[clang] Support C++20 Modules in clang-repl (PR #79261)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 created https://github.com/llvm/llvm-project/pull/79261

This comes from when I playing around clang-repl with moduels : )

I succeeded to import std with https://libcxx.llvm.org/Modules.html and calling `std::printf` after this patch.

I want to put the documentation part to https://clang.llvm.org/docs/StandardCPlusPlusModules.html in a separate commit.

>From 70cb9ac882605c3557dc41ea13a4b8945c59e9c2 Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Wed, 24 Jan 2024 14:21:25 +0800
Subject: [PATCH] Support C++20 Modules in clang-repl

---
 clang/docs/ReleaseNotes.rst               |  2 ++
 clang/include/clang/Basic/LangOptions.def |  2 +-
 clang/lib/Serialization/ASTReaderDecl.cpp |  6 ++---
 clang/test/Interpreter/cxx20-modules.cppm | 31 +++++++++++++++++++++++
 4 files changed, 37 insertions(+), 4 deletions(-)
 create mode 100644 clang/test/Interpreter/cxx20-modules.cppm

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index fc7511e9136734..db3d74e124e7d1 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -191,6 +191,8 @@ Crash and bug fixes
 Improvements
 ^^^^^^^^^^^^
 
+- Support importing C++20 modules in clang-repl.
+
 Moved checkers
 ^^^^^^^^^^^^^^
 
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 8fc75e1cca0399..1e671a7c460163 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -485,7 +485,7 @@ VALUE_LANGOPT(FuchsiaAPILevel, 32, 0, "Fuchsia API level")
 // on large _BitInts.
 BENIGN_VALUE_LANGOPT(MaxBitIntWidth, 32, 128, "Maximum width of a _BitInt")
 
-LANGOPT(IncrementalExtensions, 1, 0, " True if we want to process statements"
+COMPATIBLE_LANGOPT(IncrementalExtensions, 1, 0, " True if we want to process statements"
         "on the global scope, ignore EOF token and continue later on (thus "
         "avoid tearing the Lexer and etc. down). Controlled by "
         "-fincremental-extensions.")
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index a149d82153037f..867f4c47eaeceb 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3286,10 +3286,10 @@ DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
   if (auto *OID = dyn_cast<ObjCInterfaceDecl>(DC))
     return OID->getDefinition();
 
-  // We can see the TU here only if we have no Sema object. In that case,
-  // there's no TU scope to look in, so using the DC alone is sufficient.
+  // We can see the TU here only if we have no Sema object. It is possible
+  // we're in clang-repl so we still need to get the primary context.
   if (auto *TU = dyn_cast<TranslationUnitDecl>(DC))
-    return TU;
+    return TU->getPrimaryContext();
 
   return nullptr;
 }
diff --git a/clang/test/Interpreter/cxx20-modules.cppm b/clang/test/Interpreter/cxx20-modules.cppm
new file mode 100644
index 00000000000000..bc2b722f6b5197
--- /dev/null
+++ b/clang/test/Interpreter/cxx20-modules.cppm
@@ -0,0 +1,31 @@
+// UNSUPPORTED: system-aix
+//
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang -std=c++20 %t/mod.cppm --precompile \
+// RUN:     -o %t/mod.pcm
+// RUN: %clang %t/mod.pcm -c -o %t/mod.o
+// RUN: %clang -shared %t/mod.o -o %t/libmod.so
+//
+// RUN: cat %t/import.cpp | env LD_LIBRARY_PATH=%t:$LD_LIBRARY_PATH \
+// RUN:     clang-repl -Xcc=-std=c++20 -Xcc=-fmodule-file=M=%t/mod.pcm \
+// RUN:     | FileCheck %t/import.cpp
+
+//--- mod.cppm
+export module M;
+export const char* Hello() {
+    return "Hello Interpreter for Modules!";
+}
+
+//--- import.cpp
+
+%lib libmod.so
+
+import M;
+
+extern "C" int printf(const char *, ...);
+printf("%s\n", Hello());
+
+// CHECK: Hello Interpreter for Modules!