diff --git a/libcxx/include/__format/format_context.h b/libcxx/include/__format/format_context.h
--- a/libcxx/include/__format/format_context.h
+++ b/libcxx/include/__format/format_context.h
@@ -144,6 +144,14 @@
 #endif
 };
 
+template <typename _OutIt, typename _CharT>
+basic_format_context(_OutIt,
+                     basic_format_args<basic_format_context<_OutIt, _CharT>>,
+                     optional<_VSTD::locale>&& __loc = nullopt) -> basic_format_context<_OutIt, _CharT>;
+template <typename _OutIt, typename _CharT>
+basic_format_context(_OutIt, basic_format_args<basic_format_context<_OutIt, _CharT>>)
+    -> basic_format_context<_OutIt, _CharT>;
+
 #endif //_LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__format/format_parse_context.h b/libcxx/include/__format/format_parse_context.h
--- a/libcxx/include/__format/format_parse_context.h
+++ b/libcxx/include/__format/format_parse_context.h
@@ -89,6 +89,9 @@
   size_t __num_args_;
 };
 
+template <typename _CharT>
+basic_format_parse_context(basic_string_view<_CharT>, size_t) -> basic_format_parse_context<_CharT>;
+
 using format_parse_context = basic_format_parse_context<char>;
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 using wformat_parse_context = basic_format_parse_context<wchar_t>;
diff --git a/libcxx/include/__format/format_string.h b/libcxx/include/__format/format_string.h
--- a/libcxx/include/__format/format_string.h
+++ b/libcxx/include/__format/format_string.h
@@ -32,6 +32,9 @@
   uint32_t __value;
 };
 
+template <typename _CharT>
+__parse_number_result(const __parse_number_result<_CharT>&) -> __parse_number_result<_CharT>;
+
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI constexpr __parse_number_result<_CharT>
 __parse_number(const _CharT* __begin, const _CharT* __end);
diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -699,6 +699,9 @@
   const _CharT* __last_;
 };
 
+template <typename _CharT>
+__column_width_result(__column_width_result<_CharT> const&) -> __column_width_result<_CharT>;
+
 /// Since a column width can be two it's possible that the requested column
 /// width can't be achieved. Depending on the intended usage the policy can be
 /// selected.
diff --git a/libcxx/include/__functional/operations.h b/libcxx/include/__functional/operations.h
--- a/libcxx/include/__functional/operations.h
+++ b/libcxx/include/__functional/operations.h
@@ -372,6 +372,8 @@
         {return __x < __y;}
 };
 
+less()->less<void>;
+
 #if _LIBCPP_STD_VER > 11
 template <>
 struct _LIBCPP_TEMPLATE_VIS less<void>
diff --git a/libcxx/include/__iterator/back_insert_iterator.h b/libcxx/include/__iterator/back_insert_iterator.h
--- a/libcxx/include/__iterator/back_insert_iterator.h
+++ b/libcxx/include/__iterator/back_insert_iterator.h
@@ -59,6 +59,9 @@
     _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 _Container* __get_container() const { return container; }
 };
 
+template <typename _Container>
+back_insert_iterator(_Container&) -> back_insert_iterator<_Container>;
+
 template <class _Container>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
 back_insert_iterator<_Container>
diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -1409,6 +1409,9 @@
   Cleanup cleanup_;
 };
 
+template <typename Cleanup>
+scope_exit(Cleanup const&) -> scope_exit<Cleanup>;
+
 uintmax_t remove_all_impl(int parent_directory, const path& p, error_code& ec) {
   // First, try to open the path as a directory.
   const int options = O_CLOEXEC | O_RDONLY | O_DIRECTORY | O_NOFOLLOW;
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -794,6 +794,9 @@
 
   # Prevent bugs that can happen with llvm's brace style.
   add_flag_if_supported("-Wmisleading-indentation" MISLEADING_INDENTATION_FLAG)
+
+  # Enable -Wctad-maybe-unsupported to catch unintended use of CTAD.
+  add_flag_if_supported("-Wctad-maybe-unsupported" CTAD_MAYBE_UNSPPORTED_FLAG)
 endif (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE AND NOT LLVM_ENABLE_WARNINGS)
diff --git a/llvm/include/llvm/CodeGen/RDFGraph.h b/llvm/include/llvm/CodeGen/RDFGraph.h
--- a/llvm/include/llvm/CodeGen/RDFGraph.h
+++ b/llvm/include/llvm/CodeGen/RDFGraph.h
@@ -934,6 +934,8 @@
     const DataFlowGraph &G;
   };
 
+  template <typename T> Print(const T &, const DataFlowGraph &) -> Print<T>;
+
   template <typename T>
   struct PrintNode : Print<NodeAddr<T>> {
     PrintNode(const NodeAddr<T> &x, const DataFlowGraph &g)