Please add design doc explaining constraint checking of well-known generics

[gafter]

Associated with #24322, the following new guidelines should be documented in https://github.com/dotnet/roslyn/tree/master/docs/compilers/Design

From: @cston
Sent: Friday, January 19, 2018 2:32 PM
Subject: RE: Lookup of custom AsyncMethodBuilder members should check type parameter constraints

After several hallway discussions, we’re converging on the following approach:

For well-known generic types and methods introduced in the compiler for C#7.0 or later, the compiler will check any and all constraints when substituting type arguments at each use-site. The compiler will not require any specific constraints for well-known types or methods though. There will be no change for well-known types and methods added to the compiler before C#7.0 – that is, the compiler will continue to ignore any constraints at the use-site for those types and methods.

Generic types and methods from C#7.0 or later:
Generic types:

1. System.ValueTuple<T1, …>
2. Custom Task method builder
3. Span
4. ReadOnlySpan

Generic methods:
5. Custom Task and Task method builders:
a. Start(ref TStateMachine)
b. AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter, ref TStateMachine)
c. AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter, ref TStateMachine)

For 15.6, I’ll look at checking constraints for #5 only since we have an existing bug (#21500). For the other cases, the generic types, I’ll open 15.7 bugs to check constraints. And for any well-known types or members added to future releases of the compiler, constraint checking should be added at the time the well-known member is added to the compiler.