[clang][Interp] Skip union members in default initializers

Author: tbaederr

clang/lib/AST/Interp/ByteCodeExprGen.cpp

@@ -1061,6 +1061,10 @@ bool ByteCodeExprGen<Emitter>::visitInitList(ArrayRef<const Expr *> Inits,
              R->getField(InitIndex)->Decl->isUnnamedBitField())
         ++InitIndex;
 
+      // Potentially skip ahead. This is especially relevant in unions.
+      if (const auto *D = dyn_cast<CXXDefaultInitExpr>(Init))
+        InitIndex = D->getField()->getFieldIndex();
+
       if (!this->emitDupPtr(E))
         return false;
 

clang/lib/AST/Interp/EvaluationResult.cpp

@@ -101,6 +101,10 @@ static bool CheckFieldsInitialized(InterpState &S, SourceLocation Loc,
     Pointer FieldPtr = BasePtr.atField(F.Offset);
     QualType FieldType = F.Decl->getType();
 
+    // Don't check inactive union members.
+    if (R->isUnion() && !FieldPtr.isActive())
+      continue;
+
     if (FieldType->isRecordType()) {
       Result &= CheckFieldsInitialized(S, Loc, FieldPtr, FieldPtr.getRecord());
     } else if (FieldType->isIncompleteArrayType()) {
@@ -115,10 +119,6 @@ static bool CheckFieldsInitialized(InterpState &S, SourceLocation Loc,
       DiagnoseUninitializedSubobject(S, Loc, F.Decl);
       Result = false;
     }
-
-    // Only the first member of a union needs to be initialized.
-    if (R->isUnion())
-      break;
   }
 
   // Check Fields in all bases

clang/test/AST/Interp/unions.cpp

@@ -10,3 +10,11 @@ constexpr U a = {12};
 static_assert(a.a == 12, "");
 static_assert(a.b == 0, ""); // both-error {{not an integral constant expression}} \
                              // both-note {{read of member 'b' of union with active member 'a'}}
+union U1 {
+  int i;
+  float f = 3.0f;
+};
+constexpr U1 u1{};
+static_assert(u1.f == 3.0, "");
+static_assert(u1.i == 1, ""); // both-error {{not an integral constant expression}} \
+                              // both-note {{read of member 'i' of union with active member 'f'}}