[Clang] Fix the assertion condition after b8d1f3d

Thanks to the example provided by @MagentaTreehouse, I realized the
assertion I added didn't cover all valid cases like, when inheriting
from a class template specialization, the source of a synthesized
template parameter might be an implicit specialization, whose inner
function template is thus living at depth 0, for which we don’t want
it to overflow too.

Author: zyn0217

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -376,12 +376,14 @@ struct ConvertConstructorToDeductionGuideTransform {
         if (NestedPattern)
           Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
         auto [Depth, Index] = getDepthAndIndex(Param);
-        // Depth can still be 0 if FTD belongs to an explicit class template
-        // specialization with an empty template parameter list. In that case,
-        // we don't want the NewDepth to overflow, and it should remain 0.
+        // Depth can be 0 if FTD belongs to a class template specialization with
+        // an empty template parameter list (i.e. either an explicit full
+        // specialization or an implicit specialization) In that case, we don't
+        // want the NewDepth to overflow, and it should remain 0.
         assert(Depth ||
-               cast<ClassTemplateSpecializationDecl>(FTD->getDeclContext())
-                   ->isExplicitSpecialization());
+               isa<ClassTemplateDecl *>(
+                   cast<ClassTemplateSpecializationDecl>(FTD->getDeclContext())
+                       ->getSpecializedTemplateOrPartial()));
         NamedDecl *NewParam = transformTemplateParameter(
             SemaRef, DC, Param, Args, Index + Depth1IndexAdjustment,
             Depth ? Depth - 1 : 0);

clang/test/SemaTemplate/deduction-guide.cpp

@@ -723,3 +723,29 @@ void test() { NewDeleteAllocator abc(42); } // expected-error {{no viable constr
 // CHECK-NEXT:  `-ParmVarDecl {{.+}} 'T'
 
 } // namespace GH128691
+
+namespace GH132616_DeductionGuide {
+
+template <class T> struct A {
+  template <class U>
+  A(U);
+};
+
+template <typename>
+struct B: A<int> {
+  using A::A;
+};
+
+template <class T>
+B(T) -> B<T>;
+
+B b(24);
+
+// CHECK-LABEL: Dumping GH132616_DeductionGuide::<deduction guide for B>:
+// CHECK-NEXT: FunctionTemplateDecl {{.+}} implicit <deduction guide for B>
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.+}} typename depth 0 index 0
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.+}} class depth 0 index 1 U
+// CHECK-NEXT: `-CXXDeductionGuideDecl {{.+}} implicit <deduction guide for B> 'auto (U) -> B<type-parameter-0-0>'
+// CHECK-NEXT:  `-ParmVarDecl {{.+}} 'U'
+
+} // namespace GH132616_DeductionGuide