diff --git a/libcxx/include/__string b/libcxx/include/__string
--- a/libcxx/include/__string
+++ b/libcxx/include/__string
@@ -153,7 +153,6 @@
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::reserve(size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(basic_string const&, size_type, size_type)) \
@@ -165,6 +164,8 @@
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by(size_type, size_type, size_type, size_type, size_type, size_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by_and_replace(size_type, size_type, size_type, size_type, size_type, size_type, value_type const*)) \
+  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_maybe_alias(value_type const*, size_type)) \
+  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_maybe_alias(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_no_alias<false>(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_no_alias<true>(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::push_back(value_type)) \
@@ -177,7 +178,6 @@
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1571,12 +1571,34 @@
                                size_type __n_copy,  size_type __n_del,
                                size_type __n_add, const value_type* __p_new_stuff);
 
+    // `__assign_inline` contains the implementation called by the externally
+    // instantiated __assign_no_alias() and__assign_maybe_alias() methods.
+    template <bool __is_short, bool __maybe_aliased>
+    inline void __assign_inline(const value_type* __s, size_type __n);
+
+    // `__assign_in_place` directly assigns the given input string
+    // into the current instance where the caller has verified both
+    // that the it fits capacity, as well as that there is no aliasing.
+    inline void __assign_in_place(const value_type* __s, size_type __n) {
+      pointer __p = __is_long() ? __get_long_pointer() : __get_short_pointer();
+      __is_long() ? __set_long_size(__n) : __set_short_size(__n);
+      traits_type::copy(_VSTD::__to_address(__p), __s, __n);
+      traits_type::assign(__p[__n], value_type());
+    }
+
     // __assign_no_alias is invoked for assignment operations where we
     // have proof that the input does not alias the current instance.
     // For example, operator=(basic_string) performs a 'self' check.
     template <bool __is_short>
     void __assign_no_alias(const value_type* __s, size_type __n);
 
+    // `__assign_maybe_alias` is invoked for assignment operations where
+    // we lack proof that the input does not alias the current instance.
+    // For example, assign(const char *s) may be invoked with the 's'
+    // value referring to the contents in 'this' string instance.
+    void __assign_maybe_alias(const value_type* __s);
+    void __assign_maybe_alias(const value_type* __s, size_type __n);
+
     _LIBCPP_INLINE_VISIBILITY
     void __erase_to_end(size_type __pos);
 
@@ -2209,14 +2231,18 @@
 // assign
 
 template <class _CharT, class _Traits, class _Allocator>
-template <bool __is_short>
-void basic_string<_CharT, _Traits, _Allocator>::__assign_no_alias(
+template <bool __is_short, bool __maybe_aliased>
+void basic_string<_CharT, _Traits, _Allocator>::__assign_inline(
     const value_type* __s, size_type __n) {
   size_type __cap = __is_short ? __min_cap : __get_long_cap();
   if (__n < __cap) {
     pointer __p = __is_short ? __get_short_pointer() : __get_long_pointer();
     __is_short ? __set_short_size(__n) : __set_long_size(__n);
-    traits_type::copy(_VSTD::__to_address(__p), __s, __n);
+    if (__maybe_aliased) {
+      traits_type::move(_VSTD::__to_address(__p), __s, __n);
+    } else {
+      traits_type::copy(_VSTD::__to_address(__p), __s, __n);
+    }
     traits_type::assign(__p[__n], value_type());
     __invalidate_iterators_past(__n);
   } else {
@@ -2225,26 +2251,43 @@
   }
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+template <bool __is_short>
+void basic_string<_CharT, _Traits, _Allocator>::__assign_no_alias(
+    const value_type* __s, size_type __n) {
+  __assign_inline<__is_short, false>(__s, __n);
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+void basic_string<_CharT, _Traits, _Allocator>::__assign_maybe_alias(
+    const value_type* __s, size_type __n) {
+  __is_long() ? __assign_inline<false, true>(__s, __n)
+              : __assign_inline<true, true>(__s, __n);
+}
+
+template <class _CharT, class _Traits, class _Allocator>
+void basic_string<_CharT, _Traits, _Allocator>::__assign_maybe_alias(
+    const value_type* __s) {
+  size_type __n = traits_type::length(__s);
+  __is_long() ? __assign_inline<false, true>(__s, __n)
+              : __assign_inline<true, true>(__s, __n);
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
-basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_type __n)
-{
-    _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
-    size_type __cap = capacity();
-    if (__cap >= __n)
-    {
-        value_type* __p = _VSTD::__to_address(__get_pointer());
-        traits_type::move(__p, __s, __n);
-        traits_type::assign(__p[__n], value_type());
-        __set_size(__n);
-        __invalidate_iterators_past(__n);
-    }
-    else
-    {
-        size_type __sz = size();
-        __grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
+basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s,
+                                                  size_type __n) {
+  _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
+  if (_LIBCPP_BUILTIN_CONSTANT_P(__n)) {
+    if (__n < __min_cap) {
+      __assign_in_place(__s, __n);
+    } else {
+      __assign_maybe_alias(__s, __n);
     }
-    return *this;
+  } else {
+    __assign_maybe_alias(__s, __n);
+  }
+  return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
@@ -2428,13 +2471,22 @@
     return assign(__sv.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
-
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s)
 {
-    _LIBCPP_ASSERT(__s != nullptr, "string::assign received nullptr");
-    return assign(__s, traits_type::length(__s));
+  _LIBCPP_ASSERT(__s != nullptr, "string::assign received nullptr");
+  if (_LIBCPP_BUILTIN_CONSTANT_P(__s[0])) {
+    const size_type __n = traits_type::length(__s);
+    if (__n < __min_cap) {
+      __assign_in_place(__s, __n);
+    } else {
+      __assign_maybe_alias(__s, __n);
+    }
+  } else {
+    __assign_maybe_alias(__s);
+  }
+  return *this;
 }
 
 // append