* 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

e.g. `ReturnType`, `ElementType`, etc.

- Rename `TypeSymbolWithAnnotations` to `TypeWithAnnotations`
- Rename its type field from `TypeSymbol` to `Type`
- Fields of Symbols that are of type `TypeWithAnnotations` that are currently named `Type` would be renamed `TypeWithAnnotations`
Fixes #33736

* Eliminate unspeakable NullableAnnotation values.
* Dynamic invocations result in an oblivious state
Fixes #33638
* Fix up expected diagnostics
Due to #33924
* Rename NullableAnnotation.Unknown to NullableAnnotation.Oblivious

* Add reproducer for #33388

* Skip normalizing task types if binding attribute arguments

* Use InAttributeArgument property

* Add test where task-like normalization affects overload resolution

* Update AttributeArgument_TaskLikeOverloadResolution test case

* Add assert and remove apparently unused branch

* Add back comparison of remaining parameter types with InAttributeArgument guard

* Update comment on remaining parameter comparisons

* Add comment describing why we don't normalize task types in attribute arguments

The NullableWalker is revised so that the inferred state of an expression is either `NotNull` or `MaybeNull` (represented by the new type `NullableFlowState`).  There is no longer such as thing as an oblivious rvalue.  There are four kinds of lvalues:
- oblivious lvalues read as NotNull but null can be written to them
- annotated lvalues read as MaybeNull and null can be written to them
- unannotated lvalues read as NotNull and null may not be written to them
- unconstrained type parameters read as MaybeNull but null may not be written to them

In order to preserve the safety in the face of such unconstrained type parameters, we warn immediately when a null value of such a type is introduced.  This is a safety warning.  The contexts in which such a warning are given are
- `default` and `default(T)`
- `null` conversion to `T` (when `T` is known to be ref type)
- `e?.M()` when the return type is `T`
- dynamic conversion or cast to `T` when the dynamic might be null
- explicit conversion to `T`
- `e as T` when there is not an implicit conversion from the type of `e` to `T`
- a call to a method like `FirstOrDefault()` that is annotated with `[MaybeNull] (not yet implemented)

We remove the hidden diagnostics previously computed by the NullableWalker.

We add debugger display support for a number of types used by the NullableWalker.  The display for `NullableWalker` summarizes the computed nullability of variables in a nice compact form.

We remove support for definite assignment in the NullableWalker.  Some scenarios involving the use of not-definitely-assigned variables may cause cascaded diagnostics.  We might remove them in the future by initializing all vaiables to the `NotNull` state when they enter scope.

We overhaul and simplify the inplementation of conversions, the null-conditional operator, and result inference for lifted operators.

We simplify the Meet and Join operations so that they form a lattice on both the NullableAnnotation and NullableFlowState.

Catch variables are now initialized to a non-null state on entry to the catch block.

In unreachable code, every expression produces a non-null rvalue.  An erroneous expression produces a non-null rvalue (to suppress cascaded diagnostics).

Most nullability checking of tuple values should occur during the
nullable walking phase. This is because the test relies on the inference
of the values, not the explicit types in the code. The only time we need
to validate tuple nullability constarints during initial binding is when
we're binding a named type.

The work for generic `unmanaged struct` constraints ended up threading
through the correct `Compilation` object in many of the cases where I
was using `Compilation` to calculate whether or not nullability needed
to be checked for constraints. Used that whenever possible.

This makes nullability an explicit parameter to constraint checking
rather than inferring it from the `ConversionsBase` argument. The reason
is that these really represent separate operations: checking nullability
in conversions and validating nullability in constraint checking.

closes #32953

Fixed a bug in constraint checking where the wrong Compilation
would be used.

* Add ITypeSymbol.IsUnmanagedType and ITypeSymbol.IsRefLikeType

Fixes #30427 and #30426

Pattern-based 'await foreach' should find parameterless 'GetAsyncEnumerator' and 'MoveNextAsync' (#32184)

Closes #30171.
Closes #29838.