Compiler implicitely imports symbols in module

[Pangoraw]

Initally reported in fonsp/Pluto.jl#1973. When executing a simple block with a for loop in the REPL, the compiler will use isdefined(Main,s) which will have a side effect of implicitely importing the variable in Main and making impossible to assign to it in the given expression.

julia> begin
           first = 1
           for _ in false
               first
           end
       end
ERROR: cannot assign a value to variable Base.first from module Main
Stacktrace:
 [1] top-level scope
   @ ./REPL[1]:2

This is where I suspect the compiler uses isdefined(Main, :first) and isconst(Main, :first) for this given expression (from a gdb stacktrace):

julia/base/compiler/abstractinterpretation.jl

Lines 2001 to 2002 in 98b4b06

	if isdefined(M,s)
	if isconst(M,s)

The issue is that isdefined(m, s) will resolve the binding and implicitely import it inside m as a side effect. The same is also true for isconst(m, s). Maybe their usage should not have this side-effect in this particular case ?

[simeonschaub]

This is a tricky one, because even if we didn't resolve the binding, we would have inlined the wrong first here (Base.first instead of first). Related: #43852 (comment)

[vchuravy]

We should probably still guard the isdefined with a Base.isbindingresolved?

[simeonschaub]

I thought so at first too, but that would mean something like f(x) = sin(x) would not be type stable unless you actually used sin in the same module before. The real solution would probably be to resolve all bindings right after lowering.