[NFC] [clang] add tests for merging of UsingShadowDecl

Author: mizvekov

clang/test/Modules/cxx20-decls.cppm

@@ -0,0 +1,35 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 -I %t %t/A.cppm -emit-module-interface -o %t/A.pcm -verify
+// RUN: %clang_cc1 -std=c++20 -I %t %t/B.cpp -fmodule-file=A=%t/A.pcm -fsyntax-only -verify -ast-dump-all -ast-dump-filter baz | FileCheck %s
+
+//--- foo.h
+namespace baz {
+  using foo = char;
+  using baz::foo;
+}
+
+//--- A.cppm
+// expected-no-diagnostics
+module;
+#include "foo.h"
+export module A;
+
+//--- B.cpp
+// expected-no-diagnostics
+#include "foo.h"
+import A;
+// Since modules are loaded lazily, force loading by performing a lookup.
+using xxx = baz::foo;
+
+// CHECK-LABEL: Dumping baz:
+// CHECK-NEXT: NamespaceDecl 0x[[BAZ_REDECL_ADDR:[^ ]*]] prev 0x[[BAZ_ADDR:[^ ]*]]
+// CHECK:      TypeAliasDecl 0x[[ALIAS_REDECL_ADDR:[^ ]*]] prev 0x[[ALIAS_ADDR:[^ ]*]]
+// FIXME: UsingShadowDecl should have been merged
+// CHECK:      UsingShadowDecl 0x{{[^ ]*}} <{{.*}}> col:{{.*}} imported in A.<global> hidden implicit TypeAlias 0x[[ALIAS_REDECL_ADDR]] 'foo'
+
+// CHECK-LABEL: Dumping baz:
+// CHECK-NEXT: NamespaceDecl 0x[[BAZ_ADDR]] <{{.*}}> line:{{.*}} baz
+// CHECK:      UsingShadowDecl 0x[[SHADOW_ADDR:[^ ]*]] <{{.*}}> col:{{.*}} implicit TypeAlias 0x[[ALIAS_ADDR]] 'foo'