Handle error when type parameter of mapped type uses private type

Fixes #30201

src/compiler/diagnosticMessages.json

@@ -2927,6 +2927,10 @@
         "category": "Error",
         "code": 4102
     },
+    "Type parameter '{0}' of exported mapped object type is using private name '{1}'.": {
+        "category": "Error",
+        "code": 4103
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",

src/compiler/transformers/declarations/diagnostics.ts

@@ -391,6 +391,10 @@ namespace ts {
                     diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_interface_has_or_is_using_private_name_1;
                     break;
 
+                case SyntaxKind.MappedType:
+                    diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_mapped_object_type_is_using_private_name_1;
+                    break;
+
                 case SyntaxKind.ConstructorType:
                 case SyntaxKind.ConstructSignature:
                     diagnosticMessage = Diagnostics.Type_parameter_0_of_constructor_signature_from_exported_interface_has_or_is_using_private_name_1;

tests/baselines/reference/declarationEmitMappedPrivateTypeTypeParameter.errors.txt

@@ -0,0 +1,15 @@
+/FromFactor.ts(2,15): error TS2304: Cannot find name 'StringKeyOf'.
+/FromFactor.ts(2,15): error TS4103: Type parameter 'TName' of exported mapped object type is using private name 'StringKeyOf'.
+
+
+==== /Helpers.ts (0 errors) ====
+    export type StringKeyOf<TObj> = Extract<string, keyof TObj>;
+    
+==== /FromFactor.ts (2 errors) ====
+    export type RowToColumns<TColumns> = {
+        [TName in StringKeyOf<TColumns>]: any;
+                  ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'StringKeyOf'.
+                  ~~~~~~~~~~~
+!!! error TS4103: Type parameter 'TName' of exported mapped object type is using private name 'StringKeyOf'.
+    }

tests/baselines/reference/declarationEmitMappedPrivateTypeTypeParameter.js

@@ -0,0 +1,20 @@
+//// [tests/cases/compiler/declarationEmitMappedPrivateTypeTypeParameter.ts] ////
+
+//// [Helpers.ts]
+export type StringKeyOf<TObj> = Extract<string, keyof TObj>;
+
+//// [FromFactor.ts]
+export type RowToColumns<TColumns> = {
+    [TName in StringKeyOf<TColumns>]: any;
+}
+
+//// [Helpers.js]
+"use strict";
+exports.__esModule = true;
+//// [FromFactor.js]
+"use strict";
+exports.__esModule = true;
+
+
+//// [Helpers.d.ts]
+export declare type StringKeyOf<TObj> = Extract<string, keyof TObj>;

tests/baselines/reference/declarationEmitMappedPrivateTypeTypeParameter.symbols

@@ -0,0 +1,16 @@
+=== /Helpers.ts ===
+export type StringKeyOf<TObj> = Extract<string, keyof TObj>;
+>StringKeyOf : Symbol(StringKeyOf, Decl(Helpers.ts, 0, 0))
+>TObj : Symbol(TObj, Decl(Helpers.ts, 0, 24))
+>Extract : Symbol(Extract, Decl(lib.es5.d.ts, --, --))
+>TObj : Symbol(TObj, Decl(Helpers.ts, 0, 24))
+
+=== /FromFactor.ts ===
+export type RowToColumns<TColumns> = {
+>RowToColumns : Symbol(RowToColumns, Decl(FromFactor.ts, 0, 0))
+>TColumns : Symbol(TColumns, Decl(FromFactor.ts, 0, 25))
+
+    [TName in StringKeyOf<TColumns>]: any;
+>TName : Symbol(TName, Decl(FromFactor.ts, 1, 5))
+>TColumns : Symbol(TColumns, Decl(FromFactor.ts, 0, 25))
+}

tests/baselines/reference/declarationEmitMappedPrivateTypeTypeParameter.types

@@ -0,0 +1,10 @@
+=== /Helpers.ts ===
+export type StringKeyOf<TObj> = Extract<string, keyof TObj>;
+>StringKeyOf : Extract<string, keyof TObj>
+
+=== /FromFactor.ts ===
+export type RowToColumns<TColumns> = {
+>RowToColumns : RowToColumns<TColumns>
+
+    [TName in StringKeyOf<TColumns>]: any;
+}

tests/cases/compiler/declarationEmitMappedPrivateTypeTypeParameter.ts

@@ -0,0 +1,8 @@
+// @declaration: true
+// @filename: /Helpers.ts
+export type StringKeyOf<TObj> = Extract<string, keyof TObj>;
+
+// @filename: /FromFactor.ts
+export type RowToColumns<TColumns> = {
+    [TName in StringKeyOf<TColumns>]: any;
+}