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