[clang] [Clang] Fix null-pointer assertion reading CtorClosureDefaultArgs (PR #207917)

[Hans Wennborg via cfe-commits]

================
@@ -0,0 +1,29 @@
+// Test that MS ABI copy-constructor closures (built when throwing a class by
+// value whose copy constructor has default arguments) survive a PCH
+// round-trip. BuildCtorClosureDefaultArgs deliberately leaves the first
+// default-arg slot null for a copy ctor closure (the closure itself supplies
+// that argument).
+//
+// Test this without pch.
+// RUN: %clang_cc1 -fcxx-exceptions -fms-extensions -triple i386-pc-win32 -std=c++20 -include %s -emit-llvm -o - %s | FileCheck %s
+//
+// Test with pch.
+// RUN: %clang_cc1 -fcxx-exceptions -fms-extensions -triple i386-pc-win32 -std=c++20 -emit-pch -o %t %s
+// RUN: %clang_cc1 -fcxx-exceptions -fms-extensions -triple i386-pc-win32 -std=c++20 -include-pch %t -emit-llvm -o - %s | FileCheck %s
+
+#ifndef HEADER
+#define HEADER
+
+struct Default {
+  Default(Default &, int = 42);
+};
+
+void h(Default &d) {
+  throw d;
+}
+
+// CHECK-LABEL: define {{.*}} void @"?h@@YAXAAUDefault@@@Z"
----------------
zmodem wrote:

Should we check that the copy ctor closure gets defined as well? (`??_ODefault@@QAEXAAU0@@Z`)

https://github.com/llvm/llvm-project/pull/207917