Unclear error when return value ignored inside a conditional

[raulgrell]

The statement

if(input.keyDown[W]) GAME_DATA.position.offset(vec2(0, -deltaTime));

fails with

error: incompatible types: '&Vec2T(f32)' and 'void'
    if(input.keyDown[W]) GAME_DATA.position.offset(vec2(0, -deltaTime));
    ^
note: type '&Vec2T(f32)' here
    if(input.keyDown[W]) GAME_DATA.position.offset(vec2(0, -deltaTime));
                                                                 ^
note: type 'void' here
    if(input.keyDown[W]) GAME_DATA.position.offset(vec2(0, -deltaTime));
    ^

Where

pub fn offset(self: &Self, other: &const Self) -> &Self {
    self.data[0] += other.data[0];
    self.data[1] += other.data[1];
    return self;
}

Fixed by

// Ignoring the return value
if(input.keyDown[W]) _ = GAME_DATA.position.offset(vec2(0, -deltaTime));

or

// Wrapping in braces , which reveals the return value ignored error.     
if(input.keyPressed[W) { GAME_DATA.position.offset(vec2(0, -deltaTime)); }

or

// Redefining offset to not return the reference to self
pub fn offset(self: &Self, other: &const Self) {
    self.data[0] += other.data[0];
    self.data[1] += other.data[1];
}

The behaviour is correct, I just think the error is unergonomic.