[Clang] Ensure zero-init is not overridden when initializing a base class in a constant expression context

[shafik]

During constant evaluation when value-initializing a class if the base class was default-initialized it would undue the previously zero-initialized class members. This fixes the way we handle default initialization to avoid initializing over an already initialized member to an indeterminate value.

Fixes: #69890

[llvmbot]

@llvm/pr-subscribers-clang

Author: Shafik Yaghmour (shafik)

Changes

During constant evaluation when value-initializing a class if the base class was default-initialized it would undue the previously zero-initialized class members. This fixes the way we handle default initialization to avoid initializing over an already initialized member to an indeterminate value.

Fixes: #69890

---

Full diff: https://github.com/llvm/llvm-project/pull/70150.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+4)
• (modified) clang/lib/AST/ExprConstant.cpp (+20-13)
• (added) clang/test/CXX/dcl.decl/dcl.init/dcl.init.general/p16-cxx20.cpp (+18)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a063733e96cb74c..ae2f082ad323a78 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -622,6 +622,10 @@ Bug Fixes to C++ Support
   (`#46200 <https://github.com/llvm/llvm-project/issues/46200>`_)
   (`#57812 <https://github.com/llvm/llvm-project/issues/57812>`_)
 
+- Fix bug where we were overriding zero-initialization of class members when
+  default initializing a base class in a constant expression context. Fixes:
+  (`#69890 <https://github.com/llvm/llvm-project/issues/69890>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6b47b8a1256477d..eea0827d6f7a8a1 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4872,8 +4872,13 @@ static bool HandleBaseToDerivedCast(EvalInfo &Info, const CastExpr *E,
 
 /// Get the value to use for a default-initialized object of type T.
 /// Return false if it encounters something invalid.
-static bool getDefaultInitValue(QualType T, APValue &Result) {
+static bool handleDefaultInitValue(QualType T, APValue &Result) {
   bool Success = true;
+
+  // If there is already a value present don't overwrite it.
+  if (!Result.isAbsent())
+    return true;
+
   if (auto *RD = T->getAsCXXRecordDecl()) {
     if (RD->isInvalidDecl()) {
       Result = APValue();
@@ -4890,13 +4895,14 @@ static bool getDefaultInitValue(QualType T, APValue &Result) {
     for (CXXRecordDecl::base_class_const_iterator I = RD->bases_begin(),
                                                   End = RD->bases_end();
          I != End; ++I, ++Index)
-      Success &= getDefaultInitValue(I->getType(), Result.getStructBase(Index));
+      Success &=
+          handleDefaultInitValue(I->getType(), Result.getStructBase(Index));
 
     for (const auto *I : RD->fields()) {
       if (I->isUnnamedBitfield())
         continue;
-      Success &= getDefaultInitValue(I->getType(),
-                                     Result.getStructField(I->getFieldIndex()));
+      Success &= handleDefaultInitValue(
+          I->getType(), Result.getStructField(I->getFieldIndex()));
     }
     return Success;
   }
@@ -4906,7 +4912,7 @@ static bool getDefaultInitValue(QualType T, APValue &Result) {
     Result = APValue(APValue::UninitArray(), 0, AT->getSize().getZExtValue());
     if (Result.hasArrayFiller())
       Success &=
-          getDefaultInitValue(AT->getElementType(), Result.getArrayFiller());
+          handleDefaultInitValue(AT->getElementType(), Result.getArrayFiller());
 
     return Success;
   }
@@ -4947,7 +4953,7 @@ static bool EvaluateVarDecl(EvalInfo &Info, const VarDecl *VD) {
   if (!InitE) {
     if (VD->getType()->isDependentType())
       return Info.noteSideEffect();
-    return getDefaultInitValue(VD->getType(), Val);
+    return handleDefaultInitValue(VD->getType(), Val);
   }
   if (InitE->isValueDependent())
     return false;
@@ -6049,7 +6055,7 @@ struct StartLifetimeOfUnionMemberHandler {
       return false;
     }
     APValue Result;
-    Failed = !getDefaultInitValue(Field->getType(), Result);
+    Failed = !handleDefaultInitValue(Field->getType(), Result);
     Subobj.setUnion(Field, Result);
     return true;
   }
@@ -6401,7 +6407,7 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
     for (; !declaresSameEntity(*FieldIt, FD); ++FieldIt) {
       assert(FieldIt != RD->field_end() && "missing field?");
       if (!FieldIt->isUnnamedBitfield())
-        Success &= getDefaultInitValue(
+        Success &= handleDefaultInitValue(
             FieldIt->getType(),
             Result.getStructField(FieldIt->getFieldIndex()));
     }
@@ -6458,7 +6464,8 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
             // FIXME: This immediately starts the lifetime of all members of
             // an anonymous struct. It would be preferable to strictly start
             // member lifetime in initialization order.
-            Success &= getDefaultInitValue(Info.Ctx.getRecordType(CD), *Value);
+            Success &=
+                handleDefaultInitValue(Info.Ctx.getRecordType(CD), *Value);
         }
         // Store Subobject as its parent before updating it for the last element
         // in the chain.
@@ -6510,7 +6517,7 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
   if (!RD->isUnion()) {
     for (; FieldIt != RD->field_end(); ++FieldIt) {
       if (!FieldIt->isUnnamedBitfield())
-        Success &= getDefaultInitValue(
+        Success &= handleDefaultInitValue(
             FieldIt->getType(),
             Result.getStructField(FieldIt->getFieldIndex()));
     }
@@ -9823,7 +9830,7 @@ bool PointerExprEvaluator::VisitCXXNewExpr(const CXXNewExpr *E) {
   } else if (Init) {
     if (!EvaluateInPlace(*Val, Info, Result, Init))
       return false;
-  } else if (!getDefaultInitValue(AllocType, *Val)) {
+  } else if (!handleDefaultInitValue(AllocType, *Val)) {
     return false;
   }
 
@@ -10233,7 +10240,7 @@ bool RecordExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
     if (ZeroInit)
       return ZeroInitialization(E, T);
 
-    return getDefaultInitValue(T, Result);
+    return handleDefaultInitValue(T, Result);
   }
 
   const FunctionDecl *Definition = nullptr;
@@ -15655,7 +15662,7 @@ bool VarDecl::evaluateDestruction(
   APValue DestroyedValue;
   if (getEvaluatedValue() && !getEvaluatedValue()->isAbsent())
     DestroyedValue = *getEvaluatedValue();
-  else if (!getDefaultInitValue(getType(), DestroyedValue))
+  else if (!handleDefaultInitValue(getType(), DestroyedValue))
     return false;
 
   if (!EvaluateDestruction(getASTContext(), this, std::move(DestroyedValue),
diff --git a/clang/test/CXX/dcl.decl/dcl.init/dcl.init.general/p16-cxx20.cpp b/clang/test/CXX/dcl.decl/dcl.init/dcl.init.general/p16-cxx20.cpp
new file mode 100644
index 000000000000000..1ad205d769c3895
--- /dev/null
+++ b/clang/test/CXX/dcl.decl/dcl.init/dcl.init.general/p16-cxx20.cpp
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+
+// If the initializer is (), the object is value-initialized.
+
+// expected-no-diagnostics
+namespace GH69890 {
+struct A {
+    constexpr A() {}
+    int x;
+};
+
+struct B : A {
+    int y;
+};
+
+static_assert(B().x == 0);
+static_assert(B().y == 0);
+}

[shafik]

The clang-format error is a false positive.