[Clang][P1061] Fix invalid pack binding crash

[ricejasonf]

Fixes #134882

Consider

struct foo { char a; int b; };
constexpr foo t{'a', 1};
constexpr auto [...m] = t;

Without the constexpr qualifier, the decomposition declaration just happens to not crash in the call to DeclMustBeEmitted because it returns early because of its "discardable gval linkage". So, the fix is in flat_bindings where we cannot assume the pack binding is valid. There is also a fix along the same vein from a suggestion made by @shafik. The tests are still building on my machine, but I thought I would submit this to get eyes on it earlier since it is trivial.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Jason Rice (ricejasonf)

Changes

Fixes #134882

Consider

struct foo { char a; int b; };
constexpr foo t{'a', 1};
constexpr auto [...m] = t;

Without the constexpr qualifier, the decomposition declaration just happens to not crash in the call to DeclMustBeEmitted because it returns early because of its "discardable gval linkage". So, the fix is in flat_bindings where we cannot assume the pack binding is valid. There is also a fix along the same vein from a suggestion made by @shafik. The tests are still building on my machine, but I thought I would submit this to get eyes on it earlier since it is trivial.

---

Full diff: https://github.com/llvm/llvm-project/pull/135129.diff

3 Files Affected:

• (modified) clang/include/clang/AST/DeclCXX.h (+1-1)
• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+2-3)
• (modified) clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp (+6)

diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index 56cec07ec0293..ba6d87f8158ab 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/clang/include/clang/AST/DeclCXX.h
@@ -4281,7 +4281,7 @@ class DecompositionDecl final
         [](BindingDecl *BD) { return BD->isParameterPack(); });
 
     Bindings = Bindings.drop_front(BeforePackBindings.size());
-    if (!Bindings.empty()) {
+    if (!Bindings.empty() && Bindings.front()->getBinding()) {
       PackBindings = Bindings.front()->getBindingPackDecls();
       Bindings = Bindings.drop_front();
     }
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index e0f7ccc4674d8..4f7410c9bf0d8 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1597,11 +1597,10 @@ Decl *TemplateDeclInstantiator::VisitDecompositionDecl(DecompositionDecl *D) {
   auto *NewDD = cast_if_present<DecompositionDecl>(
       VisitVarDecl(D, /*InstantiatingVarTemplate=*/false, &NewBindingArray));
 
-  if (!NewDD || NewDD->isInvalidDecl())
+  if (!NewDD || NewDD->isInvalidDecl()) {
     for (auto *NewBD : NewBindings)
       NewBD->setInvalidDecl();
-
-  if (OldBindingPack) {
+  } else if (OldBindingPack) {
     // Mark the bindings in the pack as instantiated.
     auto Bindings = NewDD->bindings();
     BindingDecl *NewBindingPack = *llvm::find_if(
diff --git a/clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp b/clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp
index ea94757dc66b6..1818dc699c71c 100644
--- a/clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp
+++ b/clang/test/SemaCXX/cxx2c-binding-pack-nontemplate.cpp
@@ -8,4 +8,10 @@ void decompose_array() {
   // cxx23-warning@+2 {{structured binding packs are a C++2c extension}}
   // nontemplate-error@+1 {{pack declaration outside of template}}
   auto [x, ...rest, y] = arr;
+
+  // cxx26-warning@+4 {{structured binding packs are incompatible with C++ standards before C++2c}}
+  // cxx23-warning@+3 {{structured binding packs are a C++2c extension}}
+  // nontemplate-error@+2 {{decomposition declaration cannot be declared 'constexpr'}}
+  // nontemplate-error@+1 {{pack declaration outside of template}}
+  constexpr auto [x, ...rest, y] = arr;
 }

[zyn0217]

thanks

[cor3ntin]

thanks! do you need us to merge that for you?

[ricejasonf]

Yes, please, merge it when you can.

[shafik]

Thank you for the quick fix, I left a comment on the test which should be addressed in a follow up, unless I am missing something and the coverage is elsewhere already.

[shafik]

We should have both cases tested, we want as wide test coverage as possible.

Since we just saw w/ the bug this fixes the change in semantics uncovered a bug so covering both semantics is the right thing to do.

[ricejasonf]

Sorry, I haven't had time to revisit this, but in that test we are only checking the validity of the decomposition declarations so I think the addition of constexpr to that variable should not affect the checks on the other declarations. There is plenty of coverage on decompositions in the other not "nontemplate" test file. Still, I probably did not need to step on the original test like that. I will consider adding more "invalid declaration" tests and address this when I get more time. Thank you.