[BoundsSafety] Keep new function type if merged with implicit decl

clang/lib/Sema/SemaDecl.cpp

@@ -5079,6 +5079,23 @@ bool Sema::MergeCompatibleFunctionDecls(FunctionDecl *New, FunctionDecl *Old,
   if (getLangOpts().CPlusPlus)
     return MergeCXXFunctionDecl(New, Old, S);
 
+  // In BoundsSafety, we allow merging bounds attributes between function decls
+  // if the old decl is implicit. This enables us to add bounds attributes to a
+  // function decl if the old one is implicit without causing 'incompatible
+  // function decl' error. For example, we can redeclare the implicit decl of
+  // `memcmp`, and add `__sized_by()` attributes to its parameters.
+  // In attribute-only mode, those attributes are sugars though, and will be
+  // dropped by `Context.mergeTypes()` below. If both canonical types are the
+  // same, we can keep the new type to retain sugars.
+  if (getLangOpts().BoundsSafetyAttributes) {
+    if (Old->isImplicit() &&
+        Old->getType().getCanonicalType() ==
+            New->getType().getCanonicalType() &&
+        !Context.mergeTypes(Old->getType(), New->getType()).isNull()) {
+      return false;
+    }
+  }
+
   // Merge the function types so the we get the composite types for the return
   // and argument types. Per C11 6.2.7/4, only update the type if the old decl
   // was visible.

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

@@ -0,0 +1,24 @@
+// 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 <stddef.h>
+
+// CHECK:      FunctionDecl {{.+}} memcmp 'int (const void *[[single:(__single)?]] __sized_by(n), const void *[[single]] __sized_by(n), size_t)'
+// CHECK-NEXT: ParmVarDecl [[s1:0x[^ ]+]] {{.+}} s1 'const void *[[single]] __sized_by(n)':'const void *[[single]]'
+// CHECK-NEXT: ParmVarDecl [[s2:0x[^ ]+]] {{.+}} s2 'const void *[[single]] __sized_by(n)':'const void *[[single]]'
+// CHECK-NEXT: ParmVarDecl {{.+}} used n 'size_t':'unsigned long'
+// CHECK-NEXT: `-DependerDeclsAttr {{.+}} Implicit [[s1]] [[s2]] 0 0
+int memcmp(const void *__sized_by(n) s1, const void *__sized_by(n) s2, size_t n);
+
+// CHECK:      FunctionDecl {{.+}} memcpy 'void *[[single]] __sized_by(n)(void *[[single]] __sized_by(n), const void *[[single]] __sized_by(n), size_t)'
+// CHECK-NEXT: ParmVarDecl [[dst:0x[^ ]+]] {{.+}} dst 'void *[[single]] __sized_by(n)':'void *[[single]]'
+// CHECK-NEXT: ParmVarDecl [[src:0x[^ ]+]] {{.+}} src 'const void *[[single]] __sized_by(n)':'const void *[[single]]'
+// CHECK-NEXT: ParmVarDecl {{.+}} used n 'size_t':'unsigned long'
+// CHECK-NEXT: `-DependerDeclsAttr {{.+}} <<invalid sloc>> Implicit [[dst]] [[src]] 0 0
+void *__sized_by(n) memcpy(void *__sized_by(n) dst, const void *__sized_by(n) src, size_t n);