From 5b7d8df149be6554c863cdd5b73c0b0b0a7960db Mon Sep 17 00:00:00 2001
From: islandryu <65934663+islandryu@users.noreply.github.com>
Date: Sun, 13 Feb 2022 05:02:58 +0900
Subject: [PATCH] fix(eslint-plugin): [no-unnecessary-type-arguments] Use
 Symbol to check if it's the same type (#4543)

Co-authored-by: Josh Goldberg <me@joshuakgoldberg.com>
---
 .../rules/no-unnecessary-type-arguments.ts    | 18 +++++++++----
 .../no-unnecessary-type-arguments.test.ts     | 27 +++++++++++++++++++
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts
index 67244961bca..318af6c6d01 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts
@@ -37,7 +37,6 @@ export default util.createRule<[], MessageIds>({
   create(context) {
     const parserServices = util.getParserServices(context);
     const checker = parserServices.program.getTypeChecker();
-    const sourceCode = context.getSourceCode();
 
     function checkTSArgsAndParameters(
       esParameters: TSESTree.TSTypeParameterInstantiation,
@@ -47,11 +46,20 @@ export default util.createRule<[], MessageIds>({
       const i = esParameters.params.length - 1;
       const arg = esParameters.params[i];
       const param = typeParameters[i];
-
+      if (!param?.default) {
+        return;
+      }
       // TODO: would like checker.areTypesEquivalent. https://github.com/Microsoft/TypeScript/issues/13502
-      if (
-        !param?.default ||
-        param.default.getText() !== sourceCode.getText(arg)
+      const defaultType = checker.getTypeAtLocation(param.default);
+      const argTsNode = parserServices.esTreeNodeToTSNodeMap.get(arg);
+      const argType = checker.getTypeAtLocation(argTsNode);
+      if (!argType.aliasSymbol && !defaultType.aliasSymbol) {
+        if (argType.flags !== defaultType.flags) {
+          return;
+        }
+      } else if (
+        argType.aliasSymbol !== defaultType.aliasSymbol ||
+        argType.aliasTypeArguments !== defaultType.aliasTypeArguments
       ) {
         return;
       }
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-type-arguments.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-type-arguments.test.ts
index 9c642d5f88f..dc140aade44 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-arguments.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-arguments.test.ts
@@ -290,5 +290,32 @@ function bar<T = F<string>>() {}
 bar();
       `,
     },
+    {
+      code: `
+type DefaultE = { foo: string };
+type T<E = DefaultE> = { box: E };
+type G = T<DefaultE>;
+declare module 'bar' {
+  type DefaultE = { somethingElse: true };
+  type G = T<DefaultE>;
+}
+      `,
+      errors: [
+        {
+          line: 4,
+          column: 12,
+          messageId: 'unnecessaryTypeParameter',
+        },
+      ],
+      output: `
+type DefaultE = { foo: string };
+type T<E = DefaultE> = { box: E };
+type G = T;
+declare module 'bar' {
+  type DefaultE = { somethingElse: true };
+  type G = T<DefaultE>;
+}
+      `,
+    },
   ],
 });