From cdf92949bec4686a74233ae49b6a28ff99b12cbd Mon Sep 17 00:00:00 2001
From: quentin-jaquier-sonarsource
 <43733433+quentin-jaquier-sonarsource@users.noreply.github.com>
Date: Tue, 24 Sep 2019 17:46:00 +0200
Subject: [PATCH] fix(eslint-plugin): [no-unnec-type-arg] undefined symbol
 crash (#1007)

---
 .../src/rules/no-unnecessary-type-arguments.ts             | 7 ++++++-
 .../tests/rules/no-unnecessary-type-arguments.test.ts      | 1 +
 2 files changed, 7 insertions(+), 1 deletion(-)

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 040650ea05d..12d9c041dfe 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-arguments.ts
@@ -121,7 +121,12 @@ function getTypeParametersFromType(
   type: ts.EntityName | ts.Expression | ts.ClassDeclaration,
   checker: ts.TypeChecker,
 ): readonly ts.TypeParameterDeclaration[] | undefined {
-  const sym = getAliasedSymbol(checker.getSymbolAtLocation(type)!, checker);
+  const symAtLocation = checker.getSymbolAtLocation(type);
+  if (symAtLocation === undefined) {
+    return undefined;
+  }
+
+  const sym = getAliasedSymbol(symAtLocation, checker);
   if (sym === undefined || sym.declarations === undefined) {
     return undefined;
   }
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 d6ea679f823..7bb37deda8f 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
@@ -52,6 +52,7 @@ ruleTester.run('no-unnecessary-type-arguments', rule, {
       class D<TD = number> extends C { }`,
     `declare const C: unknown;
       class D<TD = number> extends C { }`,
+    `let a: A<number>`,
   ],
   invalid: [
     {