From 189162d46ecb116c420232937a7f86df913f4e79 Mon Sep 17 00:00:00 2001
From: Brad Zacher <brad.zacher@gmail.com>
Date: Sun, 13 Sep 2020 12:27:01 -0700
Subject: [PATCH] fix(eslint-plugin): [no-use-before-define] false positive for
 function type arguments (#2554)

Fixes #2527
---
 packages/eslint-plugin/src/rules/no-use-before-define.ts      | 3 ++-
 .../eslint-plugin/tests/rules/no-use-before-define.test.ts    | 4 ++++
 packages/experimental-utils/src/ts-eslint/Scope.ts            | 2 ++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/packages/eslint-plugin/src/rules/no-use-before-define.ts b/packages/eslint-plugin/src/rules/no-use-before-define.ts
index 1e26e4729f9..18525a138ce 100644
--- a/packages/eslint-plugin/src/rules/no-use-before-define.ts
+++ b/packages/eslint-plugin/src/rules/no-use-before-define.ts
@@ -260,7 +260,8 @@ export default util.createRule<Options, MessageIds>({
           variable.identifiers.length === 0 ||
           (variable.identifiers[0].range[1] <= reference.identifier.range[1] &&
             !isInInitializer(variable, reference)) ||
-          !isForbidden(variable, reference)
+          !isForbidden(variable, reference) ||
+          reference.from.type === TSESLint.Scope.ScopeType.functionType
         ) {
           return;
         }
diff --git a/packages/eslint-plugin/tests/rules/no-use-before-define.test.ts b/packages/eslint-plugin/tests/rules/no-use-before-define.test.ts
index 85dec218d11..ad66ff7858e 100644
--- a/packages/eslint-plugin/tests/rules/no-use-before-define.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-use-before-define.test.ts
@@ -343,6 +343,10 @@ const React = require('react');
         },
       },
     },
+    // https://github.com/typescript-eslint/typescript-eslint/issues/2527
+    `
+type T = (value: unknown) => value is Id;
+    `,
   ],
   invalid: [
     {
diff --git a/packages/experimental-utils/src/ts-eslint/Scope.ts b/packages/experimental-utils/src/ts-eslint/Scope.ts
index 2a7f707a429..2934a4d2756 100644
--- a/packages/experimental-utils/src/ts-eslint/Scope.ts
+++ b/packages/experimental-utils/src/ts-eslint/Scope.ts
@@ -46,9 +46,11 @@ namespace Scope {
   export type Reference = scopeManager.Reference;
   export type Variable = scopeManager.Variable | ESLintScopeVariable;
   export type Scope = scopeManager.Scope;
+  export const ScopeType = scopeManager.ScopeType;
   // TODO - in the next major, clean this up with a breaking change
   export type DefinitionType = scopeManager.Definition;
   export type Definition = scopeManager.Definition;
+  export const DefinitionType = scopeManager.DefinitionType;
 }
 
 export { Scope };