[clang][CompundLiteralExpr] Don't defer evaluation for CLEs

Previously we would defer evaluation of CLEs until LValue to RValue
conversions, which would result in creating values within wrong scope
and triggering use-after-frees.

This patch instead eagerly evaluates CLEs, within the scope requiring
them. This requires storing an extra pointer for CLE expressions with
static storage.

Author: kadircet

clang/include/clang/AST/Expr.h

@@ -3489,6 +3489,11 @@ class CompoundLiteralExpr : public Expr {
   /// The int part of the pair stores whether this expr is file scope.
   llvm::PointerIntPair<TypeSourceInfo *, 1, bool> TInfoAndScope;
   Stmt *Init;
+
+  /// Value of constant literals with static storage duration. Used only for
+  /// constant folding as CompoundLiteralExpr is not an ICE.
+  mutable APValue *StaticValue = nullptr;
+
 public:
   CompoundLiteralExpr(SourceLocation lparenloc, TypeSourceInfo *tinfo,
                       QualType T, ExprValueKind VK, Expr *init, bool fileScope)
@@ -3518,6 +3523,13 @@ class CompoundLiteralExpr : public Expr {
     TInfoAndScope.setPointer(tinfo);
   }
 
+  bool hasStaticStorage() const { return isFileScope() && isGLValue(); }
+  APValue *getOrCreateStaticValue(ASTContext &Ctx) const;
+  APValue &getStaticValue() const {
+    assert(StaticValue);
+    return *StaticValue;
+  }
+
   SourceLocation getBeginLoc() const LLVM_READONLY {
     // FIXME: Init should never be null.
     if (!Init)

clang/lib/AST/Expr.cpp

@@ -5467,3 +5467,12 @@ ConvertVectorExpr *ConvertVectorExpr::Create(
   return new (Mem) ConvertVectorExpr(SrcExpr, TI, DstType, VK, OK, BuiltinLoc,
                                      RParenLoc, FPFeatures);
 }
+
+APValue *CompoundLiteralExpr::getOrCreateStaticValue(ASTContext &Ctx) const {
+  assert(hasStaticStorage());
+  if (!StaticValue) {
+    StaticValue = new (Ctx) APValue;
+    Ctx.addDestruction(StaticValue);
+  }
+  return StaticValue;
+}

clang/lib/AST/ExprConstant.cpp

@@ -4596,10 +4596,6 @@ handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv, QualType Type,
         return false;
       }
 
-      APValue Lit;
-      if (!Evaluate(Lit, Info, CLE->getInitializer()))
-        return false;
-
       // According to GCC info page:
       //
       // 6.28 Compound Literals
@@ -4622,7 +4618,12 @@ handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv, QualType Type,
         }
       }
 
-      CompleteObject LitObj(LVal.Base, &Lit, Base->getType());
+      APValue *Lit =
+          CLE->hasStaticStorage()
+              ? &CLE->getStaticValue()
+              : Info.CurrentCall->getTemporary(Base, LVal.Base.getVersion());
+
+      CompleteObject LitObj(LVal.Base, Lit, Base->getType());
       return extractSubobject(Info, Conv, LitObj, LVal.Designator, RVal, AK);
     } else if (isa<StringLiteral>(Base) || isa<PredefinedExpr>(Base)) {
       // Special-case character extraction so we don't have to construct an
@@ -9125,9 +9126,25 @@ bool
 LValueExprEvaluator::VisitCompoundLiteralExpr(const CompoundLiteralExpr *E) {
   assert((!Info.getLangOpts().CPlusPlus || E->isFileScope()) &&
          "lvalue compound literal in c++?");
-  // Defer visiting the literal until the lvalue-to-rvalue conversion. We can
-  // only see this when folding in C, so there's no standard to follow here.
-  return Success(E);
+  APValue *Lit;
+  // If CompountLiteral has static storage, its value can be used outside
+  // this expression. So evaluate it once and store it in ASTContext.
+  if (E->hasStaticStorage()) {
+    Lit = E->getOrCreateStaticValue(Info.Ctx);
+    Result.set(E);
+    // Reset any previously evaluated state, otherwise evaluation below might
+    // fail.
+    // FIXME: Should we just re-use the previously evaluated value instead?
+    *Lit = APValue();
+  } else {
+    Lit = &Info.CurrentCall->createTemporary(E, E->getInitializer()->getType(),
+                                             ScopeKind::FullExpression, Result);
+  }
+  if (!EvaluateInPlace(*Lit, Info, Result, E->getInitializer())) {
+    *Lit = APValue();
+    return false;
+  }
+  return true;
 }
 
 bool LValueExprEvaluator::VisitCXXTypeidExpr(const CXXTypeidExpr *E) {

clang/test/AST/static-compound-literals.cpp

@@ -0,0 +1,12 @@
+// Test that we can successfully compile this code, especially under ASAN.
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+// expected-no-diagnostics
+struct Foo {
+  Foo* f;
+  operator bool() const { return true; }
+};
+constexpr Foo f((Foo[]){});
+int foo() {
+  if (Foo(*f.f)) return 1;
+  return 0;
+}