I though SE-0335: Introduce existential any is in there, but it's not.
What is the benefit of SE-0315 Type placeholders (formerly, "Placeholder types")?
The example:
// This is OK--the compiler can infer the key type as `Int`.
let dict: [_: String] = [0: "zero", 1: "one", 2: "two"]
The compiler can just infer both the key and value type:
// why not just let the compiler infer the whole thing?
let dict = [0: "zero", 1: "one", 2: "two"]