[BoundsSafety] Don't drop va_list typedef during function merging (llvm#10771)

rdar://151611200

(cherry picked from commit 9d03aa6)

Author: patrykstefanski

clang/lib/AST/ASTContext.cpp

@@ -3707,6 +3707,21 @@ QualType ASTContext::mergeBoundsSafetyPointerTypes(
   if (OrigDstTy.isNull())
     OrigDstTy = DstTy;
 
+  // An ugly way to keep va_list typedef in DstTy if the merge type doesn't
+  // change.
+  // TODO: We need a general way of not stripping sugars.
+  QualType DesugaredDstTy;
+  if (const auto *TDT = dyn_cast<TypedefType>(DstTy))
+    DesugaredDstTy = TDT->desugar();
+  else if (const auto *ET = dyn_cast<ElaboratedType>(DstTy))
+    DesugaredDstTy = ET->desugar();
+  if (!DesugaredDstTy.isNull()) {
+    QualType MergeTy = mergeBoundsSafetyPointerTypes(DesugaredDstTy, SrcTy,
+                                                     MergeFunctor, OrigDstTy);
+    if (MergeTy == DesugaredDstTy)
+      return DstTy;
+  }
+
   // FIXME: a brittle hack to avoid skipping ValueTerminatedType outside
   // this PtrAutoAttr AttributedType.
   bool RecoverPtrAuto = false;

clang/test/BoundsSafety/AST/merge-vsnprintf.c

@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fbounds-safety -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fbounds-safety -x c++ -fexperimental-bounds-safety-cxx -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fbounds-safety -x objective-c -fexperimental-bounds-safety-objc -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fexperimental-bounds-safety-attributes -x c -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fexperimental-bounds-safety-attributes -x c++ -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fexperimental-bounds-safety-attributes -x objective-c -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-ios -fsyntax-only -fexperimental-bounds-safety-attributes -x objective-c++ -ast-dump %s 2>&1 | FileCheck %s
+
+#include <ptrcheck.h>
+#include <stdarg.h>
+#include <stddef.h>
+
+#define __printf(string_index, first_to_check)                                 \
+  __attribute__((__format__(__printf__, string_index, first_to_check)))
+
+#ifndef __cplusplus
+#define __restrict restrict
+#endif
+
+// CHECK: ParmVarDecl {{.+}} foo_args 'va_list':'char *'
+int vsnprintf(char *__restrict __counted_by(__size) __str, size_t __size,
+              const char *__restrict __format, va_list foo_args) __printf(3, 0);