relates to:

• SR-1687 Swift does not warn about shadowing
• SR-8524 Shadowing in protocols leads to difficult to find bugs

Issue Description:

When pattern matching, it's common to bind a variable or constant. It's uncommon but legal to use a bound value as an argument. Consider the following enumeration and values:

// An enum with one, two, or three associated values
enum Value<T> { case one(T), two(T, T), three(T, T, T) }

// An example with two associated values
let example2: Value<Character> = .two("a", "b")

// A bound symbol
let oldValue = "x"

This code's goal is to conditionally bind newValue and pattern match the value stored in the oldValue symbol. The first example succeeds. The second example compiles and runs but does not match the coder's intent. Using an external let creates a new oldValue shadow instead of pattern matching oldValue's stored value.

// Safe
if case .two(let newValue, oldValue) = example2 { 
    ... 
}

// Syntactically legal but incorrect
if case let .two(newValue, oldValue) = example2 { 
    ... 
}

The compiler should detect same-name shadowing of an already-bound variable during pattern matching and emit a compile-time warning.

References:

• Draft Discussion https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170227/033308.html

• Redirection to bug report https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170227/033369.html