don't warn on using Foo: _

NEWS.md

@@ -15,6 +15,7 @@ Language changes
 is considered a bug fix ([#47102])
 
   - The `hash` algorithm and its values have changed. Most `hash` specializations will remain correct and require no action. Types that reimplement the core hashing logic independently, such as some third-party string packages do, may require a migration to the new algorithm. ([#57509])
+  - `using Foo: _` for a module or package `Foo` no longer issues a warning. This syntax can be used to load `Foo` without bringing any of its names (even the name `Foo` itself) into your namespace.
 
 Compiler/Runtime improvements
 -----------------------------

src/module.c

@@ -1232,10 +1232,15 @@ JL_DLLEXPORT void jl_module_import(jl_task_t *ct, jl_module_t *to, jl_module_t *
     jl_walk_binding_inplace(&ownerb, &ownerbpart, ct->world_age);
 
     if (jl_bkind_is_some_guard(jl_binding_kind(ownerbpart))) {
-        jl_printf(JL_STDERR,
-                  "WARNING: Imported binding %s.%s was undeclared at import time during import to %s.\n",
-                  jl_symbol_name(from->name), jl_symbol_name(s),
-                  jl_symbol_name(to->name));
+        if (strcmp(jl_symbol_name(s), "_") == 0) {
+            // importing `_` loads the package but does not import a binding
+            return;
+        } else {
+            jl_printf(JL_STDERR,
+                    "WARNING: Imported binding %s.%s was undeclared at import time during import to %s.\n",
+                    jl_symbol_name(from->name), jl_symbol_name(s),
+                    jl_symbol_name(to->name));
+        }
     }
 
     jl_binding_t *bto = jl_get_module_binding(to, asname, 1);

test/syntax.jl

@@ -2752,6 +2752,19 @@ import .TestImportAs.Mod2 as M2
 @test !@isdefined(Mod2)
 @test M2 === TestImportAs.Mod2
 
+# `using Foo: _` is special cased to not warn so it can be used to load
+# a module only for its side effects. (#58667)
+side_effect_triggered = Ref(false)
+module Mod58667
+__init__() = (parentmodule(@__MODULE__).side_effect_triggered[] = true; nothing)
+end
+@test_nowarn using .Mod58667: _
+@test side_effect_triggered[] # module was loaded!
+# does not warn if you import again:
+@test_nowarn using .Mod58667: _
+# but importing a non-existent non-`_` binding does warn:
+@test_warn "WARNING: Imported binding Mod58667.y was undeclared at import time during import to Main." using .Mod58667: y
+
 # 57702: nearby bindings shouldn't cause us to closure-convert in import/using
 module OddImports
 using Test