Warn if an already loaded package is attempted to be loaded from a di…

…fferent path (JuliaLang#44329)

base/loading.jl

@@ -1502,6 +1502,7 @@ function _tryrequire_from_serialized(modkey::PkgId, build_id::UInt128)
  assert_havelock(require_lock)
  loaded = nothing
  if root_module_exists(modkey)
+ warn_if_already_loaded_different(modkey)
  loaded = root_module(modkey)
  else
  loaded = start_loading(modkey)
@@ -1532,6 +1533,7 @@ function _tryrequire_from_serialized(modkey::PkgId, path::String, ocachepath::Un
  assert_havelock(require_lock)
  loaded = nothing
  if root_module_exists(modkey)
+ warn_if_already_loaded_different(modkey)
  loaded = root_module(modkey)
  else
  loaded = start_loading(modkey)
@@ -1975,11 +1977,50 @@ function __require_prelocked(uuidkey::PkgId, env=nothing)
  # After successfully loading, notify downstream consumers
  run_package_callbacks(uuidkey)
  else
+ warn_if_already_loaded_different(uuidkey)
  newm = root_module(uuidkey)
  end
  return newm
 end
 
+const already_warned_path_change_pkgs = Set{UUID}()
+# warns if the loaded version of a module is different to the one that locate_package wants to load
+function warn_if_already_loaded_different(uuidkey::PkgId)
+ uuidkey.uuid ∈ already_warned_path_change_pkgs && return
+ pkgorig = get(pkgorigins, uuidkey, nothing)
+ if pkgorig !== nothing && pkgorig.path !== nothing
+ new_path = locate_package(uuidkey)
+ if !samefile(fixup_stdlib_path(pkgorig.path), new_path)
+ if isnothing(pkgorig.version)
+ v = get_pkgversion_from_path(dirname(dirname(pkgorig.path)))
+ cur_vstr = isnothing(v) ? "" : "v$v "
+ else
+ cur_vstr = "v$v "
+ end
+ new_v = get_pkgversion_from_path(dirname(dirname(new_path)))
+ new_vstr = isnothing(new_v) ? "" : "v$new_v "
+ warnstr = """
+ $uuidkey is already loaded from a different path:
+ loaded: $cur_vstr$(repr(pkgorig.path))
+ requested: $new_vstr$(repr(new_path))
+ """
+ if isempty(already_warned_path_change_pkgs)
+ warnstr *= """
+ This might indicate a change of environment has happened between package loads and can mean that
+ incompatible packages have been loaded"""
+ if JLOptions().startupfile < 2 #(0 = auto, 1 = yes)
+ warnstr *= """. If this happened due to a startup.jl consider starting julia
+ directly in the project via the `--project` arg."""
+ else
+ warnstr *= "."
+ end
+ end
+ @warn warnstr
+ push!(already_warned_path_change_pkgs, uuidkey.uuid)
+ end
+ end
+end
+
 mutable struct PkgOrigin
  path::Union{String,Nothing}
  cachepath::Union{String,Nothing}