[ruby-core:114649] [Ruby master Bug#19857] Eval coverage is reset after each `eval`.

Issue #19857 has been updated by mame (Yusuke Endoh).


Thanks. In your case of `ERB.new`, the `test` function does not actually use its arguments. Consider a more realistic example:

```
def test(value = nil)
  template = ERB.new(File.read(path).gsub("VALUE") { value || "default" })
  template.result(binding)
end
```

In this case, if we ever pass a multi-line string to `test`, automatic merging of the coverage results would bring trouble. I think it is safer to take only the coverage of the first or last version of code when trying to compile with the same pathname, instead of automatically merging them.

----------------------------------------
Bug #19857: Eval coverage is reset after each `eval`.
https://bugs.ruby-lang.org/issues/19857#change-104478

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
----------------------------------------
It seems like `eval` based coverage is reset every time eval is invoked.

```ruby
#!/usr/bin/env ruby

require 'coverage'

def measure(flag)
  c = Class.new
  c.class_eval(<<~RUBY, "foo.rb", 1)
    def foo(flag)
      if flag
        puts "foo"
      else
        puts "bar"
      end
    end
  RUBY
  
  return c.new.foo(flag)
end

Coverage.start(lines: true, eval: true)

# Depending on the order of these two operations, different coverage is calculated, because the evaluation of the code is considered different, even if the content/path is the same.
measure(false)
measure(true)

p Coverage.result
```

Further investigation is required.



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