[clang][bytecode] Override InConstantContext flag for immediate calls (#109967)

And fix the diagnostics for __builtin_is_constant_evaluated(). We can be
in a non-constant context, but calling an immediate function always
makes the context constant for the duration of that call.

clang/lib/AST/ByteCode/Interp.cpp

@@ -1136,6 +1136,7 @@ bool Call(InterpState &S, CodePtr OpPC, const Function *Func,
   InterpFrame *FrameBefore = S.Current;
   S.Current = NewFrame.get();
 
+  InterpStateCCOverride CCOverride(S, Func->getDecl()->isImmediateFunction());
   APValue CallResult;
   // Note that we cannot assert(CallResult.hasValue()) here since
   // Ret() above only sets the APValue if the curent frame doesn't

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -136,16 +136,17 @@ static bool retPrimValue(InterpState &S, CodePtr OpPC, APValue &Result,
 static bool interp__builtin_is_constant_evaluated(InterpState &S, CodePtr OpPC,
                                                   const InterpFrame *Frame,
                                                   const CallExpr *Call) {
+  unsigned Depth = S.Current->getDepth();
+  auto isStdCall = [](const FunctionDecl *F) -> bool {
+    return F && F->isInStdNamespace() && F->getIdentifier() &&
+           F->getIdentifier()->isStr("is_constant_evaluated");
+  };
+  const InterpFrame *Caller = Frame->Caller;
   // The current frame is the one for __builtin_is_constant_evaluated.
   // The one above that, potentially the one for std::is_constant_evaluated().
   if (S.inConstantContext() && !S.checkingPotentialConstantExpression() &&
-      Frame->Caller && S.getEvalStatus().Diag) {
-    auto isStdCall = [](const FunctionDecl *F) -> bool {
-      return F && F->isInStdNamespace() && F->getIdentifier() &&
-             F->getIdentifier()->isStr("is_constant_evaluated");
-    };
-    const InterpFrame *Caller = Frame->Caller;
-
+      S.getEvalStatus().Diag &&
+      (Depth == 1 || (Depth == 2 && isStdCall(Caller->getCallee())))) {
     if (Caller->Caller && isStdCall(Caller->getCallee())) {
       const Expr *E = Caller->Caller->getExpr(Caller->getRetPC());
       S.report(E->getExprLoc(),

clang/lib/AST/ByteCode/InterpState.cpp

@@ -19,6 +19,13 @@ InterpState::InterpState(State &Parent, Program &P, InterpStack &Stk,
                          Context &Ctx, SourceMapper *M)
     : Parent(Parent), M(M), P(P), Stk(Stk), Ctx(Ctx), Current(nullptr) {}
 
+bool InterpState::inConstantContext() const {
+  if (ConstantContextOverride)
+    return *ConstantContextOverride;
+
+  return Parent.InConstantContext;
+}
+
 InterpState::~InterpState() {
   while (Current) {
     InterpFrame *Next = Current->Caller;

clang/lib/AST/ByteCode/InterpState.h

@@ -77,7 +77,7 @@ public:
   bool noteUndefinedBehavior() override {
     return Parent.noteUndefinedBehavior();
   }
-  bool inConstantContext() const { return Parent.InConstantContext; }
+  bool inConstantContext() const;
   bool hasActiveDiagnostic() override { return Parent.hasActiveDiagnostic(); }
   void setActiveDiagnostic(bool Flag) override {
     Parent.setActiveDiagnostic(Flag);
@@ -116,6 +116,7 @@ public:
 
 private:
   friend class EvaluationResult;
+  friend class InterpStateCCOverride;
   /// AST Walker state.
   State &Parent;
   /// Dead block chain.
@@ -124,6 +125,7 @@ private:
   SourceMapper *M;
   /// Allocator used for dynamic allocations performed via the program.
   DynamicAllocator Alloc;
+  std::optional<bool> ConstantContextOverride;
 
 public:
   /// Reference to the module containing all bytecode.
@@ -144,6 +146,26 @@ public:
       SeenGlobalTemporaries;
 };
 
+class InterpStateCCOverride final {
+public:
+  InterpStateCCOverride(InterpState &Ctx, bool Value)
+      : Ctx(Ctx), OldCC(Ctx.ConstantContextOverride) {
+    // We only override this if the new value is true.
+    Enabled = Value;
+    if (Enabled)
+      Ctx.ConstantContextOverride = Value;
+  }
+  ~InterpStateCCOverride() {
+    if (Enabled)
+      Ctx.ConstantContextOverride = OldCC;
+  }
+
+private:
+  bool Enabled;
+  InterpState &Ctx;
+  std::optional<bool> OldCC;
+};
+
 } // namespace interp
 } // namespace clang
 

clang/test/CodeGenCXX/cxx2a-consteval.cpp

@@ -6,6 +6,14 @@
 // RUN: %clang_cc1 -emit-llvm %s -Dconsteval="" -std=c++2a -triple x86_64-unknown-linux-gnu -o %t.ll
 // RUN: FileCheck -check-prefix=EXPR -input-file=%t.ll %s
 
+// RUN: %clang_cc1 -emit-llvm %s -std=c++2a -triple x86_64-unknown-linux-gnu -o %t.ll -fexperimental-new-constant-interpreter
+// RUN: FileCheck -check-prefix=EVAL -input-file=%t.ll %s
+// RUN: FileCheck -check-prefix=EVAL-STATIC -input-file=%t.ll %s
+// RUN: FileCheck -check-prefix=EVAL-FN -input-file=%t.ll %s
+//
+// RUN: %clang_cc1 -emit-llvm %s -Dconsteval="" -std=c++2a -triple x86_64-unknown-linux-gnu -o %t.ll -fexperimental-new-constant-interpreter
+// RUN: FileCheck -check-prefix=EXPR -input-file=%t.ll %s
+
 // there is two version of symbol checks to ensure
 // that the symbol we are looking for are correct
 // EVAL-NOT: @__cxx_global_var_init()