Warn if imported pkgs are updated. Fix #18239 (#18248)

Add test for #18248.

Remove redundant loop.

Refactor test.

Wrap line.

base/pkg/entry.jl

@@ -523,6 +523,7 @@ function resolve(
 
     # try applying changes, roll back everything if anything fails
     changed = []
+    imported = String[]
     try
         for (pkg,(ver1,ver2)) in changes
             if ver1 === nothing
@@ -535,6 +536,10 @@ function resolve(
                 up = ver1 <= ver2 ? "Up" : "Down"
                 info("$(up)grading $pkg: v$ver1 => v$ver2")
                 Write.update(pkg, Read.sha1(pkg,ver2))
+                pkgsym = Symbol(pkg)
+                if isdefined(Main, pkgsym) && isa(getfield(Main, pkgsym), Module)
+                    push!(imported, "- $pkg")
+                end
             end
             push!(changed,(pkg,(ver1,ver2)))
         end
@@ -553,6 +558,10 @@ function resolve(
         end
         rethrow(err)
     end
+    if !isempty(imported)
+        warn(join(["The following packages have been updated but were already imported:",
+            imported..., "Restart Julia to use the updated versions."], "\n"))
+    end
     # re/build all updated/installed packages
     build(map(x->x[1], filter(x -> x[2][2] !== nothing, changes)))
 end

test/pkg.jl

@@ -541,4 +541,23 @@ temp_pkg_dir() do
         @test ret === nothing && out == ""
         @test contains(err, nothingtodomsg)
     end
+
+    # issue #18239
+    let package = "Example"
+        Pkg.free(package)
+        Pkg.rm(package)  # Remove package if installed
+
+        metadata_dir = Pkg.dir("METADATA")
+        const old_commit = "83ff7116e51fc9cdbd7e67affbd344b9f5c9dbf2"
+
+        # Reset METADATA to the second to last update of Example.jl
+        LibGit2.with(LibGit2.GitRepo, metadata_dir) do repo
+            LibGit2.reset!(repo, LibGit2.Oid(old_commit), LibGit2.Consts.RESET_HARD)
+        end
+
+        Pkg.add(package)
+        msg = readstring(ignorestatus(`$(Base.julia_cmd()) --startup-file=no -e
+            "redirect_stderr(STDOUT); using Example; Pkg.update(\"$package\")"`))
+        @test contains(msg, "- $package\nRestart Julia to use the updated versions.")
+    end
 end