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

[naveen-seth]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Naveen Seth Hanig (naveen-seth)

Changes

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.

The assertions failing this were logically sound, and this change ensures that such initializations are handled correctly during semantic analysis to avoid failing any assertion.

---

Full diff: https://github.com/llvm/llvm-project/pull/167171.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaInit.cpp (+4-1)
• (added) clang/test/SemaCXX/crash-union-designated-initializer.cpp (+23)

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index cc6ddf568d346..13fb22ebfe0ef 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/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;
       }
     }
diff --git a/clang/test/SemaCXX/crash-union-designated-initializer.cpp b/clang/test/SemaCXX/crash-union-designated-initializer.cpp
new file mode 100644
index 0000000000000..7ba40311eac0b
--- /dev/null
+++ b/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}}
+}