[Clang] Fix an integer overflow issue in computing CTAD's parameter depth (llvm#128704)

There were some cases where we computed incorrect template parameter
depths for synthesized CTAD, invalid as they might be, we still
shouldn't crash anyway.

Technically the only scenario in which the inner function template's
depth is 0 is when it lives within an explicit template specialization,
where the template parameter list is empty.

Fixes llvm#128691

Author: zyn0217

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -377,8 +377,15 @@ 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.
+        assert(Depth ||
+               cast<ClassTemplateSpecializationDecl>(FTD->getDeclContext())
+                   ->isExplicitSpecialization());
         NamedDecl *NewParam = transformTemplateParameter(
-            SemaRef, DC, Param, Args, Index + Depth1IndexAdjustment, Depth - 1);
+            SemaRef, DC, Param, Args, Index + Depth1IndexAdjustment,
+            Depth ? Depth - 1 : 0);
         if (!NewParam)
           return nullptr;
         // Constraints require that we substitute depth-1 arguments

clang/test/SemaTemplate/deduction-guide.cpp

@@ -691,3 +691,35 @@ Test test(42);
 // CHECK-NEXT: | `-ParmVarDecl {{.*}} 'auto:1'
 
 } // namespace GH122134
+
+namespace GH128691 {
+
+template <typename = void>
+class NewDeleteAllocator;
+
+template <>
+struct NewDeleteAllocator<> {
+  template <typename T>
+  NewDeleteAllocator(T); // expected-note {{candidate template ignored}} \
+                         // expected-note {{implicit deduction guide declared as}}
+};
+
+template <typename>
+struct NewDeleteAllocator : NewDeleteAllocator<> { // expected-note {{candidate template ignored}} \
+                                                   // expected-note {{implicit deduction guide declared as}}
+  using NewDeleteAllocator<>::NewDeleteAllocator;
+};
+
+void test() { NewDeleteAllocator abc(42); } // expected-error {{no viable constructor or deduction guide}}
+
+// CHECK-LABEL: Dumping GH128691::<deduction guide for NewDeleteAllocator>:
+// CHECK-NEXT: FunctionTemplateDecl {{.+}} <deduction guide for NewDeleteAllocator>
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.+}} typename depth 0 index 0
+// CHECK-NEXT: | `-TemplateArgument type 'void'
+// CHECK-NEXT: |   |-inherited from TemplateTypeParm {{.+}} depth 0 index 0
+// CHECK-NEXT: |   `-BuiltinType {{.+}} 'void'
+// CHECK-NEXT: |-TemplateTypeParmDecl {{.+}} typename depth 0 index 1 T
+// CHECK-NEXT: `-CXXDeductionGuideDecl {{.+}} <deduction guide for NewDeleteAllocator> 'auto (T) -> NewDeleteAllocator<type-parameter-0-0>'
+// CHECK-NEXT:  `-ParmVarDecl {{.+}} 'T'
+
+} // namespace GH128691