Swift: Cannot convert value of type 'X' to expected argument type 'Y'
The value’s type doesn’t match what the function or property expects.
Exact message you may see
Cannot convert value of typeto expected argument typeCannot convert value of type 'String?' to expected argument type 'String'
Seen on:
iOS
Meaning
Common with optionals (String? vs String), numeric types (Int vs Double/CGFloat), closures with wrong signatures, and Binding vs value in SwiftUI.
Common causes
- Optional passed where a non-optional is needed
- Different numeric types without conversion
- Closure signature mismatch
- Passing a value instead of a Binding ($state)
⚡ Quick fix
- Unwrap optionals (if let/guard/??)
- Convert explicitly: Double(x), CGFloat(y)
- Use $ for bindings in SwiftUI
Detailed fix by platform
Swift
- swift
let width: CGFloat = CGFloat(columns) * 44 Toggle("Notifications", isOn: $settings.enabled)
How to diagnose
- Types — Actual vs expected
- Context — Optional/numeric/binding?
🔧 Still not fixed?
Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:
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Other ways to find it
Where this error sits
Origin: On your device. Errors that are often confused with or caused by this one (click to open):
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Last updated 14 Oct 2026