From 2ad26487b6a1a8933b6b5be9cbd59a6da4220185 Mon Sep 17 00:00:00 2001
From: Boaz Brickner <brickner@google.com>
Date: Thu, 13 Nov 2025 21:53:03 +0100
Subject: [PATCH] C++ Interop: Don't crash when trying to call a C++ function
 with undeduced return type (#6363)

This crashed on trying to build a C++ thunk because the error wasn't
propagated.

Part of #5436.
---
 toolchain/check/cpp/import.cpp                |  3 +++
 .../interop/cpp/function/return.carbon        | 20 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/toolchain/check/cpp/import.cpp b/toolchain/check/cpp/import.cpp
index a2c06bcdb4aa..0a2a8a89ca50 100644
--- a/toolchain/check/cpp/import.cpp
+++ b/toolchain/check/cpp/import.cpp
@@ -1546,6 +1546,9 @@ static auto GetReturnPattern(Context& context, SemIR::LocId loc_id,
     // void.
     return SemIR::InstId::None;
   }
+  if (type_inst_id == SemIR::ErrorInst::TypeInstId) {
+    return SemIR::ErrorInst::InstId;
+  }
   auto pattern_type_id = GetPatternType(context, type_id);
   clang::SourceLocation return_type_loc =
       clang_decl->getReturnTypeSourceRange().getBegin();
diff --git a/toolchain/check/testdata/interop/cpp/function/return.carbon b/toolchain/check/testdata/interop/cpp/function/return.carbon
index de70ef916347..c01773e4108f 100644
--- a/toolchain/check/testdata/interop/cpp/function/return.carbon
+++ b/toolchain/check/testdata/interop/cpp/function/return.carbon
@@ -35,6 +35,26 @@ fn F() {
   //@dump-sem-ir-end
 }
 
+// ============================================================================
+// Undeduced return type
+// ============================================================================
+
+// --- fail_return_undeduced_auto.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+auto UndeducedReturn();
+''';
+
+fn Call() {
+  // CHECK:STDERR: fail_return_undeduced_auto.carbon:[[@LINE+4]]:3: error: semantics TODO: `Unsupported: return type: auto` [SemanticsTodo]
+  // CHECK:STDERR:   Cpp.UndeducedReturn();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  Cpp.UndeducedReturn();
+}
+
 // CHECK:STDOUT: --- import_multiple.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {