From 72c2e418a4180f6db5795ebb42cbc095f5c53c37 Mon Sep 17 00:00:00 2001
From: Taeheon Kim <skyrabbits1@gmail.com>
Date: Wed, 15 Dec 2021 16:01:01 +0900
Subject: [PATCH] fix(eslint-plugin): [strict-boolean-expression] false
 positive for truthy boolean (#4275)

---
 .../src/rules/strict-boolean-expressions.ts   | 25 ++++++++++++++-----
 .../rules/strict-boolean-expressions.test.ts  | 14 +++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts b/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts
index 88ec00846f3..bf8d6709840 100644
--- a/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts
+++ b/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts
@@ -233,7 +233,7 @@ export default util.createRule<Options, MessageId>({
         wantedTypes.every(type => types.has(type));
 
       // boolean
-      if (is('boolean')) {
+      if (is('boolean') || is('truthy boolean')) {
         // boolean is always okay
         return;
       }
@@ -251,6 +251,11 @@ export default util.createRule<Options, MessageId>({
         return;
       }
 
+      // Known edge case: boolean `true` and nullish values are always valid boolean expressions
+      if (is('nullish', 'truthy boolean')) {
+        return;
+      }
+
       // nullable boolean
       if (is('nullish', 'boolean')) {
         if (!options.allowNullableBoolean) {
@@ -708,6 +713,7 @@ export default util.createRule<Options, MessageId>({
     type VariantType =
       | 'nullish'
       | 'boolean'
+      | 'truthy boolean'
       | 'string'
       | 'truthy string'
       | 'number'
@@ -732,12 +738,19 @@ export default util.createRule<Options, MessageId>({
       ) {
         variantTypes.add('nullish');
       }
+      const booleans = types.filter(type =>
+        tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike),
+      );
 
-      if (
-        types.some(type =>
-          tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike),
-        )
-      ) {
+      // If incoming type is either "true" or "false", there will be one type
+      // object with intrinsicName set accordingly
+      // If incoming type is boolean, there will be two type objects with
+      // intrinsicName set "true" and "false" each because of tsutils.unionTypeParts()
+      if (booleans.length === 1) {
+        tsutils.isBooleanLiteralType(booleans[0], true)
+          ? variantTypes.add('truthy boolean')
+          : variantTypes.add('boolean');
+      } else if (booleans.length === 2) {
         variantTypes.add('boolean');
       }
 
diff --git a/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts b/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts
index c98102a1f17..335c2c71346 100644
--- a/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts
+++ b/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts
@@ -167,6 +167,20 @@ function f(arg: 1 | null) {
     `
 function f(arg: 1 | 2 | null) {
   if (arg) console.log(arg);
+}
+    `,
+    `
+interface Options {
+  readonly enableSomething?: true;
+}
+
+function f(opts: Options): void {
+  if (opts.enableSomething) console.log('Do something');
+}
+    `,
+    `
+declare const x: true | null;
+if (x) {
 }
     `,
     {