local variables of loops are retained in subsequent iterations

[heliosdrm]

(Detected here: https://discourse.julialang.org/t/running-tests-vs-debugging-gives-different-results/52554)

This function normally fails, because b is not defined in the second iteration of the loop.

function foo()
    a = 1
    for i = 1:2 
        if i == 1
            b = 10
        else
            a = b
        end
    end
    return a + 1
end

However, in debug mode (e.g. @run foo()) it does not fail, since b is defined when the offending line is to be run:

julia> @breakpoint foo() 7
foo() in Main at /home/meliana/prueba/test.jl:1:7

julia> @run foo()
Hit breakpoint:
In foo() at /home/meliana/prueba/test.jl:1
  3      for i = 1:2 
  4          if i == 1
  5              b = 10
  6          else
> 7              a = b
  8          end
  9      end
 10      return a + 1
 11  end

About to run: _J5
1|debug> fr
[1] foo() at /home/meliana/prueba/test.jl:1
  | a::Int64 = 1
  | ::Tuple{Int64,Int64} = (2, 2)
  | i::Int64 = 2
  | b::Int64 = 10

If I understand it right, this seems to be happening because in the first iteration b got into the running frame, and while the interpreter is inside the loop, it takes the values for its local variables (including b) from the frame.

On the other hand, that's not happening here:

function bar()
    let            
        a = 1
    end
    return a + 1
end

julia> @enter bar()
In bar() at /home/meliana/prueba/test.jl:1
 1  function bar()
 2      let            
 3          a = 1
 4      end
>5      return a + 1
 6  end

About to run: (+)(Main.a, 1)
1|debug> fr
[1] bar() at /home/meliana/prueba/test.jl:1
  | a::Int64 = 1
1|debug> c
ERROR: UndefVarError: a not defined
Stacktrace:
 [1] bar() at /home/meliana/prueba/test.jl:5

In this second example, the variable a also remains in the frame after exiting the let block, but since the last line is outside the part of the code that contains its scope, the interpreter does not look for the value of a in the frame anymore (thus the line "about to run" refers to Main.a).

[KristofferC]

Closing as dup of JuliaDebug/JuliaInterpreter.jl#444.