whole struct value assignment can fail

[francescoalemanno]

const Pair = struct {
    x: i64,
    y: i64,
    const Self = @This();
    pub fn swap(self: Self) Self {
        return Self{ .x = self.y, .y = self.x };
    }
};

pub fn main() !void {
    var p = Pair{ .x = 1, .y = 10 };
    p = p.swap();
    print("{}", .{p});
}

expected result:

Pair{ .x = 10, .y = 1 }

actual result:

Pair{ .x = 10, .y = 10 }

on zig 0.9.0-dev.927+eb5e4ac49

[travisstaloch]

duplicate of #4021