[clang-tools-extra] [clang-tidy] Add redundant qualified alias check (PR #180404)

[Daniil Dudkin via cfe-commits]

================
@@ -0,0 +1,131 @@
+// RUN: %check_clang_tidy -std=c++11-or-later %s readability-redundant-qualified-alias %t
+// RUN: %check_clang_tidy -check-suffix=NS -std=c++11-or-later %s readability-redundant-qualified-alias %t -- \
+// RUN:   -config='{CheckOptions: { readability-redundant-qualified-alias.OnlyNamespaceScope: true }}'
+
+namespace n1 {
+struct Foo {};
+struct Bar {};
+struct Attr {};
+struct Commented {};
+struct Elab {};
+struct MacroEq {};
+struct MacroType {};
+struct PtrType {};
+struct LocalType {};
+} // namespace n1
+
+namespace n2 {
+namespace n3 {
+struct Deep {};
+} // namespace n3
+} // namespace n2
+
+namespace td {
+typedef n1::Foo TypedefFoo;
+} // namespace td
+
+struct GlobalType {};
+
+using Foo = n1::Foo;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-MESSAGES-NS: :[[@LINE-2]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-FIXES: using n1::Foo;
+
+using Bar = ::n1::Bar;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-MESSAGES-NS: :[[@LINE-2]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-FIXES: using ::n1::Bar;
+
+using Attr = n1::Attr __attribute__((aligned(8)));
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-MESSAGES-NS: :[[@LINE-2]]:7: warning: type alias is redundant; use a using-declaration instead [readability-redundant-qualified-alias]
+// CHECK-FIXES: using n1::Attr __attribute__((aligned(8)));
----------------
unterumarmung wrote:

Yes, that syntax is accepted by both Clang and GCC:

```cpp
using NS::Type __attribute__((...));
```

I verified it directly with `clang++ -std=c++20 -fsyntax-only` and `g++ -std=c++20 -fsyntax-only`.

https://github.com/llvm/llvm-project/pull/180404