[Clang][Sema] Extract ellipsis location from CXXFoldExpr for reattaching constraints on NTTPs

[zyn0217]

We build up a CXXFoldExpr for immediately declared constraints as per C++20 [temp.param]/4. This is done by
formImmediatelyDeclaredConstraint where an EllipsisLoc is essential to determine whether this is a pack.

On the other hand, when attempting to instantiate a class template, member templates might not be instantiated immediately, so we leave them intact. For function templates with NTTPs, we reattach constraints if possible so that they can be evaluated later. To properly form that, we attempted to extract an ellipsis location if the param per se was a parameter pack. Unfortunately, for the following NTTP case, we seemingly failed to handle:

template <Constraint auto... Pack>
void member();

The NTTPD Pack is neither an ExpandedParameterPack nor a PackExpansion (its type does not expand anything). As a result, we end up losing track of the constraints on packs, although we have them inside the associated CXXFoldExpr.

This patch fixes that by extracting the ellipsis location out of the previous constraint expression. Closes #63837.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

We build up a CXXFoldExpr for immediately declared constraints as per C++20 [temp.param]/4. This is done by
formImmediatelyDeclaredConstraint where an EllipsisLoc is essential to determine whether this is a pack.

On the other hand, when attempting to instantiate a class template, member templates might not be instantiated immediately, so we leave them intact. For function templates with NTTPs, we reattach constraints if possible so that they can be evaluated later. To properly form that, we attempted to extract an ellipsis location if the param per se was a parameter pack. Unfortunately, for the following NTTP case, we seemingly failed to handle:

template <Constraint auto... Pack>
void member();

The NTTPD Pack is neither an ExpandedParameterPack nor a PackExpansion (its type does not expand anything). As a result, we end up losing track of the constraints on packs, although we have them inside the associated CXXFoldExpr.

This patch fixes that by extracting the ellipsis location out of the previous constraint expression. Closes #63837.

---

Full diff: https://github.com/llvm/llvm-project/pull/78080.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+13-8)
• (modified) clang/test/SemaTemplate/concepts.cpp (+21)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 154412144ef97a..04b2fac2c74713 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -857,6 +857,9 @@ Bug Fixes to C++ Support
   (`#64607 <https://github.com/llvm/llvm-project/issues/64607>`_)
   (`#64086 <https://github.com/llvm/llvm-project/issues/64086>`_)
 
+- Fixed a crash where we lost uninstantiated constraints on placeholder NTTP packs. Fixes:
+  (`#63837 <https://github.com/llvm/llvm-project/issues/63837>`_)
+
 - Fixed a regression where clang forgets how to substitute into constraints on template-template
   parameters. Fixes: 
   (`#57410 <https://github.com/llvm/llvm-project/issues/57410>`_) and
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d768bb72e07c09..208a7b120c419b 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3056,16 +3056,21 @@ Decl *TemplateDeclInstantiator::VisitNonTypeTemplateParmDecl(
         D->getPosition(), D->getIdentifier(), T, D->isParameterPack(), DI);
 
   if (AutoTypeLoc AutoLoc = DI->getTypeLoc().getContainedAutoTypeLoc())
-    if (AutoLoc.isConstrained())
+    if (AutoLoc.isConstrained()) {
+      SourceLocation EllipsisLoc;
+      if (IsExpandedParameterPack)
+        EllipsisLoc =
+            DI->getTypeLoc().getAs<PackExpansionTypeLoc>().getEllipsisLoc();
+      else if (auto *Constraint = dyn_cast_if_present<CXXFoldExpr>(
+                   D->getPlaceholderTypeConstraint()))
+        EllipsisLoc = Constraint->getEllipsisLoc();
       // Note: We attach the uninstantiated constriant here, so that it can be
-      // instantiated relative to the top level, like all our other constraints.
-      if (SemaRef.AttachTypeConstraint(
-              AutoLoc, Param, D,
-              IsExpandedParameterPack
-                ? DI->getTypeLoc().getAs<PackExpansionTypeLoc>()
-                    .getEllipsisLoc()
-                : SourceLocation()))
+      // instantiated relative to the top level, like all our other
+      // constraints.
+      if (SemaRef.AttachTypeConstraint(AutoLoc, /*NewConstrainedParm=*/Param,
+                                       /*OrigConstrainedParm=*/D, EllipsisLoc))
         Invalid = true;
+    }
 
   Param->setAccess(AS_public);
   Param->setImplicit(D->isImplicit());
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index e98ebcc9203a43..bac209a28da912 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -1064,3 +1064,24 @@ void cand(T t)
 
 void test() { cand(42); }
 }
+
+namespace GH63837 {
+
+template<class> concept IsFoo = true;
+
+template<class> struct Struct {
+  template<IsFoo auto... xs>
+  void foo() {}
+
+  template<auto... xs> requires (... && IsFoo<decltype(xs)>)
+  void bar() {}
+
+  template<IsFoo auto... xs>
+  static inline int field = 0;
+};
+
+template void Struct<void>::foo<>();
+template void Struct<void>::bar<>();
+template int Struct<void>::field<1, 2>;
+
+}