[clang][Sema] Fix crash on designated initializer of union member in subobject

Fixes #166327

Clang previously hit an assertion in C++ mode when a nested
initializer list was followed by a designated initializer that
referred to a union member of the same subobject.
This fixes the assertion.

Author: naveen-seth

clang/lib/Sema/SemaInit.cpp

@@ -2402,8 +2402,11 @@ void InitListChecker::CheckStructUnionTypes(
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
-        if (StructuredList)
+        if (StructuredList) {
           StructuredList->setInitializedFieldInUnion(*Field);
+          StructuredList->resizeInits(SemaRef.Context,
+                                      StructuredList->getNumInits() + 1);
+        }
         break;
       }
     }

clang/test/SemaCXX/crash-union-designated-initializer.cpp

@@ -0,0 +1,23 @@
+// Ensures that Clang does not crash in C++ mode, when a nested initializer
+// is followed by a designated initializer for a union member of that same
+// subobject.
+// See issue #166327.
+
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+auto main(void) -> int {
+  struct Point {
+    float x;
+    float y;
+    union {
+      int idx;
+      char label;
+    } extra;
+  };
+
+  struct SavePoint {
+    struct Point p;
+  };
+
+  SavePoint save = {.p = {.x = 3.0, .y = 4.0}, .p.extra.label = 'p'}; // expected-warning {{nested designators are a C99 extension}}
+}