From b42300cfa33d2a47205978db91a539ee174d8907 Mon Sep 17 00:00:00 2001
From: David Blaikie <dblaikie@gmail.com>
Date: Sat, 6 Jun 2026 05:07:33 +1000
Subject: [PATCH] Ensure exported entities aren't remapped/duplicated (#7304)

This ensures the clang_decls map is used as a cache - without this,
visiting the same entity twice could cause it to be
re-exported/duplicated. See attached test case.
---
 toolchain/check/cpp/generate_ast.cpp          | 25 ++++++++++++++++---
 .../interop/cpp/function/export/alias.carbon  | 23 +++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)
 create mode 100644 toolchain/check/testdata/interop/cpp/function/export/alias.carbon

diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index 2e21f243dda2..c62c85d75d9e 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -446,15 +446,34 @@ auto CarbonExternalASTSource::MapInstIdToClangDeclOrType(LookupResult lookup)
 auto CarbonExternalASTSource::GetOrExportFunctionToCpp(
     SemIR::InstId target_inst_id, SemIR::FunctionId function_id)
     -> clang::FunctionDecl* {
-  const SemIR::Function& function = context_->functions().Get(function_id);
+  SemIR::Function& function = context_->functions().Get(function_id);
   auto clang_decl_id = context_->clang_decls().Lookup(function.first_decl_id());
   if (clang_decl_id.has_value()) {
     return cast<clang::FunctionDecl>(
         context_->clang_decls().Get(clang_decl_id).key.decl);
   }
 
-  return ExportFunctionToCpp(*context_, SemIR::LocId(target_inst_id),
-                             function_id);
+  auto* clang_function_decl =
+      ExportFunctionToCpp(*context_, SemIR::LocId(target_inst_id), function_id);
+
+  if (!clang_function_decl) {
+    return nullptr;
+  }
+
+  SemIR::ClangDeclSignature thunk_signature;
+  thunk_signature.kind = SemIR::ClangDeclSignature::Normal;
+  thunk_signature.num_params =
+      static_cast<int32_t>(clang_function_decl->getNumParams());
+  thunk_signature.passing_modes.assign(
+      thunk_signature.num_params,
+      SemIR::ClangDeclSignature::PassingMode::ByValue);
+  context_->clang_decls().Add(
+      {.key = SemIR::ClangDeclKey::ForFunctionDecl(
+           clang_function_decl,
+           context_->clang_decl_signatures().Add(std::move(thunk_signature))),
+       .inst_id = function.first_decl_id()});
+
+  return clang_function_decl;
 }
 
 auto CarbonExternalASTSource::BuildCarbonNamespace() -> void {
diff --git a/toolchain/check/testdata/interop/cpp/function/export/alias.carbon b/toolchain/check/testdata/interop/cpp/function/export/alias.carbon
new file mode 100644
index 000000000000..b5c20264835d
--- /dev/null
+++ b/toolchain/check/testdata/interop/cpp/function/export/alias.carbon
@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/function/export/alias.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/function/export/alias.carbon
+
+// --- alias_identical_address.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+fn F();
+alias G = F;
+
+inline Cpp '''
+static_assert(&Carbon::F == &Carbon::G);
+''';