=> should need to be explicitly imported to be extended

[LilithHafner]

julia> 1 => 2
1 => 2

julia> (a => b) = 1 => 2 # Failed attempt to unpack a pair
Error showing value of type UnionAll:
ERROR: 
SYSTEM (REPL): showing an error caused an error
ERROR: 
SYSTEM (REPL): caught exception of type StackOverflowError while trying to handle a nested exception; giving up

julia> ...inconsistent state leading to Julia crash...

Should behave like + and other builtins:

julia> 1 => 2
1 => 2

julia> (a => b) = 1 => 2 # Failed attempt to unpack a pair
ERROR: error in method definition: function Base.=> must be explicitly imported to be extended
Stacktrace:
 [1] top-level scope
   @ none:0
 [2] top-level scope
   @ REPL[2]:1

julia> ...all is well...

[LilithHafner]

See https://discourse.julialang.org/t/pair-iteration-specification/72794

[knuesel]

=> and + do get the same treatment, the "must be explicitely imported" error occurs only if the Base function was used previously in the Julia session.

(Edit: I misread the error, they only get the same treatment if the Base version wasn't used previously, otherwise there is indeed a bug with =>)

[KristofferC]

Dup of #25744