[Clang] Fix a partial ordering bug involving CTAD injected template arguments (#149782)

The synthesized deduction guides use injected template arguments for
distinguishment of explicit and implicit deduction guides.

In partial ordering, we may substitute into these injected types when
checking consistency. Properly substituting them needs the instantiated
class template specializations which isn't the case at that point. So
instead, we check their template specialization types.

No release note because I think we want a backport, after baking it for
a couple of days.

Fixes #134613

(cherry picked from commit 07faafe)

Author: zyn0217

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5525,6 +5525,15 @@ static TemplateDeductionResult CheckDeductionConsistency(
   // 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.
+
+  // Workaround: Implicit deduction guides use InjectedClassNameTypes, whereas
+  // the explicit guides don't. The substitution doesn't transform these types,
+  // so let it transform their specializations instead.
+  bool IsDeductionGuide = isa<CXXDeductionGuideDecl>(FTD->getTemplatedDecl());
+  if (IsDeductionGuide) {
+    if (auto *Injected = P->getAs<InjectedClassNameType>())
+      P = Injected->getInjectedSpecializationType();
+  }
   QualType InstP = S.SubstType(P.getCanonicalType(), MLTAL, FTD->getLocation(),
                                FTD->getDeclName(), &IsIncompleteSubstitution);
   if (InstP.isNull() && !IsIncompleteSubstitution)
@@ -5539,9 +5548,15 @@ static TemplateDeductionResult CheckDeductionConsistency(
   if (auto *PA = dyn_cast<PackExpansionType>(A);
       PA && !isa<PackExpansionType>(InstP))
     A = PA->getPattern();
-  if (!S.Context.hasSameType(
-          S.Context.getUnqualifiedArrayType(InstP.getNonReferenceType()),
-          S.Context.getUnqualifiedArrayType(A.getNonReferenceType())))
+  auto T1 = S.Context.getUnqualifiedArrayType(InstP.getNonReferenceType());
+  auto T2 = S.Context.getUnqualifiedArrayType(A.getNonReferenceType());
+  if (IsDeductionGuide) {
+    if (auto *Injected = T1->getAs<InjectedClassNameType>())
+      T1 = Injected->getInjectedSpecializationType();
+    if (auto *Injected = T2->getAs<InjectedClassNameType>())
+      T2 = Injected->getInjectedSpecializationType();
+  }
+  if (!S.Context.hasSameType(T1, T2))
     return TemplateDeductionResult::NonDeducedMismatch;
   return TemplateDeductionResult::Success;
 }

clang/test/SemaTemplate/deduction-guide.cpp

@@ -966,3 +966,19 @@ Expand<Type, Invocable<>> _{};
 // CHECK-NEXT:   | `-ParmVarDecl {{.+}} 'T...' pack
 
 }
+
+namespace GH134613 {
+template <typename R> struct Foo {
+  using value_type = R;
+
+  Foo() = default;
+  Foo(Foo<Foo<R>> &&rhs) {}
+};
+
+void main() {
+  auto r1 = Foo(Foo<Foo<int>>{});
+
+  static_assert(__is_same(decltype(r1)::value_type, int));
+}
+
+}