Handle parameter type error for index signature in declaration emit

src/compiler/declarationEmitter.ts

@@ -1625,6 +1625,12 @@ namespace ts {
                             Diagnostics.Parameter_0_of_call_signature_from_exported_interface_has_or_is_using_name_1_from_private_module_2 :
                             Diagnostics.Parameter_0_of_call_signature_from_exported_interface_has_or_is_using_private_name_1;
 
+                    case SyntaxKind.IndexSignature:
+                        // Interfaces cannot have parameter types that cannot be named
+                        return symbolAccessibilityResult.errorModuleName ?
+                            Diagnostics.Parameter_0_of_index_signature_from_exported_interface_has_or_is_using_name_1_from_private_module_2 :
+                            Diagnostics.Parameter_0_of_index_signature_from_exported_interface_has_or_is_using_private_name_1;
+
                     case SyntaxKind.MethodDeclaration:
                     case SyntaxKind.MethodSignature:
                         if (hasModifier(node.parent, ModifierFlags.Static)) {

src/compiler/diagnosticMessages.json

@@ -2300,6 +2300,14 @@
         "category": "Message",
         "code": 4090
     },
+    "Parameter '{0}' of index signature from exported interface has or is using name '{1}' from private module '{2}'.": {
+        "category": "Error",
+        "code": 4091
+    },
+    "Parameter '{0}' of index signature from exported interface has or is using private name '{1}'.": {
+        "category": "Error",
+        "code": 4092
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",

tests/baselines/reference/declarationEmitIndexTypeNotFound.errors.txt

@@ -0,0 +1,17 @@
+tests/cases/compiler/declarationEmitIndexTypeNotFound.ts(3,6): error TS1023: An index signature parameter type must be 'string' or 'number'.
+tests/cases/compiler/declarationEmitIndexTypeNotFound.ts(3,13): error TS2304: Cannot find name 'TypeNotFound'.
+tests/cases/compiler/declarationEmitIndexTypeNotFound.ts(3,13): error TS4092: Parameter 'index' of index signature from exported interface has or is using private name 'TypeNotFound'.
+
+
+==== tests/cases/compiler/declarationEmitIndexTypeNotFound.ts (3 errors) ====
+
+    export interface Test {
+        [index: TypeNotFound]: any;
+         ~~~~~
+!!! error TS1023: An index signature parameter type must be 'string' or 'number'.
+                ~~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'TypeNotFound'.
+                ~~~~~~~~~~~~
+!!! error TS4092: Parameter 'index' of index signature from exported interface has or is using private name 'TypeNotFound'.
+    }
+

tests/baselines/reference/declarationEmitIndexTypeNotFound.js

@@ -0,0 +1,9 @@
+//// [declarationEmitIndexTypeNotFound.ts]
+
+export interface Test {
+    [index: TypeNotFound]: any;
+}
+
+
+//// [declarationEmitIndexTypeNotFound.js]
+"use strict";

tests/cases/compiler/declarationEmitIndexTypeNotFound.ts

@@ -0,0 +1,5 @@
+// @declaration: true
+
+export interface Test {
+    [index: TypeNotFound]: any;
+}