Index: clang/lib/AST/Interp/ByteCodeExprGen.cpp
===================================================================
--- clang/lib/AST/Interp/ByteCodeExprGen.cpp
+++ clang/lib/AST/Interp/ByteCodeExprGen.cpp
@@ -1410,6 +1410,7 @@
 
         if (!this->emitPopPtr(Initializer))
           return false;
+        ++InitIndex;
       } else {
         // Initializer for a direct base class.
         if (const Record::Base *B = R->getBase(Init->getType())) {
@@ -1421,6 +1422,8 @@
 
           if (!this->emitPopPtr(Initializer))
             return false;
+          // Base initializers don't increase InitIndex, since they don't count
+          // into the Record's fields.
         } else {
           const Record::Field *FieldToInit = R->getField(InitIndex);
           // Non-primitive case. Get a pointer to the field-to-initialize
@@ -1433,9 +1436,9 @@
 
           if (!this->emitPopPtr(Initializer))
             return false;
+          ++InitIndex;
         }
       }
-      ++InitIndex;
     }
 
     return true;
Index: clang/test/AST/Interp/cxx20.cpp
===================================================================
--- clang/test/AST/Interp/cxx20.cpp
+++ clang/test/AST/Interp/cxx20.cpp
@@ -583,3 +583,19 @@
   constexpr Outer O;
   static_assert(O.bar() == 12);
 }
+
+namespace BaseAndFieldInit {
+  struct A {
+    int a;
+  };
+
+  struct B : A {
+    int b;
+  };
+
+  struct C : B {
+    int c;
+  };
+
+  constexpr C c = {1,2,3};
+}