Disallow named arguments for methods of a type union where parameter names differ

[chalcolith]

Working on learning more about the compiler...

This fixes the bug noted in #394, where named parameters can be used with methods in a union type whose parameter names differ.

The change makes it a compile-time error to call a method on a union type using named arguments when the methods in the union types have differently-named parameters.

Was:
This fixes the bug noted in #394, where named parameters can be used with
methods in a union type whose parameter names differ.

The change makes it a compile-time error to call a method on a union type
(whether using named parameters or not) when the methods in the union types
have differently-named parameters.

[SeanTAllen]

our else block handling is wildly inconsistent throughout the codebase.

we should put together a c coding style

[jemc]

whether using named parameters or not

I think this is overly aggressive. Note that the rules discussed in the referenced ticket were:

• When calling on a union without passing arguments by name the names don't matter.
• When calling on a union passing any arguments by name ALL parameter names must match on all types in the union.

[chalcolith]

From what I can tell the function lookup happens before we know that positional or named parameters are in use. Is there a way to annotate the function type AST node so that when the function call is processed we can reject it if the names are different?

[chalcolith]

I have added a check for the use of named arguments before triggering a compiler error. I am not very confident given my limited knowledge of the compiler internals that this will catch all the necessary cases.