[Tuple Specification] numerous additions needed for discards and deconstruction

[BillWagner]

The description of deconstruction is not complete:

1. The grammar production needs to express both initialization and assignment on the LHS of a deconstruction.
2. The RHS of a deconstruction could be any type that supports deconstruction (tuple or any type with a candidate accessible Deconstruct method.) That isn't specified in the grammar.
3. The scope and definite assignment state of the destination of a deconstruction must be specified.
4. Can deconstruction be a compound assignment? (Stated another way, what is the type of a deconstruction assignment expression?
5. The example that shows a Deconstruct method needs to be replaced by specification language for the rules of a Deconstruct method.

Deconstruction is mentioned in the same breath as assignment. If deconstruction can only occur on the LHS of a (simple?, compound?) assignment, should deconstruction be covered under assignment rather than in its own section? If the two are in separate sections, the expression grammar needs to have the new production, tuple_deconstruction_expression, plugged into it. Support for tuple deconstruction could be the beginning of a more general pattern support mechanism to come later. If that is the case, perhaps we should admit that and start using pattern-related terminology and organization. If so, that suggests tuple deconstruction should be separate from the assignment operator, in a (pre-)patterns section that will be expanded over future spec versions.

[michael-hawker]

Related to #1358?

I believe this is what would be needed for the is pattern to work, right?

i.e. I have this currently:

var info = e.Parameter as (IEnumerable<ToolkitSampleMetadata> Samples, IEnumerable<ToolkitFrontMatter> Docs, bool AreDocsFirst)?;

if (info is not null)
{

But would rather have:

if (e.Parameter is (IEnumerable<ToolkitSampleMetadata> Samples, IEnumerable<ToolkitFrontMatter> Docs, bool AreDocsFirst) info)
{

Instead today I get CS1061 and CS8129 errors about Deconstruct missing:

[BillWagner]

This has been added in the C# 7 standard