[clang][Interp] Implement zero-init for record types

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -924,8 +924,31 @@ bool ByteCodeExprGen<Emitter>::VisitImplicitValueInitExpr(const ImplicitValueIni
   if (std::optional<PrimType> T = classify(QT))
     return this->visitZeroInitializer(*T, QT, E);
 
-  if (QT->isRecordType())
-    return false;
+  if (QT->isRecordType()) {
+    const RecordDecl *RD = QT->getAsRecordDecl();
+    assert(RD);
+    if (RD->isInvalidDecl())
+      return false;
+    if (RD->isUnion()) {
+      // C++11 [dcl.init]p5: If T is a (possibly cv-qualified) union type, the
+      // object's first non-static named data member is zero-initialized
+      // FIXME
+      return false;
+    }
+
+    if (const auto *CXXRD = dyn_cast<CXXRecordDecl>(RD);
+        CXXRD && CXXRD->getNumVBases() > 0) {
+      // TODO: Diagnose.
+      return false;
+    }
+
+    const Record *R = getRecord(QT);
+    if (!R)
+      return false;
+
+    assert(Initializing);
+    return this->visitZeroRecordInitializer(R, E);
+  }
 
   if (QT->isIncompleteArrayType())
     return true;

clang/test/AST/Interp/records.cpp

@@ -90,8 +90,7 @@ struct Ints2 {
   int a = 10;
   int b;
 };
-constexpr Ints2 ints22; // both-error {{without a user-provided default constructor}} \
-                        // expected-error {{must be initialized by a constant expression}}
+constexpr Ints2 ints22; // both-error {{without a user-provided default constructor}}
 
 constexpr Ints2 I2 = Ints2{12, 25};
 static_assert(I2.a == 12, "");
@@ -1409,3 +1408,15 @@ namespace VirtualBases {
     static_assert((X*)(Y1*)&z != (X*)(Y2*)&z, "");
   }
 }
+
+namespace ZeroInit {
+  struct S3 {
+    S3() = default;
+    S3(const S3&) = default;
+    S3(S3&&) = default;
+    constexpr S3(int n) : n(n) {}
+    int n;
+  };
+  constexpr S3 s3d; // both-error {{default initialization of an object of const type 'const S3' without a user-provided default constructor}}
+  static_assert(s3d.n == 0, "");
+}