Apply a conservative approach by filtering out results for accesses to
captured nullable values, when there is an (implicit) call to the capturing
callable which is `null`-guarded. For example:
```
bool M(int? i, IEnumerable<int> @is)
{
if (i.HasValue)
return @is.Any(j => j == i.Value); // GOOD
return false;
}
```
Write accesses in assignments, such as the access to `x` in `x = 0` are not
evaluated, so they should not have entries in the control flow graph. However,
qualifiers (and indexer arguments) should still be evaluated, for example in
```
x.Foo.Bar = 0;
```
the CFG should be `x --> x.Foo --> 0 --> x.Foo.Bar = 0` (as opposed to
`x --> x.Foo --> x.Foo.Bar --> 0 --> x.Foo.Bar = 0`, prior to this change).
A special case is assignments via acessors (properties, indexers, and event
adders), where we do want to include the access in the control flow graph,
as it represents the accessor call:
```
x.Prop = 0;
```
But instead of `x --> x.set_Prop --> 0 --> x.Prop = 0` the CFG should be
`x --> 0 --> x.set_Prop --> x.Prop = 0`, as the setter is called *after* the
assigned value has been evaluated.
An even more special case is tuple assignments via accessors:
```
(x.Prop1, y.Prop2) = (0, 1);
```
Here the CFG should be
`x --> y --> 0 --> 1 --> x.set_Prop1 --> y.set_Prop2 --> (x.Prop1, y.Prop2) = (0, 1)`.
The recent change to `AccessorCall` on dd99525566 resulted
in some bad join-orders, so I have (partly) reverted them. This means that the issues
orignally addressed by that change are now reintroduced, and I plan to instead apply a
fix to the CFG, which--unlike the original fix--should be able to handle multi-property-tuple
assignments.
The syntactic node assiociated with accessor calls was previously always the
underlying member access. For example, in
```
x.Prop = y.Prop;
```
the implicit call to `x.set_Prop()` was at the syntactic node `x.Prop`, while the
implicit call to `y.get_Prop()` was at the syntactic node `y.Prop`.
However, this breaks the invariant that arguments to calls dominate the call itself,
as the argument `y.Prop` for the implicit `value` parameter in `x.set_Prop()` will
be evaluated after the call (the left-hand side in an assignment is evaluated before
the right-hand side).
The solution is to redefine the access call to `x.set_Prop()` to point to the whole
assignment `x.Prop = y.Prop`, instead of the access `x.Prop`. For reads, we still want
to associate the accessor call with the member access.
A corner case arises when multiple setters are called in a tuple assignment:
```
(x.Prop1, x.Prop2) = (0, 1)
```
In this case, we cannot associate the assignment with both `x.set_Prop1()` and
`x.set_Prop2()`, so we instead revert to using the underlying member accesses as
before.
For example, in
```
void M(object x)
{
var y = x == null ? 1 : 2;
if (y == 2)
x.ToString();
}
```
the guard `y == 2` implies that the guard `x == null` must be false,
as the assignment of `2` to `y` is unique.
