[Clang] [Modules] Crash when specifying `-Xclang -emit-module-interface`

[koplas]

To reproduce, set CLANG_BIN to your clang binary and run this script:

#!/bin/bash
set -e

# ==== 🔧 Configuration ====

CLANG_BIN="/usr/local/bin/clang-21" # Set this to your clang binary

OUT_DIR="./build"
PCM_DIR="$OUT_DIR/pcm"
mkdir -p "$PCM_DIR"

# main.cppm
cat <<EOF >main.cppm
export module a;
EOF

# ==== 📦 Precompile modules ====

echo "🔨 Precompiling main.cppm"
"$CLANG_BIN" -std=c++20 \
  -Xclang -emit-module-interface \
  main.cppm -o "$PCM_DIR/main.pcm"

echo "✅ Done"

It will now crash:

🔨 Precompiling main.cppm
clang-21: /home/paul/Documents/llvm-project/clang/lib/Frontend/FrontendAction.cpp:687: bool clang::FrontendAction::BeginSourceFile(CompilerInstance &, const FrontendInputFile &): Assertion `hasASTFileSupport() && "This action does not have AST file support!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /usr/local/bin/clang-21 -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -dumpdir ./build/pcm/main.pcm- -disable-free -clear-ast-before-backend -main-file-name main.cppm -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/paul/delete-me -fcoverage-compilation-dir=/home/paul/delete-me -resource-dir /usr/local/lib/clang/21 -std=c++20 -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -emit-module-interface -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/main-7a4a7d.o -x pcm /tmp/main-dc5e6d.pcm
 #0 0x000055704d652b89 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/paul/Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:804:11
 #1 0x000055704d65312b PrintStackTraceSignalHandler(void*) /home/paul/Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:880:1
 #2 0x000055704d650ebc llvm::sys::RunSignalHandlers() /home/paul/Documents/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #3 0x000055704d6538eb SignalHandler(int, siginfo_t*, void*) /home/paul/Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:418:7
 #4 0x00007f54f464bcd0 (/usr/lib/libc.so.6+0x3dcd0)
 #5 0x00007f54f46a5624 (/usr/lib/libc.so.6+0x97624)
 #6 0x00007f54f464bba0 raise (/usr/lib/libc.so.6+0x3dba0)
 #7 0x00007f54f4633582 abort (/usr/lib/libc.so.6+0x25582)
 #8 0x00007f54f46334eb __assert_perror_fail (/usr/lib/libc.so.6+0x254eb)
 #9 0x000055704e9fcedd clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /home/paul/Documents/llvm-project/clang/lib/Frontend/FrontendAction.cpp:689:50
#10 0x000055704e90b8d3 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /home/paul/Documents/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1054:9
#11 0x000055704ebe341d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /home/paul/Documents/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:300:8
#12 0x0000557048fdd198 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /home/paul/Documents/llvm-project/clang/tools/driver/cc1_main.cpp:294:13
#13 0x0000557048fced73 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) /home/paul/Documents/llvm-project/clang/tools/driver/driver.cpp:218:5
#14 0x0000557048fcdaa9 clang_main(int, char**, llvm::ToolContext const&) /home/paul/Documents/llvm-project/clang/tools/driver/driver.cpp:259:5
#15 0x0000557049004bfa main /home/paul/Documents/llvm-project/build_debug/tools/clang/tools/driver/clang-driver.cpp:17:10
#16 0x00007f54f4635488 (/usr/lib/libc.so.6+0x27488)
#17 0x00007f54f463554c __libc_start_main (/usr/lib/libc.so.6+0x2754c)
#18 0x0000557048fcd4f5 _start (/usr/local/bin/clang-21+0x1e004f5)
clang-21: error: unable to execute command: Aborted (core dumped)
clang-21: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 21.0.0git (https://github.com/llvm/llvm-project.git 64555e3d48223c4c5a851f0cbe0278ec6f2241ba)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Build config: +unoptimized, +assertions
clang-21: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-21: note: diagnostic msg: /tmp/main-fbfd35.cppm
clang-21: note: diagnostic msg: /tmp/main-fbfd35.sh
clang-21: note: diagnostic msg:

********************

I found this issue while investigating #58540.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Paul Schwabauer (koplas)

To reproduce, set `CLANG_BIN` to your clang binary and run this script: ```sh #!/bin/bash set -e

==== 🔧 Configuration ====

CLANG_BIN="/usr/local/bin/clang-21" # Set this to your clang binary

OUT_DIR="./build"
PCM_DIR="$OUT_DIR/pcm"
mkdir -p "$PCM_DIR"

a.hpp

cat <<EOF >a.hpp
EOF

b.cppm

cat <<EOF >b.cppm
export module b;
export import "a.hpp";
EOF

echo "🔨 Precompiling a.hpp header unit"
"$CLANG_BIN" -std=c++20 -fmodule-header a.hpp -o "$PCM_DIR/a.pcm"

