[C++20] [Modules] Add mangling number for lambda in non-internal module unit context

Close https://github.com/llvm/llvm-project/issues/59513
Close https://github.com/llvm/llvm-project/issues/110146

As we discussed, this is related to ABI:
https://github.com/itanium-cxx-abi/cxx-abi/issues/186

I was intending to fix this after it gets merged into the ItaniumC++ABI
formally. But it looks like ItaniumC++ABI doesn't update it yet and
there are more issue reports for it.

Luckily Richard had a clear direction guide here though. So I think it
should be good to do this without a formal ItaniumC++ABI wording.

The diff of the patch is slightly larger than it was by a simple
refacoration to simple the control flow a little bit.

3 files changed, 75 insertions, 13 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index ceb32ee..292406f 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -292,7 +292,8 @@ Sema::getCurrentMangleNumberContext(const DeclContext *DC) {
     DataMember,
     InlineVariable,
     TemplatedVariable,
-    Concept
+    Concept,
+    NonInlineInModulePurview
   } Kind = Normal;
 
   bool IsInNonspecializedTemplate =
@@ -301,29 +302,50 @@ Sema::getCurrentMangleNumberContext(const DeclContext *DC) {
   // Default arguments of member function parameters that appear in a class
   // definition, as well as the initializers of data members, receive special
   // treatment. Identify them.
-  if (ManglingContextDecl) {
+  Kind = [&]() {
+    if (!ManglingContextDecl)
+      return Normal;
+
+    if (auto *ND = dyn_cast<NamedDecl>(ManglingContextDecl)) {
+      // See discussion in https://github.com/itanium-cxx-abi/cxx-abi/issues/186
+      //
+      // zygoloid:
+      //    Yeah, I think the only cases left where lambdas don't need a
+      //    mangling are when they have (effectively) internal linkage or appear
+      //    in a non-inline function in a non-module translation unit.
+      Module *M = ManglingContextDecl->getOwningModule();
+      if (M && M->getTopLevelModule()->isNamedModuleUnit() &&
+          ND->isExternallyVisible())
+        return NonInlineInModulePurview;
+    }
+
     if (ParmVarDecl *Param = dyn_cast<ParmVarDecl>(ManglingContextDecl)) {
       if (const DeclContext *LexicalDC
           = Param->getDeclContext()->getLexicalParent())
         if (LexicalDC->isRecord())
-          Kind = DefaultArgument;
+          return DefaultArgument;
     } else if (VarDecl *Var = dyn_cast<VarDecl>(ManglingContextDecl)) {
       if (Var->getMostRecentDecl()->isInline())
-        Kind = InlineVariable;
-      else if (Var->getDeclContext()->isRecord() && IsInNonspecializedTemplate)
-        Kind = TemplatedVariable;
-      else if (Var->getDescribedVarTemplate())
-        Kind = TemplatedVariable;
-      else if (auto *VTS = dyn_cast<VarTemplateSpecializationDecl>(Var)) {
+        return InlineVariable;
+
+      if (Var->getDeclContext()->isRecord() && IsInNonspecializedTemplate)
+        return TemplatedVariable;
+
+      if (Var->getDescribedVarTemplate())
+        return TemplatedVariable;
+
+      if (auto *VTS = dyn_cast<VarTemplateSpecializationDecl>(Var)) {
         if (!VTS->isExplicitSpecialization())
-          Kind = TemplatedVariable;
+          return TemplatedVariable;
       }
     } else if (isa<FieldDecl>(ManglingContextDecl)) {
-      Kind = DataMember;
+      return DataMember;
     } else if (isa<ImplicitConceptSpecializationDecl>(ManglingContextDecl)) {
-      Kind = Concept;
+      return Concept;
     }
-  }
+
+    return Normal;
+  }();
 
   // Itanium ABI [5.1.7]:
   //   In the following contexts [...] the one-definition rule requires closure
@@ -342,6 +364,7 @@ Sema::getCurrentMangleNumberContext(const DeclContext *DC) {
     return std::make_tuple(nullptr, nullptr);
   }
 
+  case NonInlineInModulePurview:
   case Concept:
     // Concept definitions aren't code generated and thus aren't mangled,
     // however the ManglingContextDecl is important for the purposes of
diff --git a/clang/test/Modules/lambda-in-module-purview-2.cppm b/clang/test/Modules/lambda-in-module-purview-2.cppm
new file mode 100644
index 0000000..7c971ff
--- /dev/null
+++ b/clang/test/Modules/lambda-in-module-purview-2.cppm
@@ -0,0 +1,22 @@
+// 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/use.cpp -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+
+//--- lambda.h
+auto cmp = [](auto l, auto r) {
+  return l < r;
+};
+
+//--- a.cppm
+module;
+#include "lambda.h"
+export module a;
+export using ::cmp;
+
+//--- use.cpp
+// expected-no-diagnostics
+import a;
+auto x = cmp;
diff --git a/clang/test/Modules/lambda-in-module-purview.cppm b/clang/test/Modules/lambda-in-module-purview.cppm
new file mode 100644
index 0000000..4f76543
--- /dev/null
+++ b/clang/test/Modules/lambda-in-module-purview.cppm
@@ -0,0 +1,17 @@
+// 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/use.cpp -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+
+//--- a.cppm
+export module a;
+export auto cmp = [](auto l, auto r) {
+  return l < r;
+};
+
+//--- use.cpp
+// expected-no-diagnostics
+import a;
+auto x = cmp;