From 5bd4ec0a21bfc0f1482f8824cd7f8a5a37a74334 Mon Sep 17 00:00:00 2001
From: ttozzi <devttozzi@gmail.com>
Date: Wed, 15 Oct 2025 03:07:46 +0900
Subject: [PATCH] Fix invalid rewrite in UseExplicitNilCheckInConditions by
 skipping when explicit optional type annotation is present

---
 Documentation/RuleDocumentation.md                 |  4 ++++
 .../Rules/UseExplicitNilCheckInConditions.swift    |  7 ++++++-
 .../UseExplicitNilCheckInConditionsTests.swift     | 14 ++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Documentation/RuleDocumentation.md b/Documentation/RuleDocumentation.md
index 510cc0704..4a0f0bf1f 100644
--- a/Documentation/RuleDocumentation.md
+++ b/Documentation/RuleDocumentation.md
@@ -490,6 +490,10 @@ than binding and immediately discarding the value.
 For example, `if let _ = someValue { ... }` is forbidden. Use `if someValue != nil { ... }`
 instead.
 
+Note: If the conditional binding carries an explicit type annotation (e.g. `if let _: S? = expr`),
+we skip the transformation. Such annotations can be necessary to drive generic type inference
+when a function mentions a type only in its return position.
+
 Lint: `let _ = expr` inside a condition list will yield a lint error.
 
 Format: `let _ = expr` inside a condition list will be replaced by `expr != nil`.
diff --git a/Sources/SwiftFormat/Rules/UseExplicitNilCheckInConditions.swift b/Sources/SwiftFormat/Rules/UseExplicitNilCheckInConditions.swift
index 6e7e3e6c0..5c2f6bdae 100644
--- a/Sources/SwiftFormat/Rules/UseExplicitNilCheckInConditions.swift
+++ b/Sources/SwiftFormat/Rules/UseExplicitNilCheckInConditions.swift
@@ -19,6 +19,10 @@ import SwiftSyntaxBuilder
 /// For example, `if let _ = someValue { ... }` is forbidden. Use `if someValue != nil { ... }`
 /// instead.
 ///
+/// Note: If the conditional binding carries an explicit type annotation (e.g. `if let _: S? = expr`),
+/// we skip the transformation. Such annotations can be necessary to drive generic type inference
+/// when a function mentions a type only in its return position.
+///
 /// Lint: `let _ = expr` inside a condition list will yield a lint error.
 ///
 /// Format: `let _ = expr` inside a condition list will be replaced by `expr != nil`.
@@ -29,7 +33,8 @@ public final class UseExplicitNilCheckInConditions: SyntaxFormatRule {
     case .optionalBinding(let optionalBindingCondition):
       guard
         let initializerClause = optionalBindingCondition.initializer,
-        isDiscardedAssignmentPattern(optionalBindingCondition.pattern)
+        isDiscardedAssignmentPattern(optionalBindingCondition.pattern),
+        optionalBindingCondition.typeAnnotation == nil
       else {
         return node
       }
diff --git a/Tests/SwiftFormatTests/Rules/UseExplicitNilCheckInConditionsTests.swift b/Tests/SwiftFormatTests/Rules/UseExplicitNilCheckInConditionsTests.swift
index d526df02b..28cca0205 100644
--- a/Tests/SwiftFormatTests/Rules/UseExplicitNilCheckInConditionsTests.swift
+++ b/Tests/SwiftFormatTests/Rules/UseExplicitNilCheckInConditionsTests.swift
@@ -160,4 +160,18 @@ final class UseExplicitNilCheckInConditionsTests: LintOrFormatRuleTestCase {
       ]
     )
   }
+
+  func testTypeAnnotations() {
+    assertFormatting(
+      UseExplicitNilCheckInConditions.self,
+      input: """
+        if let _: F = foo() {}
+        if let _: S? = foo() {}
+        """,
+      expected: """
+        if let _: F = foo() {}
+        if let _: S? = foo() {}
+        """
+    )
+  }
 }