4c505a73a30 100644
--- a/tests/baselines/reference/reexportMissingDefault3.symbols
+++ b/tests/baselines/reference/reexportMissingDefault3.symbols
@@ -9,6 +9,5 @@ export { b } from "./b";
 >b : Symbol(b, Decl(a.ts, 0, 8))
 
 export { default as a } from "./b";
->default : Symbol("b", Decl(b.ts, 0, 0))
 >a : Symbol(a, Decl(a.ts, 1, 8))
 
diff --git a/tests/baselines/reference/reexportMissingDefault3.types b/tests/baselines/reference/reexportMissingDefault3.types
index 6ad7f16080a..b6908bed4be 100644
--- a/tests/baselines/reference/reexportMissingDefault3.types
+++ b/tests/baselines/reference/reexportMissingDefault3.types
@@ -3,6 +3,7 @@
 === b.ts ===
 export const b = null;
 >b : any
+>  : ^^^
 
 === a.ts ===
 export { b } from "./b";
@@ -10,8 +11,8 @@ export { b } from "./b";
 >  : ^^^
 
 export { default as a } from "./b";
->default : typeof import("b")
->        : ^^^^^^^^^^^^^^^^^^
->a : typeof import("b")
->  : ^^^^^^^^^^^^^^^^^^
+>default : any
+>        : ^^^
+>a : any
+>  : ^^^