Fix invalid rewrite in UseExplicitNilCheckInConditions by skipping when explicit optional type annotation is present

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`.

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
  }

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() {}
+ """
+ )
+ }
 }