==== 📦 Precompile modules ====

echo "🔨 Precompiling b.cppm"
"$CLANG_BIN" -std=c++20 -fcxx-modules -fmodules
-Xclang -emit-module-interface
-fmodule-file="$PCM_DIR/a.pcm"
-c b.cppm -o "$PCM_DIR/b.pcm"

echo "✅ Done"

It will now crash:

🔨 Precompiling a.hpp
🔨 Precompiling b.cppm
b.cppm:2:8: warning: the implementation of header units is in an experimental phase [-Wexperimental-header-units]
2 | export import "a.hpp";
| ^
1 warning generated.
clang-21: /home//Documents/llvm-project/clang/lib/Frontend/FrontendAction.cpp:687: bool clang::FrontendAction::BeginSourceFile(CompilerInstance &, const FrontendInputFile &): Assertion `hasASTFileSupport() && "This action does not have AST file support!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /usr/local/bin/clang-21 -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -disable-free -clear-ast-before-backend -main-file-name b.cppm -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home//delete-me -fcoverage-compilation-dir=/home/l/delete-me -resource-dir /usr/local/lib/clang/21 -std=c++20 -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fmodules -fimplicit-module-maps -fmodules-cache-path=/home//.cache/clang/ModuleCache -fmodule-file=./build/pcm/a.pcm -fmodules-validate-system-headers -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -fcolor-diagnostics -emit-module-interface -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o ./build/pcm/b.pcm -x pcm /tmp/b-8749f9.pcm
#0 0x000055fa339a4b89 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home//Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:804:11
#1 0x000055fa339a512b PrintStackTraceSignalHandler(void) /home//Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:880:1
#2 0x000055fa339a2ebc llvm::sys::RunSignalHandlers() /home//Documents/llvm-project/llvm/lib/Support/Signals.cpp:105:5
#3 0x000055fa339a58eb SignalHandler(int, siginfo_t, void) /home//Documents/llvm-project/llvm/lib/Support/Unix/Signals.inc:418:7
#4 0x00007fcd0ae4bcd0 (/usr/lib/libc.so.6+0x3dcd0)
#5 0x00007fcd0aea5624 (/usr/lib/libc.so.6+0x97624)
#6 0x00007fcd0ae4bba0 raise (/usr/lib/libc.so.6+0x3dba0)
#7 0x00007fcd0ae33582 abort (/usr/lib/libc.so.6+0x25582)
#8 0x00007fcd0ae334eb __assert_perror_fail (/usr/lib/libc.so.6+0x254eb)
#9 0x000055fa34d4eedd clang::FrontendAction::BeginSourceFile(clang::CompilerInstance&, clang::FrontendInputFile const&) /home//Documents/llvm-project/clang/lib/Frontend/FrontendAction.cpp:689:50
#10 0x000055fa34c5d8d3 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /home//Documents/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1054:9
#11 0x000055fa34f3541d clang::ExecuteCompilerInvocation(clang::CompilerInstance) /home//Documents/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:300:8
#12 0x000055fa2f32f198 cc1_main(llvm::ArrayRef<char const>, char const*, void*) /home//Documents/llvm-project/clang/tools/driver/cc1_main.cpp:294:13
#13 0x000055fa2f320d73 ExecuteCC1Tool(llvm::SmallVectorImpl<char const>&, llvm::ToolContext const&) /home//Documents/llvm-project/clang/tools/driver/driver.cpp:218:5
#14 0x000055fa2f31faa9 clang_main(int, char*, llvm::ToolContext const&) /home//Documents/llvm-project/clang/tools/driver/driver.cpp:259:5
#15 0x000055fa2f356bfa main /home//Documents/llvm-project/build_debug/tools/clang/tools/driver/clang-driver.cpp:17:10
#16 0x00007fcd0ae35488 (/usr/lib/libc.so.6+0x27488)
#17 0x00007fcd0ae3554c __libc_start_main (/usr/lib/libc.so.6+0x2754c)
#18 0x000055fa2f31f4f5 _start (/usr/local/bin/clang-21+0x1e004f5)
clang-21: error: unable to execute command: Aborted (core dumped)
clang-21: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 21.0.0git (https://github.com/llvm/llvm-project.git 64555e3)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Build config: +unoptimized, +assertions
clang-21: note: diagnostic msg:

---

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-21: note: diagnostic msg: /tmp/b-a6db61.cppm
clang-21: note: diagnostic msg: /tmp/b-a6db61.cache
clang-21: note: diagnostic msg: /tmp/b-a6db61.sh
clang-21: note: diagnostic msg:

---

I found this issue while investigating #<!-- -->58540.
</details>