Take default export spaces into account when checking merged declarations.

src/compiler/checker.ts

@@ -10682,10 +10682,18 @@ namespace ts {
             // to denote disjoint declarationSpaces (without making new enum type).
             let exportedDeclarationSpaces = SymbolFlags.None;
             let nonExportedDeclarationSpaces = SymbolFlags.None;
+            let defaultExportedDeclarationFlags = SymbolFlags.None;
             for (let d of symbol.declarations) {
                 let declarationSpaces = getDeclarationSpaces(d);
-                if (getEffectiveDeclarationFlags(d, NodeFlags.Export)) {
-                    exportedDeclarationSpaces |= declarationSpaces;
+                let effectiveDeclarationFlags = getEffectiveDeclarationFlags(d, NodeFlags.Export | NodeFlags.Default);
+
+                if (effectiveDeclarationFlags & NodeFlags.Export) {
+                    if (effectiveDeclarationFlags & NodeFlags.Default) {
+                        defaultExportedDeclarationFlags |= declarationSpaces;
+                    }
+                    else {
+                        exportedDeclarationSpaces |= declarationSpaces;
+                    }
                 }
                 else {
                     nonExportedDeclarationSpaces |= declarationSpaces;
@@ -10693,11 +10701,16 @@ namespace ts {
             }
 
             let commonDeclarationSpace = exportedDeclarationSpaces & nonExportedDeclarationSpaces;
+            let commonDeclarationSpaceForDefault = defaultExportedDeclarationFlags & (exportedDeclarationSpaces | nonExportedDeclarationSpaces);
 
-            if (commonDeclarationSpace) {
+            if (commonDeclarationSpace || commonDeclarationSpaceForDefault) {
                 // declaration spaces for exported and non-exported declarations intersect
                 for (let d of symbol.declarations) {
-                    if (getDeclarationSpaces(d) & commonDeclarationSpace) {
+                    let declarationSpaces = getDeclarationSpaces(d);
+                    if (declarationSpaces & commonDeclarationSpaceForDefault) {
+                        error(d.name, Diagnostics.Merged_declaration_0_cannot_include_a_default_export_declaration, declarationNameToString(d.name));
+                    }
+                    else if (declarationSpaces & commonDeclarationSpace) {
                         error(d.name, Diagnostics.Individual_declarations_in_merged_declaration_0_must_be_all_exported_or_all_local, declarationNameToString(d.name));
                     }
                 }

src/compiler/diagnosticInformationMap.generated.ts

@@ -424,6 +424,7 @@ namespace ts {
         JSX_element_class_does_not_support_attributes_because_it_does_not_have_a_0_property: { code: 2607, category: DiagnosticCategory.Error, key: "JSX element class does not support attributes because it does not have a '{0}' property" },
         The_global_type_JSX_0_may_not_have_more_than_one_property: { code: 2608, category: DiagnosticCategory.Error, key: "The global type 'JSX.{0}' may not have more than one property" },
         Cannot_emit_namespaced_JSX_elements_in_React: { code: 2650, category: DiagnosticCategory.Error, key: "Cannot emit namespaced JSX elements in React" },
+        Merged_declaration_0_cannot_include_a_default_export_declaration: { code: 2651, category: DiagnosticCategory.Error, key: "Merged declaration '{0}' cannot include a default export declaration." },
         Import_declaration_0_is_using_private_name_1: { code: 4000, category: DiagnosticCategory.Error, key: "Import declaration '{0}' is using private name '{1}'." },
         Type_parameter_0_of_exported_class_has_or_is_using_private_name_1: { code: 4002, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported class has or is using private name '{1}'." },
         Type_parameter_0_of_exported_interface_has_or_is_using_private_name_1: { code: 4004, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported interface has or is using private name '{1}'." },

src/compiler/diagnosticMessages.json

@@ -1685,6 +1685,10 @@
         "category": "Error",
         "code": 2650
     },
+    "Merged declaration '{0}' cannot include a default export declaration.": {
+        "category": "Error",
+        "code": 2651
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000