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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/109065.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+5-2) 
- (modified) clang/test/SemaTemplate/GH18291.cpp (+9) 


``````````diff
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

``````````

</details>


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