Warn when comparing Char and String

[Timeroot]

More than once I've made a mistake like,

for line = lines
 headchar = line[1]
 if headchar == "X"
   break
 end
 ...
end

... and then it never stops because it starts with 'X', not "X". It usually takes me a minute to realize why it's not working.

How feasible would it be to special-case a warning when checking a character and string for equality? Sort of similar to how

5 in Dict(5 => "yes", 3 => "no")

produces a special warning,

AbstractDict collections only contain Pairs;
Either look for e.g. A=>B instead, or use the `keys` or `values`
function if you are looking for a key or value respectively.

Stacktrace:
 [1] error(s::String)
   @ Base .\error.jl:33
 [2] in(p::Int64, a::Dict{Int64, String})
   @ Base .\abstractdict.jl:28

I'm not familiar enough with the dispatch performance, but I could imagine that adding this check in the wrong place could impact performance heavily (by incurring a check+dispatch on every single instance of == anywhere). If the performance impact is minimal though, it might be handy.

[Seelengrab]

Sadly a duplicate of #40717