Fixes #40197

* Refactor tuple implementation

* Tweaks

* Address PR feedback

* Pass tuple data in constructors

* Address more feedback

* Tweak comparison

* Restore constructWithTypeParameters flag to avoid overflow

* Assert that tuple parameter matches TupleUnderlyingType somewhat

* Rename to TupleData. Address some IDE feedback

* Remaining TODO2's

* Fix last IDE test!

* Factor array comparison

* Tweak WithTupleData and EqualsIgnoringTupleUnderlyingType

* Expand a test

* Remove duplicate line

* Remove an EE test for badly formed ValueTuple type

* PR feedback from Neal

* Restore an IDE test without ValueTuple

* Rename constants

* Restore formatting

Fixes #39767

Fixes #39947

This change also separates implementation of ISymbol interfaces from internal symbols in C# compiler.

Fixes #30669

* x == default is valid object equality for reference types
* Block null == default

Before C# 8, the reduction of an extension method would fail (return null) when the
method's constraints would not be satisfied.  But in C# 8 the unmanaged constraint requires
we know the language version, and therefore the compilation, to check.  That is not available
in `IMethodSymbol.ReduceExtensionMethod`.  So we dropped the constraint checking in #37022.
However this causes an unfortunate regression in the quality of the IDE experience.  So here we
restore the constraint checking, using the compilation's language version when available.
IMethodSymbol.ReduceExtensionMethod uses the C# 8 rules since it has no language version to check.

Fixes #37780

Fixes #37187

Implements https://github.com/dotnet/csharplang/issues/2389
Fixes #35958

Note that the switch expression conversion is not a standard implicit conversion.  Specifically, it should not be considered as a valid implicit conversion as input to a user-defined conversion, as that could permit two chained user-defined conversions.

Also changes binding of switch expression to ECMA so it can interact properly with switch expressions.
Fixes #14965

* Add reproducer for #34872

* Fix bug and add test

* WIP

* Test similar scenario in VB

* Add void check in ClassifyConversionFromType, update tuple tests

* Cleanup

Implements a design change where overrides are allowed to change the
type of the member as long as there is an implicit nullable reference
conversion from the overriding type to the overridden type according to
the nullable variance rules.

Fixes #23268
Fixes #30958

Closes #28834.
Closes #30061.

* Permit stackalloc in nested contexts. (#28969)
Fixes #26759

Fixes #29956

Also introduce a helper `TypeSymbol.IsVoidType()`

Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported. (#35272)

 Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported.

Fixes #30229.
Fixes #35179.

Implements the following LDM decision:

For partial types, the invariant matching from type inference and merging. A mismatch
between two non-oblivious candidates produces an error. No warnings are produced.

For partial methods, nullability has to match with exception for oblivious and we produce warnings.
For the result, we use the implementation signature inside the implementation, and the
declaration signature for the callers.

Fixes #34233

Track l-values in conditional operator
Assignment to ref variable produces safety warnings

Fixes #30987