[clang] [clang-repl] Fix module contamination after failed template instantiation (PR #204152)

[Anutosh Bhat via cfe-commits]

https://github.com/anutosh491 updated https://github.com/llvm/llvm-project/pull/204152

>From ef6f947625ee81aa81d00eaac1f6b596f9a1d8f8 Mon Sep 17 00:00:00 2001
From: anutosh491 <andersonbhat491 at gmail.com>
Date: Tue, 16 Jun 2026 19:01:28 +0530
Subject: [PATCH] Fix module contamination after failed template instantiation

---
 clang/lib/Interpreter/IncrementalAction.cpp     |  6 ++++--
 clang/lib/Interpreter/IncrementalAction.h       |  2 ++
 clang/lib/Interpreter/IncrementalParser.cpp     |  4 +++-
 .../recover-template-instantiation.cpp          | 17 +++++++++++++++++
 4 files changed, 26 insertions(+), 3 deletions(-)
 create mode 100644 clang/test/Interpreter/recover-template-instantiation.cpp

diff --git a/clang/lib/Interpreter/IncrementalAction.cpp b/clang/lib/Interpreter/IncrementalAction.cpp
index d22031c8fa893..f6bd75a5d90ed 100644
--- a/clang/lib/Interpreter/IncrementalAction.cpp
+++ b/clang/lib/Interpreter/IncrementalAction.cpp
@@ -53,7 +53,8 @@ IncrementalAction::IncrementalAction(CompilerInstance &Instance,
         }
         return Act;
       }()),
-      Interp(I), CI(Instance), Consumer(std::move(Consumer)) {}
+      Interp(I), CI(Instance), LLVMCtx(LLVMCtx), Consumer(std::move(Consumer)) {
+}
 
 std::unique_ptr<ASTConsumer>
 IncrementalAction::CreateASTConsumer(CompilerInstance & /*CI*/,
@@ -114,7 +115,8 @@ std::unique_ptr<llvm::Module> IncrementalAction::GenModule() {
              CachedInCodeGenModule->ifunc_empty())) &&
            "CodeGen wrote to a readonly module");
     std::unique_ptr<llvm::Module> M(CG->ReleaseModule());
-    CG->StartModule("incr_module_" + std::to_string(ID++), M->getContext());
+    CG->StartModule("incr_module_" + std::to_string(ID++),
+                    M ? M->getContext() : LLVMCtx);
     return M;
   }
   return nullptr;
diff --git a/clang/lib/Interpreter/IncrementalAction.h b/clang/lib/Interpreter/IncrementalAction.h
index 725cdd0c27cf4..27daf15d8c91e 100644
--- a/clang/lib/Interpreter/IncrementalAction.h
+++ b/clang/lib/Interpreter/IncrementalAction.h
@@ -13,6 +13,7 @@
 #include "clang/Frontend/MultiplexConsumer.h"
 
 namespace llvm {
+class LLVMContext;
 class Module;
 }
 
@@ -35,6 +36,7 @@ class IncrementalAction : public WrapperFrontendAction {
   bool IsTerminating = false;
   Interpreter &Interp;
   [[maybe_unused]] CompilerInstance &CI;
+  llvm::LLVMContext &LLVMCtx;
   std::unique_ptr<ASTConsumer> Consumer;
 
   /// When CodeGen is created the first llvm::Module gets cached in many places
diff --git a/clang/lib/Interpreter/IncrementalParser.cpp b/clang/lib/Interpreter/IncrementalParser.cpp
index f6d2779d64b2b..b3af640f3cb21 100644
--- a/clang/lib/Interpreter/IncrementalParser.cpp
+++ b/clang/lib/Interpreter/IncrementalParser.cpp
@@ -85,7 +85,9 @@ IncrementalParser::ParseOrWrapTopLevelDecl() {
   DiagnosticsEngine &Diags = S.getDiagnostics();
   if (Diags.hasErrorOccurred()) {
     CleanUpPTU(C.getTranslationUnitDecl());
-
+    // Discard any partial CodeGen state to avoid contaminating the next cell.
+    Consumer->HandleTranslationUnit(C);
+    Act->GenModule();
     Diags.Reset(/*soft=*/true);
     Diags.getClient()->clear();
     return llvm::make_error<llvm::StringError>("Parsing failed.",
diff --git a/clang/test/Interpreter/recover-template-instantiation.cpp b/clang/test/Interpreter/recover-template-instantiation.cpp
new file mode 100644
index 0000000000000..7f67ef5f131f8
--- /dev/null
+++ b/clang/test/Interpreter/recover-template-instantiation.cpp
@@ -0,0 +1,17 @@
+// REQUIRES: host-supports-jit
+// RUN: cat %s | clang-repl 2>&1 | FileCheck %s
+
+// Verify that clang-repl recovers cleanly after a deferred template
+// instantiation error. The failed cell must not contaminate the CodeGen module
+// used by subsequent cells.
+
+template <typename T> T f(T a) { static_assert(sizeof(T) == 0, "unsupported type"); return a; }
+
+f(1.0);
+// CHECK: error: static assertion failed
+
+int x = 10;
+x
+// CHECK: (int) 10
+
+%quit