Properly diagnose constant evaluation issues at TU scope

[aaron.ballman]

Summary

We were not creating an evaluation context for the TU scope, so we never popped an evaluation context for it. Popping the evaluation context triggers a number of diagnostics, including warnings about immediate invocations that we were previously missing.

Note: I think we have an additional issue that we should solve, but not as part of this patch. I don't think Clang is properly modeling static initialization as happening before constant expression evaluation. I think structure members members are zero initialized per http://eel.is/c++draft/basic.start.static#1, https://eel.is/c++draft/basic.start.static#2.sentence-2, and http://eel.is/c++draft/dcl.init#general-6.2 and the new test case actually should be accepted. However, it's also worth noting that other compilers behave the way this patch makes Clang behave: https://godbolt.org/z/T7noqhdPr

[cor3ntin]

There is already an evaluation context created in Sema but it's never popped back.
And because we rely heavily on there always being at least one evaluation context (ExprEvalContexts.back() is used all over the place for example), i think it make somewhat sense to have 2.

So this looks good to me. I wonder if we need a comment?

clang/test/SemaCXX/cxx2a-consteval.cpp

699

I know there are more patches coming up and i don't remember what's broken or not, but if at all possible, can you add a test for a variable template instantiation too?

[aaron.ballman]

Added a comment and an additional test case.

[aaron.ballman]

Thanks for the reviews! I've committed in 1c55f05c6a6b58f8cc7b15a37e79753fb8abe3e3.

clang/docs/ReleaseNotes.rst

 
 C++ Language Changes in Clang
 -----------------------------
 
 - ...
 
 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
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Implemented `P2128R6: Multidimensional subscript operator <https://wg21.link/P2128R6>`_.
 - Implemented `P0849R8: auto(x): decay-copy in the language <https://wg21.link/P0849R8>`_.
 
 CUDA Language Changes in Clang

clang/lib/Parse/ParseAST.cpp

void clang::ParseAST(Sema &S, bool PrintStats, bool SkipFunctionBodies) {
   // after the pragma, there won't be any tokens or a Lexer.
   bool HaveLexer = S.getPreprocessor().getCurrentLexer();
 
   if (HaveLexer) {
     llvm::TimeTraceScope TimeScope("Frontend");
     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
       // is due to a top-level semicolon, an action override, or a parse error
       // skipping something.
       if (ADecl && !Consumer->HandleTopLevelDecl(ADecl.get()))
         return;
     }

clang/lib/Sema/Sema.cpp

if (getLangOpts().ObjC)
     NSAPIObj.reset(new NSAPI(Context));
 
   if (getLangOpts().CPlusPlus)
     FieldCollector.reset(new CXXFieldCollector());
 
   // 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);
 
   // Initialization of data sharing attributes stack for OpenMP
   InitDataSharingAttributesStack();
 
   std::unique_ptr<sema::SemaPPCallbacks> Callbacks =

clang/test/SemaCXX/cxx2a-consteval.cpp

struct A {
   }
 
   void c() {
     a() + d(); // expected-error {{call to consteval function 'PR48235::A::a' is not a constant expression}} \
                // expected-note {{use of 'this' pointer is only allowed within the evaluation of a call to a 'constexpr' member function}}
   }
 };
 } // 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