Expand definition deprecation warning to include constexpr statements.

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

Differential Revision: https://reviews.llvm.org/D126664

clang/docs/ReleaseNotes.rst

@@ -241,6 +241,9 @@ Improvements to Clang's diagnostics
   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
 -------------------------------------------------

clang/lib/Sema/SemaDecl.cpp

@@ -4508,15 +4508,15 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   }
 
   // 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;
     }
   }

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

@@ -5,4 +5,9 @@ namespace {
     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}}
 }