Operands of "non-regular" versions of arithmetic ops don't infer whereas regular ops operands do

[DustinByfuglien]

For programmer point of view there is no visible reason that operands of "non-regular" versions of arithmetic operators (saturating, overflowing, ...) can't be inferred in some situations when "regular" do it.

fn main() {
    let mut sum = 0;
    sum = sum + 1_usize;                // Ok
    let mut sum = 0;
    sum = sum.saturating_add(1_usize);  // Error
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0689]: can't call method `saturating_add` on ambiguous numeric type `{integer}`
 --> src/main.rs:5:15
  |
5 |     sum = sum.saturating_add(1_usize);  // Error
  |               ^^^^^^^^^^^^^^
  |
help: you must specify a type for this binding, like `i32`
  |
4 |     let mut sum: i32 = 0;
  |         ^^^^^^^^^^^^

It could be nice if saturating version of sum can be inferred to usize too.

[bluss]

Duplicate of #24124