Index: docs/DesignDocs/VisibilityMacros.rst
===================================================================
--- docs/DesignDocs/VisibilityMacros.rst
+++ docs/DesignDocs/VisibilityMacros.rst
@@ -110,6 +110,35 @@
   the extern template declaration) as exported on Windows, as discussed above.
   On all other platforms, this macro has an empty definition.
 
+**_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS**
+  Mark a symbol as hidden so it will not be exported from shared libraries. This
+  is intended specifically for method templates of either classes marked with
+  `_LIBCPP_TYPE_VIS` or classes with an extern template instantiation
+  declaration marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS`.
+
+  When building libc++ with hidden visibility, we want explicit template
+  instantiations to export members, which is consistent with existing Windows
+  behavior. We also want classes annotated with `_LIBCPP_TYPE_VIS` to export
+  their members, which is again consistent with existing Windows behavior.
+  Both these changes are necessary for clients to be able to link against a
+  libc++ DSO built with hidden visibility without encountering missing symbols.
+
+  An unfortunate side effect, however, is that method templates of classes
+  either marked `_LIBCPP_TYPE_VIS` or with extern template instantiation
+  declarations marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` also get default
+  visibility when instantiated. These methods are often implicitly instantiated
+  inside other libraries which use the libc++ headers, and will therefore end up
+  being exported from those libraries, since those implicit instantiations will
+  receive default visibility. This is not acceptable for libraries that wish to
+  control their visibility, and led to PR30642.
+
+  Consequently, all such problematic method templates are explicitly marked
+  either hidden (via this macro) or inline, so that they don't leak into client
+  libraries. The problematic methods were found by running
+  `bad-visibility-finder <https://github.com/smeenai/bad-visibility-finder>`_
+  against the libc++ headers after making `_LIBCPP_TYPE_VIS` and
+  `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility.
+
 **_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY**
   Mark a member function of a class template as visible and always inline. This
   macro should only be applied to member functions of class templates that are
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -591,6 +591,7 @@
 #define _LIBCPP_EXTERN_VIS          _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
 #define _LIBCPP_HIDDEN
+#define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
 #define _LIBCPP_TEMPLATE_VIS
 #define _LIBCPP_FUNC_VIS_ONLY
 #define _LIBCPP_ENUM_VIS
@@ -614,6 +615,8 @@
 #endif
 #endif
 
+#define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS _LIBCPP_HIDDEN
+
 #ifndef _LIBCPP_FUNC_VIS
 #if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #define _LIBCPP_FUNC_VIS __attribute__ ((__visibility__("default")))
@@ -668,7 +671,7 @@
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) && __has_attribute(__type_visibility__)
-#    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __attribute__ ((__type_visibility__("default")))
+#    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __attribute__ ((__visibility__("default")))
 #  else
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  endif
Index: include/locale
===================================================================
--- include/locale
+++ include/locale
@@ -623,16 +623,19 @@
     ~num_get() {}
 
     template <class _Fp>
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     iter_type __do_get_floating_point
                             (iter_type __b, iter_type __e, ios_base& __iob,
                              ios_base::iostate& __err, _Fp& __v) const;
 
     template <class _Signed>
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     iter_type __do_get_signed
                             (iter_type __b, iter_type __e, ios_base& __iob,
                              ios_base::iostate& __err, _Signed& __v) const;
 
     template <class _Unsigned>
+    _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
     iter_type __do_get_unsigned
                             (iter_type __b, iter_type __e, ios_base& __iob,
                              ios_base::iostate& __err, _Unsigned& __v) const;
Index: include/string
===================================================================
--- include/string
+++ include/string
@@ -792,6 +792,7 @@
     basic_string(const basic_string& __str, size_type __pos,
                  const _Allocator& __a = _Allocator());
     template<class _Tp>
+        _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
         basic_string(const _Tp& __t, size_type __pos, size_type __n,
                      const allocator_type& __a = allocator_type(),
                      typename enable_if<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value, void>::type* = 0);