Merge pull request #26121 from mattmccutchen/issue-23999

"Could not find a declaration file for module" error needs to use the unmangled package name where appropriate.

src/compiler/checker.ts

@@ -2244,8 +2244,9 @@ namespace ts {
                 ? chainDiagnosticMessages(
                     /*details*/ undefined,
                     typesPackageExists(packageId.name)
-                        ? Diagnostics.If_the_0_package_actually_exposes_this_module_consider_sending_a_pull_request_to_amend_https_Colon_Slash_Slashgithub_com_SlashDefinitelyTyped_SlashDefinitelyTyped_Slashtree_Slashmaster_Slashtypes_Slash_0
-                        : Diagnostics.Try_npm_install_types_Slash_0_if_it_exists_or_add_a_new_declaration_d_ts_file_containing_declare_module_0,
+                        ? Diagnostics.If_the_0_package_actually_exposes_this_module_consider_sending_a_pull_request_to_amend_https_Colon_Slash_Slashgithub_com_SlashDefinitelyTyped_SlashDefinitelyTyped_Slashtree_Slashmaster_Slashtypes_Slash_1
+                        : Diagnostics.Try_npm_install_types_Slash_1_if_it_exists_or_add_a_new_declaration_d_ts_file_containing_declare_module_0,
+                    packageId.name,
                     getMangledNameForScopedPackage(packageId.name))
                 : undefined;
             errorOrSuggestion(isError, errorNode, chainDiagnosticMessages(

src/compiler/diagnosticMessages.json

@@ -3901,7 +3901,7 @@
         "category": "Error",
         "code": 7034
     },
-    "Try `npm install @types/{0}` if it exists or add a new declaration (.d.ts) file containing `declare module '{0}';`": {
+    "Try `npm install @types/{1}` if it exists or add a new declaration (.d.ts) file containing `declare module '{0}';`": {
         "category": "Error",
         "code": 7035
     },
@@ -3921,7 +3921,7 @@
         "category": "Error",
         "code": 7039
     },
-    "If the '{0}' package actually exposes this module, consider sending a pull request to amend 'https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/{0}`": {
+    "If the '{0}' package actually exposes this module, consider sending a pull request to amend 'https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/{1}`": {
         "category": "Error",
         "code": 7040
     },