Be more specific on the the special case of condition expressions of if expressions

[ttzytt]

The grammar for if expressions is described as:

Notice that there is a special case for the condition expression "except struct expression." Not sure if I'm correct, but I think this meant to specify the cases that result in ambiguity, such as

if A{} {
}

Where A is a struct. In this case, I assume the parser/compiler cannot determine whether A itself is a variable and the following {} is the block, or alternatively, A{} as a whole is an expression.

However, there are other cases leading to compile errors even if the condition expression is not struct expression, below are two examples:

use std::ops;
struct A;
struct B;

impl ops::Add<B> for A{
    type Output = bool;
    fn add(self, rhs : B) -> bool{true}
}

impl A{
    fn to_bool(self) -> bool {true}
}

fn main(){
    if A{}.to_bool() {()} // ex1
    if A{} + B{} {()} //ex2
}

In ex1 a method call expression is shown, and in ex2 an operator expression is shown.

Adding paratheses around the two expressions will resolve the error.

I feel like this needs to be better explained in the reference other than "except struct expression," as the expression does not have to be struct expression to induce error.

If I understood the problem correctly, I'm willing to submit a PR for this using similar examples, please point me out if the original description is intentional.

[ehuss]

Thanks for the report! I agree that the current documentation is incomplete/lacking/wrong. This is currently tracked in #569, so closing as a duplicate of that.