[clang] [clang] Use canonical type for substitution which might be incomplete (PR #109065)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov created https://github.com/llvm/llvm-project/pull/109065

When checking deduction consistency, a substitution can be incomplete such that only sugar parts refer to non-deduced template parameters.

This would not otherwise lead to an inconsistent deduction, so this patch makes it so we canonicalize the types before substitution in order to avoid that possibility, for now.

When we are able to produce substitution failure diagnostics for partial ordering, we might want to improve the TemplateInstantiator so that it does not fail in that case.

This fixes a regression on top of #100692, which was reported on the PR. This was never released, so there are no release notes.

>From eae5075a033c44b32d452233628a152d0e08fec8 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Tue, 17 Sep 2024 20:44:52 -0300
Subject: [PATCH] [clang] Use canonical type for substitution which might be
 incomplete

When checking deduction consistency, a substitution can be incomplete
such that only sugar parts refer to non-deduced template parameters.

This would not otherwise lead to an inconsistent deduction, so
this patch makes it so we canonicalize the types before substitution
in order to avoid that possibility, for now.

When we are able to produce substitution failure diagnostics for
partial ordering, we might want to improve the TemplateInstantiator
so that it does not fail in that case.

This fixes a regression on top of #100692, which was reported on
the PR. This was never released, so there are no release notes.
---
 clang/lib/Sema/SemaTemplateDeduction.cpp | 7 +++++--
 clang/test/SemaTemplate/GH18291.cpp      | 9 +++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index b50648d5752ce5..7d83b86a007337 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -5505,8 +5505,11 @@ static TemplateDeductionResult CheckDeductionConsistency(
   Sema::ArgumentPackSubstitutionIndexRAII PackIndex(
       S, ArgIdx != -1 ? ::getPackIndexForParam(S, FTD, MLTAL, ArgIdx) : -1);
   bool IsIncompleteSubstitution = false;
-  QualType InstP = S.SubstType(P, MLTAL, FTD->getLocation(), FTD->getDeclName(),
-                               &IsIncompleteSubstitution);
+  // FIXME: A substitution can be incomplete on a non-structural part of the
+  // type. Use the canonical type for now, until the TemplateInstantiator can
+  // deal with that.
+  QualType InstP = S.SubstType(P.getCanonicalType(), MLTAL, FTD->getLocation(),
+                               FTD->getDeclName(), &IsIncompleteSubstitution);
   if (InstP.isNull() && !IsIncompleteSubstitution)
     return TemplateDeductionResult::SubstitutionFailure;
   if (!CheckConsistency)
diff --git a/clang/test/SemaTemplate/GH18291.cpp b/clang/test/SemaTemplate/GH18291.cpp
index 820564ffa6f1a0..2e9754b6561740 100644
--- a/clang/test/SemaTemplate/GH18291.cpp
+++ b/clang/test/SemaTemplate/GH18291.cpp
@@ -112,3 +112,12 @@ namespace static_vs_nonstatic {
     }
   } // namespace explicit_obj_param
 } // namespace static_vs_nonstatic
+
+namespace incomplete_on_sugar {
+  template <unsigned P, class T> void f(T[P]) = delete;
+  template <unsigned P> void f(int[][P]);
+  void test() {
+    int array[1][8];
+    f<8>(array);
+  }
+} // namespace incomplete_on_sugar