https://issues.dlang.org/show_bug.cgi?id=24671

          Issue ID: 24671
           Summary: Improve error message on override mismatch
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: qs.il.paperinik@gmail.com

When trying to override an inherited method, there are three kinds of error:
- No base class method with the given name exists.
- There are base class methods with that name, but all are final.
- There is at least one overridable base class method with that name, but the
signature is incompatible.

In the first case, try typo correction.

In the second case, tell the user there are no non-final methods available. Also try typo correction.

In the third case, when signatures are incompatible, print both the signatures
and point out the difference in a diff style manner. That is, in the derived
class’s method that tries to override, use red for stuff to be removed and
green for stuff that would have to be added.
If there is an overload in the base class that has exactly as many parameters
as the would-be overload, only do the diff for the overloads with the right
number of parameters; otherwise do the diff for all base class overloads with
the right name.

Example case:
```
Error: function `pure nothrow @nogc @safe int app.D.f(int delegate(int) pure
nothrow @nogc @safe body)` does not override any function, did you mean to
override `pure nothrow @nogc @safe int app.B.f(scope int delegate(int) pure
nothrow @nogc @safe)`?
```
Better:
```
Error: function `pure nothrow @nogc @safe int app.D.f(int delegate(int) pure
nothrow @nogc @safe body)` does not override any function, did you mean to
override `pure nothrow @nogc @safe int app.B.f(scope int delegate(int) pure
nothrow @nogc @safe)`?
Did you intend to override like this:
pure nothrow @nogc @safe int f(scope int delegate(int) pure nothrow @nogc
@safe)
                               ^^^^^ (addition)
```

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