[clang][bytecode] Change diagnostics for self-initialization (#141006)

Change the diagnostics when reading from the variable we're currently
initializing do be the same as the one the current interpreter emits.

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -669,6 +669,21 @@ bool CheckInitialized(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
 
   if (const auto *VD = Ptr.getDeclDesc()->asVarDecl();
       VD && (VD->isConstexpr() || VD->hasGlobalStorage())) {
+
+    if (!S.getLangOpts().CPlusPlus23 && VD == S.EvaluatingDecl) {
+      if (!S.getLangOpts().CPlusPlus14 &&
+          !VD->getType().isConstant(S.getASTContext())) {
+        // Diagnose as non-const read.
+        diagnoseNonConstVariable(S, OpPC, VD);
+      } else {
+        const SourceInfo &Loc = S.Current->getSource(OpPC);
+        // Diagnose as "read of object outside its lifetime".
+        S.FFDiag(Loc, diag::note_constexpr_access_uninit)
+            << AK << /*IsIndeterminate=*/false;
+      }
+      return false;
+    }
+
     if (VD->getAnyInitializer()) {
       const SourceInfo &Loc = S.Current->getSource(OpPC);
       S.FFDiag(Loc, diag::note_constexpr_var_init_non_constant, 1) << VD;

clang/test/AST/ByteCode/cxx11.cpp

@@ -23,6 +23,16 @@ int array2[recurse2]; // both-warning {{variable length arrays in C++}} \
                       // expected-error {{variable length array declaration not allowed at file scope}} \
                       // ref-warning {{variable length array folded to constant array as an extension}}
 
+constexpr int b = b; // both-error {{must be initialized by a constant expression}} \
+                     // both-note {{read of object outside its lifetime is not allowed in a constant expression}}
+
+
+[[clang::require_constant_initialization]] int c = c; // both-error {{variable does not have a constant initializer}} \
+                                                      // both-note {{attribute here}} \
+                                                      // both-note {{read of non-const variable}} \
+                                                      // both-note {{declared here}}
+
+
 struct S {
   int m;
 };

clang/test/AST/ByteCode/cxx17.cpp

@@ -1,5 +1,10 @@
-// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -std=c++17 -verify=expected,both %s
-// RUN: %clang_cc1 -std=c++17 -verify=ref,both %s
+// RUN: %clang_cc1 -std=c++17 -verify=expected,both %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++17 -verify=ref,both      %s
+
+[[clang::require_constant_initialization]] int cc = cc; // both-error {{variable does not have a constant initializer}} \
+                                                        // both-note {{attribute here}} \
+                                                        // both-note {{ead of object outside its lifetime}}
+
 
 struct F { int a; int b;};
 constexpr F getF() {