Issue #16983 has been reported by pocke (Masataka Kuwabara).

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Bug #16983: RubyVM::AbstractSyntaxTree.of(method) returns meaningless node if the method is defined in eval
https://bugs.ruby-lang.org/issues/16983

* Author: pocke (Masataka Kuwabara)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-06-23T13:58:26Z master dc351ff984) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------

# Problem

`RubyVM::AST.of(method)` returns a meaningless node if the method is defined in eval.
For example:


```ruby
p 'blah'

eval <<~RUBY, binding, __FILE__, __LINE__ + 1
  def foo
  end
RUBY

method = method(:foo)
pp RubyVM::AbstractSyntaxTree.of(method)
# => (STR@3:5-3:12 "def foo\n" + "end\n")
```


I expect the node of `foo` method, or `nil`. But it returns a `STR` node.





It becomes a big problem when `AST.of` receives arbitrary methods.
Because we can't distinguish a method is defined in `eval` or not.
It means we can't believe the returned value of `AST.of` if the method may receive a method defined in `eval`.

For example:

```ruby
def do_something_for_each_method_ast(klass)
  klass.instance_methods(false).each do |m|
    ast = RubyVM::AbstractSyntaxTree.of(klass.instance_method(m))
    next unless ast

    do_something ast
  end
end

class A
  eval <<~RUBY, binding, __FILE__, __LINE__ + 1
    def foo
    end
  RUBY
end

do_something_for_each_method_ast A
```


In the example, I expect the `do_something` method receives only node for a method definition,
but it may pass a wrong node if any method is defined in `eval`.



# Cause (I guess)


I guess the cause is misleading node number.
In and out of an `eval` block uses different sequences of node number.
So if I specify `__FILE__` to `eval`, the actual file and code in `eval` may have the same node number.

For example


```ruby
p 'blah' # Node number for 'blah' is 1, file name is "test.rb"

eval <<~RUBY, binding, __FILE__, __LINE__ + 1
  def foo # Node number for `def` is also 1, file name is also "test.rb"
  end
RUBY

method = method(:foo)
# It finds a node from node number 1 by reading "test.rb", so it get the str node.
pp RubyVM::AbstractSyntaxTree.of(method)
# => (STR@3:5-3:12 "def foo\n" + "end\n")
```





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