[clang] [Clang] SFINAE on mismatching pack length during constraint satisfaction checking (PR #101879)

[via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/101879

If a fold expanded constraint would expand packs of different size, it is not a valid pack expansion and it is not satisfied. This should not produce an error.

Fixes #99430

>From 665047d94935c71230322bb4a5951a9a567a64bb Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Sun, 4 Aug 2024 12:14:31 +0200
Subject: [PATCH] [Clang] SFINAE on mismatching pack lenght during constraint
 satisfaction checking

If a fold expanded constraint would expand packs of different size,
it is not a valid pack expansion and it is not satisfied.
This should not produce an error.

Fixes #99430
---
 clang/lib/Sema/SemaConcept.cpp          |  4 ++++
 clang/test/SemaCXX/cxx2c-fold-exprs.cpp | 30 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 9e16b67284be4..c34d32002b5ad 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -531,6 +531,10 @@ static ExprResult calculateConstraintSatisfaction(
 
     std::optional<unsigned>
     EvaluateFoldExpandedConstraintSize(const CXXFoldExpr *FE) const {
+
+      // We should ignore errors in the presence of packs of different size.
+      Sema::SFINAETrap Trap(S);
+
       Expr *Pattern = FE->getPattern();
 
       SmallVector<UnexpandedParameterPack, 2> Unexpanded;
diff --git a/clang/test/SemaCXX/cxx2c-fold-exprs.cpp b/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
index 1e0bc7bcfb4e7..0674135aac483 100644
--- a/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
+++ b/clang/test/SemaCXX/cxx2c-fold-exprs.cpp
@@ -275,3 +275,33 @@ static_assert(S<int>::g<int>() == 2); // expected-error {{call to 'g' is ambiguo
 
 
 }
+
+namespace GH99430 {
+
+template <class _Ty1, class _Ty2>
+using _Synth_three_way_result = int;
+
+template <class... _Types>
+class tuple;
+
+template <int _Index>
+struct tuple_element;
+
+template <class, int...>
+struct _Three_way_comparison_result_with_tuple_like {
+  using type = int;
+};
+
+template <class... _TTypes, int... _Indices>
+  requires(requires {
+    typename _Synth_three_way_result<_TTypes, tuple_element<_Indices>>;
+  } && ...)
+
+struct _Three_way_comparison_result_with_tuple_like<tuple<_TTypes...>, _Indices...>{
+    using type = long;
+};
+
+static_assert(__is_same_as(_Three_way_comparison_result_with_tuple_like<tuple<int>, 0, 1>::type, int));
+static_assert(__is_same_as(_Three_way_comparison_result_with_tuple_like<tuple<int>, 0>::type, long));
+
+}