[clang] [C++20] [Modules] Warn for importing implementation partition unit in interface units (PR #108493)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 created https://github.com/llvm/llvm-project/pull/108493

Recently, there are multiple false positive issue reports about the reachability of implementation partition units:
- https://github.com/llvm/llvm-project/issues/105882
- https://github.com/llvm/llvm-project/issues/101348
- https://lists.isocpp.org/core/2024/08/16232.php

And according to our use experience for modules, we find it is a pretty good practice to not import implementation partition units in the interface units. It can help developers to have a pretty good mental model for when to use an implementation partition unit: that any unit in the module but not in the module interfaces can be in the implementation partition unit.

So I think it is good to add the diagnostics.

>From 2ce2ed721f1ee985aa6163331bcb1f2f666a3b62 Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Fri, 13 Sep 2024 13:18:43 +0800
Subject: [PATCH] [C++20] [Modules] Warn for importing implementation partition
 unit in interface units

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 3 +++
 clang/lib/Sema/SemaModule.cpp                    | 5 +++++
 clang/test/CXX/module/module.import/p2.cpp       | 3 +--
 clang/test/Modules/cxx20-10-3-ex1.cpp            | 1 +
 4 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index efdc058edca56d..7545a06123789d 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -439,6 +439,9 @@ def warn_deprecated_literal_operator_id: Warning<
   "is deprecated">, InGroup<DeprecatedLiteralOperator>, DefaultIgnore;
 def warn_reserved_module_name : Warning<
   "%0 is a reserved name for a module">, InGroup<ReservedModuleIdentifier>;
+def warn_import_implementation_partition_unit_in_interface_unit : Warning<
+  "it is not suggested to import implementation partition unit in interface unit">,
+  InGroup<DiagGroup<"import-implementation-partition-unit-in-interface-unit">>;
 
 def warn_parameter_size: Warning<
   "%0 is a large (%1 bytes) pass-by-value argument; "
diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 3b84e7bd4277fd..5748906748a44e 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -650,6 +650,11 @@ DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
   else
     VisibleModules.setVisible(Mod, ImportLoc);
 
+  assert((!Mod->isModulePartitionImplementation() || getCurrentModule()) &&
+         "We can only import a partition unit in a named module.");
+  if (Mod->isModulePartitionImplementation() && getCurrentModule()->isModuleInterfaceUnit())
+    Diag(ImportLoc, diag::warn_import_implementation_partition_unit_in_interface_unit);
+
   checkModuleImportContext(*this, Mod, ImportLoc, CurContext);
 
   // FIXME: we should support importing a submodule within a different submodule
diff --git a/clang/test/CXX/module/module.import/p2.cpp b/clang/test/CXX/module/module.import/p2.cpp
index ef6006811e7763..3ac76856c7cfc3 100644
--- a/clang/test/CXX/module/module.import/p2.cpp
+++ b/clang/test/CXX/module/module.import/p2.cpp
@@ -30,9 +30,8 @@ void test() {
 }
 
 //--- UseInPartA.cppm
-// expected-no-diagnostics
 export module M:partA;
-import :impl;
+import :impl; // expected-warning {{it is not suggested to import implementation partition unit in interface unit}}
 void test() {
   A a;
 }
diff --git a/clang/test/Modules/cxx20-10-3-ex1.cpp b/clang/test/Modules/cxx20-10-3-ex1.cpp
index 99b88c7e442ffd..dcdc92340366ae 100644
--- a/clang/test/Modules/cxx20-10-3-ex1.cpp
+++ b/clang/test/Modules/cxx20-10-3-ex1.cpp
@@ -37,6 +37,7 @@ module M:PartImpl;
 export module M;
                      // error: exported partition :Part is an implementation unit
 export import :PartImpl; // expected-error {{module partition implementations cannot be exported}}
+                         // expected-warning at -1 {{it is not suggested to import implementation partition unit in interface unit}}
 
 //--- std10-3-ex1-tu3.cpp
 export module M:Part;