[clang] [Sema] Avoid excessive desuraging in template deduction (PR #100144)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov created https://github.com/llvm/llvm-project/pull/100144

Not for submission yet. Will eventually aim to fix #100095. The current approach should not be correct as it would have *too* much sugar in turn. We should also make sure the final substitution arguments do not contain any sugar.

It does address the crash, though, so probably something along that diretion is needed, e.g. need to check if `decltype()` that's not type-dependent, but instantiation-dependent works correctly and follow the approach taken there.

>From 20c9e54bc2e30864592b74db6ed1eeeddeacca50 Mon Sep 17 00:00:00 2001
From: Ilya Biryukov <ibiryukov at google.com>
Date: Tue, 23 Jul 2024 17:52:19 +0200
Subject: [PATCH] [Sema] Avoid excessive desuraging in template deduction

Not for submission yet. Will eventually aim to fix #100095.
The current approach should not be correct as it would have *too* much
sugar in turn. We should also make sure the final substitution arguments
do not contain any sugar.

It does address the crash, though, so probably something along that
diretion is needed, e.g. need to check if `decltype()` that's not
type-dependent, but instantiation-dependent works correctly and follow
the approach taken there.
---
 clang/lib/Sema/SemaTemplateDeduction.cpp      | 11 ++++---
 .../test/SemaTemplate/variadic-no-mention.cpp | 30 +++++++++++++++++++
 2 files changed, 35 insertions(+), 6 deletions(-)
 create mode 100644 clang/test/SemaTemplate/variadic-no-mention.cpp

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index b7b857ebf804b..7fc595bd60225 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -688,10 +688,7 @@ DeduceTemplateSpecArguments(Sema &S, TemplateParameterList *TemplateParams,
 
   // FIXME: To preserve sugar, the TST needs to carry sugared resolved
   // arguments.
-  ArrayRef<TemplateArgument> PResolved =
-      TP->getCanonicalTypeInternal()
-          ->castAs<TemplateSpecializationType>()
-          ->template_arguments();
+  ArrayRef<TemplateArgument> PResolved = TP->template_arguments();
 
   QualType UA = A;
   std::optional<NestedNameSpecifier *> NNS;
@@ -951,10 +948,12 @@ class PackDeductionScope {
 
     // Skip over the pack elements that were expanded into separate arguments.
     // If we partially expanded, this is the number of partial arguments.
-    if (IsPartiallyExpanded)
+    if (IsPartiallyExpanded)  {
       PackElements += NumPartialPackArgs;
-    else if (IsExpanded)
+    } else if (IsExpanded) {
+      assert(FixedNumExpansions.has_value());
       PackElements += *FixedNumExpansions;
+    }
 
     for (auto &Pack : Packs) {
       if (Info.PendingDeducedPacks.size() > Pack.Index)
diff --git a/clang/test/SemaTemplate/variadic-no-mention.cpp b/clang/test/SemaTemplate/variadic-no-mention.cpp
new file mode 100644
index 0000000000000..09d860249c31d
--- /dev/null
+++ b/clang/test/SemaTemplate/variadic-no-mention.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+template <class... T>
+struct Types {};
+template <int& field>
+using Forget = int;
+template <int&... fields>
+using SeqKey = Types<Forget<fields>...>;
+
+template <typename Key, typename Value>
+struct HelperBase {
+  using ResponseParser = Key();
+  HelperBase(ResponseParser response_parser) {}
+};
+template <int&... fields>
+SeqKey<fields...> Parser();
+
+template <int&... fields>
+struct Helper : public HelperBase<SeqKey<fields...>, double> {
+  using Key = SeqKey<fields...>;
+  using Value = double;
+  using ParentClass = HelperBase<Key, Value>;
+  Helper() : ParentClass(Parser<fields...>) {}
+};
+
+void test() {
+  Helper<>();
+}
+