Better dynamic support

[bbarry]

Original: dotnet/roslyn/#3012

The generic type syntax could be used with dynamic as dynamic<T> to give tooling support about members and operators related to the usage of the variable.

This tooling could include:

1. member lookup on the instance
2. warnings/errors where it can be known that a member doesn't exist or an operation is invalid

[TonyValenti]

I've long thought that "dynamic" should be a variable modifier that would do something similar to what you indicated.

For example:

dynamic foo = SomeMethod(...); ///We can do this today
dynamic Person foo = CreatePerson(...); // This should let me Dot-access person members and DLR-access non-person members.
dynamic var foo = CreateSuperMan(...); // This should allow me to dot-access any publicly accessible members from the SuperMan class and DLR-access other members.

[bbarry]

As a related thought: dotnet/roslyn/#5306 is also looking for some compile time enforcement of dynamic types.

[TonyValenti]

Hi All,
I was also thinking about this in reference to #228 (Intersection Types).

Perhaps we should just allow listing multiple types to create intersection types. For example, here are a bunch of declarations:

var Item = ...
dynamic var Item = ...
IStudent ITeacher = ...
dynamic IStudent = ...
dynamic IStudent ITeacher IEmployable IActionable = ...

[Thaina]

@TonyValenti There are proposal about union and intersect type #344 #399 which is the same as typescript approach

Also I prefer dynamic<> syntax I have seen in #969

using CompoundType = (TU1 | TU2) & TI1;

dynamic<CompoundType> obj = ...;