zyn0217 wrote:
I remembered @LYP951018 shared me with a case involving PackIndexingExprs that
was incorrectly rejected due to the mishandling of PackExpansionExprs:
```cpp
namespace init_capture_pack {
void init_capture() {
auto L = [](auto... x) {
return [x...](auto... y) {
return [... w = y]() {
return w...[3];
};
};
};
static_assert(L()(0, 1, 2, 3)() == 3);
}
```
I have prepared for a fix on my branch
https://github.com/zyn0217/llvm-project/commit/72fc2fcf472066a78c1496119aabf23a93c06ca0,
which somehow relies on this "the unexpanded pack should have size 1" feature.
I applied this PR locally only to find we ran into a crash with the case
afterward.
```
bool clang::Sema::DiagnoseUnexpandedParameterPack(Expr *,
UnexpandedParameterPackContext): Assertion `!Unexpanded.empty() && "Unable to
find unexpanded parameter packs"' 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: ../llvm-project-Build/BuildDebug/bin/clang
-fsyntax-only -fno-crash-diagnostics -std=c++20 /clangd-test/pack-indexing.cpp
-DCRASH
1. /clangd-test/pack-indexing.cpp:11:19: current parser token ')'
2. /clangd-test/pack-indexing.cpp:1:1: parsing namespace
'init_capture_pack'
3. /clangd-test/pack-indexing.cpp:3:21: parsing function body
'init_capture_pack::init_capture'
4. /clangd-test/pack-indexing.cpp:3:21: in compound statement ('{}')
5. /clangd-test/pack-indexing.cpp:4:12: instantiating function definition
'init_capture_pack::init_capture()::(anonymous class)::operator()<>'
#11 0x00007f68109c9dad
clang::Sema::DiagnoseUnexpandedParameterPack(clang::Expr*,
clang::Sema::UnexpandedParameterPackContext)
/repo/llvm-project/clang/lib/Sema/SemaTemplateVariadic.cpp:427:43
#12 0x00007f681049bdd1 clang::Sema::BuildReturnStmt(clang::SourceLocation,
clang::Expr*, bool) /repo/llvm-project/clang/lib/Sema/SemaStmt.cpp:3782:7
#13 0x00007f681091c6a3 clang::TreeTransform<(anonymous
namespace)::TemplateInstantiator>::RebuildReturnStmt(clang::SourceLocation,
clang::Expr*) /repo/llvm-project/clang/lib/Sema/TreeTransform.h:1501:22
#14 0x00007f68109045d7 clang::TreeTransform<(anonymous
namespace)::TemplateInstantiator>::TransformReturnStmt(clang::ReturnStmt*)
/repo/llvm-project/clang/lib/Sema/TreeTransform.h:8288:23
#15 0x00007f68108be03e clang::TreeTransform<(anonymous
namespace)::TemplateInstantiator>::TransformStmt(clang::Stmt*,
clang::TreeTransform<(anonymous
namespace)::TemplateInstantiator>::StmtDiscardKind)
/repo/llvm-project-Build/BuildDebug/tools/clang/include/clang/AST/StmtNodes.inc:926:1
#16 0x00007f68108dabbb clang::TreeTransform<(anonymous
namespace)::TemplateInstantiator>::TransformCompoundStmt(clang::CompoundStmt*,
bool) /repo/llvm-project/clang/lib/Sema/TreeTransform.h:7859:38
```
https://github.com/llvm/llvm-project/pull/101385
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