[clang] Use tag name lookup for class names (PR #112166)

[Gábor Spaits via cfe-commits]

https://github.com/spaits created https://github.com/llvm/llvm-project/pull/112166

This PR would fix #16855 .

I think the correct lookup to use for class names is Tag name lookup, because it does not take namespaces into account. The current lookup does and because of this some valid programs are not accepted.

If you think that Tag name lookup is not correct for all cases when we are looking up types based on class names then we can only do tag name lookup when looking up class names for inheritance. In case of inheritance:
```
[class.derived]p2 says:

"During the lookup for a base class name, non-type names are ignored."
```

>From e9d43ef25b882071822cf3f16a988197c07967b1 Mon Sep 17 00:00:00 2001
From: Gabor Spaits <gaborspaits1 at gmail.com>
Date: Mon, 14 Oct 2024 10:12:42 +0200
Subject: [PATCH] Use tag name lookup for class names

This PR would fix #16855 .

I think the correct lookup to use for class names is Tag name lookup,
because it does not take namespaces into account. The current lookup
does and because of this some valid programs are not accepted.

If you think that Tag name lookup is not correct for all cases when
we are looking up types based on class names then we can only
do tag name lookup when looking up class names for inheritance.
In case of inheritance:
```
[class.derived]p2 says:

"During the lookup for a base class name, non-type names are ignored."
```
---
 clang/lib/Sema/SemaDecl.cpp         |  8 +++++---
 clang/test/CXX/class.derived/p2.cpp | 12 ++++++++++++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 118873bc93ad4b..5cce92ef118951 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -357,9 +357,10 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
       return nullptr;
   }
 
-  // FIXME: LookupNestedNameSpecifierName isn't the right kind of
-  // lookup for class-names.
-  LookupNameKind Kind = isClassName ? LookupNestedNameSpecifierName :
+  // In case we know that the identifier is a class name, we know that it is
+  // a type declaration (struct, class, union or enum) so we can use tag name
+  // lookup.
+  LookupNameKind Kind = isClassName ? LookupTagName :
                                       LookupOrdinaryName;
   LookupResult Result(*this, &II, NameLoc, Kind);
   if (LookupCtx) {
@@ -547,6 +548,7 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
   if (T.isNull()) {
     // If it's not plausibly a type, suppress diagnostics.
     Result.suppressDiagnostics();
+    llvm::errs() << "Not a type\n";
     return nullptr;
   }
 
diff --git a/clang/test/CXX/class.derived/p2.cpp b/clang/test/CXX/class.derived/p2.cpp
index 87e0f748615456..588436c3a8d211 100644
--- a/clang/test/CXX/class.derived/p2.cpp
+++ b/clang/test/CXX/class.derived/p2.cpp
@@ -7,3 +7,15 @@ namespace PR5840 {
   int Base = 10;
   struct Derived : Base {};
 }
+
+namespace issue_number {
+  struct x {};
+  namespace
+  {
+      namespace x
+      {
+          struct y : x
+          {};
+      }
+  }
+}
\ No newline at end of file