diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -241,6 +241,9 @@
   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
 -------------------------------------------------
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4508,15 +4508,15 @@
   }
 
   // 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;
     }
   }
diff --git a/clang/test/CXX/basic/basic.def/p2.cpp b/clang/test/CXX/basic/basic.def/p2.cpp
--- a/clang/test/CXX/basic/basic.def/p2.cpp
+++ b/clang/test/CXX/basic/basic.def/p2.cpp
@@ -5,4 +5,9 @@
     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}}
 }