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

Backports b8d1f3d.

This fixes a potential integer overflow bug that has been around for
many versions and was exposed by my patch recently. So we think it
warrants a backport

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -1058,6 +1058,7 @@ Bug Fixes to C++ Support
 - Fixed a substitution bug in transforming CTAD aliases when the type alias contains a non-pack template argument
   corresponding to a pack parameter (#GH124715)
 - Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
+- Fixed an integer overflow bug in computing template parameter depths when synthesizing CTAD guides. (#GH128691)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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