Fix: Only allow synthetic defaults for declaration files in export specifiers

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

@@ -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) {

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'.

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
+>        : ^^^
 

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'.
+    

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
+>        : ^^^
 

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'.

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
+>        : ^^^
 

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'.

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))
 

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
+>  : ^^^