Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -118,6 +118,8 @@
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
+- Diagnose consteval and constexpr issues that happen at namespace scope. This
+  partially addresses `Issue 51593 <https://github.com/llvm/llvm-project/issues/51593>`_.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
Index: clang/lib/Parse/ParseAST.cpp
===================================================================
--- clang/lib/Parse/ParseAST.cpp
+++ clang/lib/Parse/ParseAST.cpp
@@ -155,6 +155,9 @@
     P.Initialize();
     Parser::DeclGroupPtrTy ADecl;
     Sema::ModuleImportState ImportState;
+    EnterExpressionEvaluationContext PotentiallyEvaluated(
+        S, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
+
     for (bool AtEOF = P.ParseFirstTopLevelDecl(ADecl, ImportState); !AtEOF;
          AtEOF = P.ParseTopLevelDecl(ADecl, ImportState)) {
       // If we got a null return and something *was* parsed, ignore it.  This
Index: clang/lib/Sema/Sema.cpp
===================================================================
--- clang/lib/Sema/Sema.cpp
+++ clang/lib/Sema/Sema.cpp
@@ -233,6 +233,9 @@
   // Tell diagnostics how to render things from the AST library.
   Diags.SetArgToStringFn(&FormatASTNodeDiagnosticArgument, &Context);
 
+  // This evaluation context exists to ensure that there's always at least one
+  // valid evaluation context available. It is never removed from the
+  // evaluation stack.
   ExprEvalContexts.emplace_back(
       ExpressionEvaluationContext::PotentiallyEvaluated, 0, CleanupInfo{},
       nullptr, ExpressionEvaluationContextRecord::EK_Other);
Index: clang/test/SemaCXX/cxx2a-consteval.cpp
===================================================================
--- clang/test/SemaCXX/cxx2a-consteval.cpp
+++ clang/test/SemaCXX/cxx2a-consteval.cpp
@@ -689,3 +689,23 @@
   }
 };
 } // PR48235
+
+namespace NamespaceScopeConsteval {
+struct S {
+  int Val; // expected-note {{subobject declared here}}
+  consteval S() {}
+};
+
+S s1; // expected-error {{call to consteval function 'NamespaceScopeConsteval::S::S' is not a constant expression}} \
+         expected-note {{subobject of type 'int' is not initialized}}
+
+template <typename Ty>
+struct T {
+  Ty Val; // expected-note {{subobject declared here}}
+  consteval T() {}
+};
+
+T<int> t; // expected-error {{call to consteval function 'NamespaceScopeConsteval::T<int>::T' is not a constant expression}} \
+             expected-note {{subobject of type 'int' is not initialized}}
+
+} // namespace NamespaceScopeConsteval