[clang] [CIR] Support more declarations without any codegen (PR #151076)

[Morris Hafner via cfe-commits]

https://github.com/mmha created https://github.com/llvm/llvm-project/pull/151076

This patch adds or completes support for a couple of top level declaration types  that don't emit any code: Most notably these include Concepts, static_assert and type aliases.

>From fb52c398ad796aa3728b7f7c8edaf4de9776ada9 Mon Sep 17 00:00:00 2001
From: Morris Hafner <mhafner at nvidia.com>
Date: Tue, 29 Jul 2025 04:54:42 +0200
Subject: [PATCH] [CIR] Support more declarations without any codegen

This patch adds or completes support for a couple of top level declaration types  that don't emit any code: Most notably these include Concepts, static_assert and type aliases.
---
 clang/lib/CIR/CodeGen/CIRGenModule.cpp |  7 ++++++-
 clang/test/CIR/CodeGen/empty.cpp       | 27 ++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/CIR/CodeGen/empty.cpp

diff --git a/clang/lib/CIR/CodeGen/CIRGenModule.cpp b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
index 350270518156e..6f32c9e2e2f8d 100644
--- a/clang/lib/CIR/CodeGen/CIRGenModule.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenModule.cpp
@@ -1267,8 +1267,13 @@ void CIRGenModule::emitTopLevelDecl(Decl *decl) {
     break;
 
   // No code generation needed.
-  case Decl::UsingShadow:
+  case Decl::ClassTemplate:
+  case Decl::Concept:
   case Decl::Empty:
+  case Decl::FunctionTemplate:
+  case Decl::StaticAssert:
+  case Decl::TypeAliasTemplate:
+  case Decl::UsingShadow:
     break;
 
   case Decl::CXXConstructor:
diff --git a/clang/test/CIR/CodeGen/empty.cpp b/clang/test/CIR/CodeGen/empty.cpp
new file mode 100644
index 0000000000000..7e61f4b1211a3
--- /dev/null
+++ b/clang/test/CIR/CodeGen/empty.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s --check-prefix=CIR
+
+// These declarations shouldn't emit any code. Therefore the module is expected to be empty.
+
+template<typename T>
+concept some_concept = true;
+
+template<some_concept T>
+class class_template {};
+
+; // Empty declaration
+
+template<typename T>
+void function_template();
+
+static_assert(true, "top level static assert");
+
+template<typename T>
+using type_alias = T;
+
+namespace N {
+    using ::class_template; // UsingShadow
+}
+
+// CIR: module {{.*}} {
+// CIR-NEXT: }
\ No newline at end of file