diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -695,6 +695,9 @@
 - Fix handling of using-declarations in the init statements of for
   loop declarations.
   (`#63627 <https://github.com/llvm/llvm-project/issues/63627>`_)
+- Fix crash when emitting diagnostic for out of order designated initializers
+  in C++.
+  (`#63605 <https://github.com/llvm/llvm-project/issues/63605>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -2847,7 +2847,7 @@
         SemaRef.Diag(DIE->getBeginLoc(), diag::ext_designated_init_reordered)
             << KnownField << PrevField << DIE->getSourceRange();
 
-        unsigned OldIndex = NumBases + PrevField->getFieldIndex();
+        unsigned OldIndex = StructuredIndex - 1;
         if (StructuredList && OldIndex <= StructuredList->getNumInits()) {
           if (Expr *PrevInit = StructuredList->getInit(OldIndex)) {
             SemaRef.Diag(PrevInit->getBeginLoc(),
@@ -2951,8 +2951,12 @@
     // If this the first designator, our caller will continue checking
     // the rest of this struct/class/union subobject.
     if (IsFirstDesignator) {
+      if (Field != RD->field_end() && Field->isUnnamedBitfield())
+        ++Field;
+
       if (NextField)
         *NextField = Field;
+
       StructuredIndex = FieldIndex;
       return false;
     }
diff --git a/clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp b/clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp
--- a/clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp
+++ b/clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp
@@ -63,7 +63,7 @@
   .x = 1, // override-note {{previous}}
   .x = 1, // override-error {{overrides prior initialization}} override-note {{previous}}
   .y = 1, // override-note {{previous}}
-  .y = 1, // override-error {{overrides prior initialization}}
+  .y = 1, // override-error {{overrides prior initialization}} // reorder-note {{previous initialization for field 'y' is here}}
   .x = 1, // reorder-error {{declaration order}} override-error {{overrides prior initialization}} override-note {{previous}}
   .x = 1, // override-error {{overrides prior initialization}}
 };
@@ -177,3 +177,43 @@
     h({.a = 1});
   }
 }
+
+namespace GH63605 {
+struct A  {
+  unsigned x;
+  unsigned y;
+  unsigned z;
+};
+
+struct B {
+  unsigned a;
+  unsigned b;
+};
+
+struct : public A, public B {
+  unsigned : 2;
+  unsigned a : 6;
+  unsigned : 1;
+  unsigned b : 6;
+  unsigned : 2;
+  unsigned c : 6;
+  unsigned d : 1;
+  unsigned e : 2;
+} data = {
+  {.z=0,
+         // pedantic-note@-1 {{first non-designated initializer is here}}
+         // reorder-note@-2 {{previous initialization for field 'z' is here}}
+   .y=1, // reorder-error {{field 'z' will be initialized after field 'y'}}
+         // reorder-note@-1 {{previous initialization for field 'y' is here}}
+   .x=2}, // reorder-error {{field 'y' will be initialized after field 'x'}}
+  {.b=3,  // reorder-note {{previous initialization for field 'b' is here}}
+   .a=4}, // reorder-error {{field 'b' will be initialized after field 'a'}}
+    .e = 1, // reorder-note {{previous initialization for field 'e' is here}}
+            // pedantic-error@-1 {{mixture of designated and non-designated initializers in the same initializer list is a C99 extension}}
+    .d = 1, // reorder-error {{field 'e' will be initialized after field 'd'}}
+            // reorder-note@-1 {{previous initialization for field 'd' is here}}
+    .c = 1, // reorder-error {{field 'd' will be initialized after field 'c'}} // reorder-note {{previous initialization for field 'c' is here}}
+    .b = 1, // reorder-error {{field 'c' will be initialized after field 'b'}} // reorder-note {{previous initialization for field 'b' is here}}
+    .a = 1, // reorder-error {{field 'b' will be initialized after field 'a'}}
+};
+}