[clang] [C++20] [Modules] Merge lambdas from Sema to imported one (PR #103036)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

<details>
<summary>Changes</summary>

Close https://github.com/llvm/llvm-project/issues/102721

Previously we tried to merge lambdas between TUs. But we may still meet problems if we import and then include the headers containing the same lambda.

The solution in this patch is, when we assigned a lambda numbering in a sema, try to see if there is already a same lambda imported. If yes, try to merge them.

---
Full diff: https://github.com/llvm/llvm-project/pull/103036.diff


3 Files Affected:

- (modified) clang/lib/Serialization/ASTReader.cpp (-11) 
- (modified) clang/lib/Serialization/ASTReaderDecl.cpp (+28) 
- (added) clang/test/Modules/pr102721.cppm (+33) 


``````````diff
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 511e2df7ad3230..4e4f1780003c5c 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -8962,17 +8962,6 @@ void ASTReader::ReadLateParsedTemplates(
   LateParsedTemplates.clear();
 }
 
-void ASTReader::AssignedLambdaNumbering(const CXXRecordDecl *Lambda) {
-  if (Lambda->getLambdaContextDecl()) {
-    // Keep track of this lambda so it can be merged with another lambda that
-    // is loaded later.
-    LambdaDeclarationsForMerging.insert(
-        {{Lambda->getLambdaContextDecl()->getCanonicalDecl(),
-          Lambda->getLambdaIndexInContext()},
-         const_cast<CXXRecordDecl *>(Lambda)});
-  }
-}
-
 void ASTReader::LoadSelector(Selector Sel) {
   // It would be complicated to avoid reading the methods anyway. So don't.
   ReadMethodPool(Sel);
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index a9199f7e50f5dc..c8a0791bb89502 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -4506,6 +4506,34 @@ void ASTReader::loadObjCCategories(GlobalDeclID ID, ObjCInterfaceDecl *D,
   ModuleMgr.visit(Visitor);
 }
 
+void ASTReader::AssignedLambdaNumbering(const CXXRecordDecl *Lambda) {
+  if (Lambda->getLambdaContextDecl()) {
+    auto ContextAndIndexPair = std::make_pair<const Decl *, unsigned>(
+        Lambda->getLambdaContextDecl()->getCanonicalDecl(),
+        Lambda->getLambdaIndexInContext());
+
+    auto Iter = LambdaDeclarationsForMerging.find(ContextAndIndexPair);
+    // If we import first and then include second, try to merge the second
+    // one with the first one.
+    if (Iter != LambdaDeclarationsForMerging.end() &&
+        Iter->second->isFromASTFile()) {
+      NamedDecl *Slot = Iter->second;
+      ASTDeclMerger Merger(*this);
+      // The lambda shouldn't be a key declaration as the lambda wouldn't
+      // be a canonical decl for its owning module.
+      Merger.mergeRedeclarableImpl(const_cast<CXXRecordDecl *>(Lambda),
+                                   cast<TagDecl>(Slot),
+                                   /*KeyDeclID*/ GlobalDeclID());
+      return;
+    }
+
+    // Keep track of this lambda so it can be merged with another lambda that
+    // is loaded later.
+    LambdaDeclarationsForMerging.insert(
+        {ContextAndIndexPair, const_cast<CXXRecordDecl *>(Lambda)});
+  }
+}
+
 template<typename DeclT, typename Fn>
 static void forAllLaterRedecls(DeclT *D, Fn F) {
   F(D);
diff --git a/clang/test/Modules/pr102721.cppm b/clang/test/Modules/pr102721.cppm
new file mode 100644
index 00000000000000..6a84393bcbd1f6
--- /dev/null
+++ b/clang/test/Modules/pr102721.cppm
@@ -0,0 +1,33 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-module-interface -o %t/b.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/test.cc -fsyntax-only -verify \
+// RUN:   -fprebuilt-module-path=%t
+
+//--- foo.h
+inline auto x = []{};
+
+//--- a.cppm
+module;
+#include "foo.h"
+export module a;
+export using ::x;
+
+//--- b.cppm
+module;
+import a;
+#include "foo.h"
+export module b;
+export using ::x;
+
+//--- test.cc
+// expected-no-diagnostics
+import a;
+import b;
+void test() {
+  x();
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/103036