Expand definition deprecation warning to include constexpr statements.

[luken-google]

Summary

Clang currently warns on definitions downgraded to declarations
with a const modifier, but not for a constexpr modifier. This patch
updates the warning logic to warn on both inputs, and adds a test to
check the additional case as well.

See also: https://bugs.chromium.org/p/chromium/issues/detail?id=1284718

[hans]

Looks reasonable to me. It would be good if Richard or Aaron could take a look too.

[aaron.ballman]

LGTM, but please add a release note about the new diagnostic behavior.

Do you need someone to commit on your behalf? If so, what name and email address would you like used for patch attribution?

[luken-google]

Add release note.

[luken-google]

Thanks for the review! I do need someone to commit on my behalf. Please commit with name "Luke Nihlen" and email address "[e-mail]".

[luken-google]

Squash commit to include original changes.

clang/docs/ReleaseNotes.rst

- Clang now checks for stack resource exhaustion when recursively parsing
   This fixes `Issue 51642 <https://github.com/llvm/llvm-project/issues/51642>`_.
 - Unknown preprocessor directives in a skipped conditional block are now given
   a typo correction suggestion if the given directive is sufficiently similar
   to another preprocessor conditional directive. For example, if ``#esle``
   appears in a skipped block, we will warn about the unknown directive and
   suggest ``#else`` as an alternative. ``#elifdef`` and ``#elifndef`` are only
   suggested when in C2x or C++2b mode. Fixes
   `Issue 51598 <https://github.com/llvm/llvm-project/issues/51598>`_.
+- The ``-Wdeprecated`` diagnostic will now warn on out-of-line ``constexpr``
+  declarations downgraded to definitions in C++1z, in addition to the
+  existing warning on out-of-line ``const`` declarations.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
 - Improve __builtin_dump_struct:
 
   - Support bitfields in struct and union.
   - Improve the dump format, dump both bitwidth(if its a bitfield) and field

clang/lib/Sema/SemaDecl.cpp

if (!Old->getTLSKind()) {
       // declaration to determine the kind. Do we need to be compatible here?
       Diag(New->getLocation(), diag::err_thread_thread_different_kind)
         << New->getDeclName() << (New->getTLSKind() == VarDecl::TLS_Dynamic);
       Diag(OldLocation, PrevDiag);
     }
   }
 
   // C++ doesn't have tentative definitions, so go right ahead and check here.
-  if (getLangOpts().CPlusPlus &&
-      New->isThisDeclarationADefinition() == VarDecl::Definition) {
+  if (getLangOpts().CPlusPlus) {
     if (Old->isStaticDataMember() && Old->getCanonicalDecl()->isInline() &&
         Old->getCanonicalDecl()->isConstexpr()) {
       // This definition won't be a definition any more once it's been merged.
       Diag(New->getLocation(),
            diag::warn_deprecated_redundant_constexpr_static_def);
-    } else if (VarDecl *Def = Old->getDefinition()) {
-      if (checkVarDeclRedefinition(Def, New))
+    } else if (New->isThisDeclarationADefinition() == VarDecl::Definition) {
+      VarDecl *Def = Old->getDefinition();
+      if (Def && checkVarDeclRedefinition(Def, New))
         return;
     }
   }
 
   if (haveIncompatibleLanguageLinkages(Old, New)) {
     Diag(New->getLocation(), diag::err_different_language_linkage) << New;
     Diag(OldLocation, PrevDiag);
     New->setInvalidDecl();

clang/test/CXX/basic/basic.def/p2.cpp

 // RUN: %clang_cc1 -std=c++1z -verify %s -Wdeprecated
 
 namespace {
   struct A {
     static constexpr int n = 0;
   };
   const int A::n; // expected-warning {{deprecated}}
+
+  struct B {
+    static constexpr int m = 0;
+  };
+  constexpr int B::m; // expected-warning {{deprecated}}
 }