Slightly improved printing for convert errors

We often get MethodErrors involving `convert` between slightly
different types (e.g. where the programmer expected to have
constructed the correct type, but made a slight mistake).
This attempts to slightly improve the formatting of the
error message to make differences more obvious. Of course
significant work remains to make this error message more usable.

base/errorshow.jl

@@ -165,6 +165,49 @@ end
 
 typesof(args...) = Tuple{Any[ Core.Typeof(a) for a in args ]...}
 
+function print_with_compare(io::IO, @nospecialize(a::DataType), @nospecialize(b::DataType), color::Symbol)
+ if a.name === b.name
+ Base.show_type_name(io, a.name)
+ n = length(a.parameters)
+ print(io, '{')
+ for i = 1:n
+ if i > length(b.parameters)
+ printstyled(io, a.parameters[i], color=color)
+ else
+ print_with_compare(io::IO, a.parameters[i], b.parameters[i], color)
+ end
+ i < n && print(io, ',')
+ end
+ print(io, '}')
+ else
+ printstyled(io, a; color=color)
+ end
+end
+
+function print_with_compare(io::IO, @nospecialize(a), @nospecialize(b), color::Symbol)
+ if a === b
+ print(io, a)
+ else
+ printstyled(io, a; color=color)
+ end
+end
+
+function show_convert_error(io::IO, ex::MethodError, @nospecialize(arg_types_param))
+ # See #13033
+ T = striptype(ex.args[1])
+ if T === nothing
+ print(io, "First argument to `convert` must be a Type, got ", ex.args[1])
+ else
+ print_one_line = isa(T, DataType) && isa(arg_types_param[2], DataType) && T.name != arg_types_param[2].name
+ printstyled(io, "Cannot `convert` an object of type ")
+ print_one_line || printstyled(io, "\n ")
+ print_with_compare(io, arg_types_param[2], T, :light_green)
+ printstyled(io, " to an object of type ")
+ print_one_line || printstyled(io, "\n ")
+ print_with_compare(io, T, arg_types_param[2], :light_red)
+ end
+end
+
 function showerror(io::IO, ex::MethodError)
  # ex.args is a tuple type if it was thrown from `invoke` and is
  # a tuple of the arguments otherwise.
@@ -191,13 +234,7 @@ function showerror(io::IO, ex::MethodError)
  end
  if f == Base.convert && length(arg_types_param) == 2 && !is_arg_types
  f_is_function = true
- # See #13033
- T = striptype(ex.args[1])
- if T === nothing
- print(io, "First argument to `convert` must be a Type, got ", ex.args[1])
- else
- print(io, "Cannot `convert` an object of type ", arg_types_param[2], " to an object of type ", T)
- end
+ show_convert_error(io, ex, arg_types_param)
  elseif isempty(methods(f)) && isa(f, DataType) && f.abstract
  print(io, "no constructors have been defined for ", f)
  elseif isempty(methods(f)) && !isa(f, Function) && !isa(f, Type)