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

clang/lib/Sema/SemaInit.cpp

@@ -3029,10 +3029,11 @@ InitListChecker::CheckDesignatedInitializer(const InitializedEntity &Entity,
       if (StructuredList) {
         FieldDecl *CurrentField = StructuredList->getInitializedFieldInUnion();
         if (CurrentField && !declaresSameEntity(CurrentField, *Field)) {
-          assert(StructuredList->getNumInits() == 1
-                 && "A union should never have more than one initializer!");
+          const auto NumInits = StructuredList->getNumInits();
+          assert(StructuredList->getNumInits() <= 1 &&
+                 "A union should never have more than one initializer!");
 
-          Expr *ExistingInit = StructuredList->getInit(0);
+          Expr *ExistingInit = NumInits ? StructuredList->getInit(0) : nullptr;
           if (ExistingInit) {
             // We're about to throw away an initializer, emit warning.
             diagnoseInitOverride(

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}}
+}