Enable -Wctad-maybe-unsupported in LLVM build

Warns on potentially unintended use of C++17 Class Template Argument
Deduction. Use of this feature with types that aren't intended to
support it may may future refactorings of those types more difficult -
so this warning fires whenever the feature is used with a type that may
not have intended to be used with CTAD (the warning uses the existence
of at least one explicit deduction guide to indicate that a type
intentionally supports CTAD - absent that, it's assumed to not be
intended to support CTAD & produces a warning).

This is disabled in libcxx because lots of the standard library is
assumed to provide ctad-usable APIs and the false positive suppression
in the diagnostic is based on system header classification which doesn't
apply in the libcxx build itself.

Differential Revision: https://reviews.llvm.org/D131727

libcxx/CMakeLists.txt

@@ -592,6 +592,7 @@ function(cxx_add_warning_flags target)
       -Wno-user-defined-literals
       -Wno-covered-switch-default
       -Wno-suggest-override
+      -Wno-ctad-maybe-unsupported
     )
     if (LIBCXX_TARGETING_CLANG_CL)
       target_add_compile_flags_if_supported(${target} PRIVATE

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -794,6 +794,9 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
   # 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)

llvm/include/llvm/CodeGen/RDFGraph.h

@@ -934,6 +934,8 @@ namespace rdf {
     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)