You may have a broader scenario in mind, but my experience is limited to languages like Rust and C where the type is linked to an object the moment that object is declared for the first time, and can't be changed later. These languages do use the same syntax for definition and type annotation.

I will remember this conversation when I encounter a language that allows this (declaration in the first line, type annotations follow):

  var x
  x : Integer
  x = 5
  ...
  x : String
  x = "hello"