[ruby-core:98844] [Ruby master Bug#16967] Branch coverage duplicates branches inside ensure

Issue #16967 has been reported by jeremyevans0 (Jeremy Evans).

----------------------------------------
Bug #16967: Branch coverage duplicates branches inside ensure
https://bugs.ruby-lang.org/issues/16967

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-06-05T21:26:28Z master ca15b7b8ee) [x86_64-openbsd6.7]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
The following file, which should have perfect coverage, is reported as only have 50% of the branches covered:

```ruby
def a
  yield
ensure
  p 1
  p 2 if $!
end
a{}
a{raise} rescue nil
```

The branches reported by branch coverage are:

```ruby
{[:if, 0, 5, 2, 5, 11]=>
  {[:then, 1, 5, 2, 5, 5]=>1, [:else, 2, 5, 2, 5, 11]=>0},
 [:if, 3, 5, 2, 5, 11]=>
  {[:then, 4, 5, 2, 5, 5]=>0, [:else, 5, 5, 2, 5, 11]=>1}}
```

Instead of reporting 2 branches for the `if`, 4 branches are reported:

1. exception raised, if condition true
2. exception raised, if condition false
3. exception not raised, if condition true
4. exception not raised, if condition false

In this example, it is impossible to cover branches 2 and 3, because the if condition is only true if an exception is raised.

Note that ensure blocks by themselves are not considered branches.  This code results in no branches reported by branch coverage:

```ruby
def a
  yield
ensure
  p $!
end
a{}
a{raise} rescue nil
```

Nested ensure usage duplicates all branches.  This code with 3 nested ensures generates 16 branches:

```ruby
def a
  yield
ensure
  begin
  ensure
    begin
    ensure
      p 1
      p 2 if $!
    end
  end
end
a{}
a{raise} rescue nil
```

I think this is a bug in the coverage library, and that it should not duplicate branches inside ensure.  This issue is not theoretical, it affects branch coverage testing in my libraries.



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