[lldb][DWARFASTParserClang] Fix handle non-type template parameters in MakeAPValue

[zyn-li]

Summary

MakeAPValue in DWARFASTParserClang.cpp did not handle pointer-to-member non-type template parameters (e.g., template <int S::*P>), causing LLDB to silently produce incorrect results or crash.

When DWARF encodes a pointer-to-member non-type template parameter, it uses DW_TAG_template_value_parameter with a DW_AT_const_value representing the byte offset of the member within the struct. MakeAPValue is responsible for converting this value into a clang APValue, but it only handled integer/enum and floating-point types. For pointer-to-member types, it returned std::nullopt.

This caused the caller (ParseTemplateDIE) to fall back to creating a type-only TemplateArgument (kind=Type) instead of a value-carrying one. When two specializations differ only by which member they point to (e.g., Data<&S::x> vs Data<&S::y>), both produce identical TemplateArguments. Clang's findSpecialization then treats the second as a duplicate, so only one specialization exists in the AST. The second variable becomes unresolvable.

(See Debugger Evidence section below)

In more complex cases, this triggers an assertion failure in clang::CXXRecordDecl::setBases(): cast() argument of incompatible type.

Fix

Refactored MakeAPValue to comprehensively handle the Non-Type Template Parameters (NTTPs). Instead of a narrow check for integers and enums, the function now leverages CompilerType specialized queries to treat the following categories as integral values:

• Pointer-to-Member Types: Specifically resolves the reported issue where member offsets (e.g., 8 vs. 24) were being dropped.
• Pointer Types: Generalizes support for NTTPs that represent addresses of objects or functions.
• Nullptr Type: Properly handles std::nullptr_t constants.
• Reference Types: lvalue references are correctly captured as value arguments.

By converting these types into distinct clang::APValue objects, we ensure that ParseTemplateDIE no longer falls back to type-only arguments. This allows the Clang AST to uniquely identify specializations that differ only by constant offsets or addresses.

Additionally, the fix replaces the silent failure path with:

• Unsupported TypeClass entries are now logged,
• Added an lldbassert to ensure that problems are caught during development rather than silently "ruining" the template AST.

Known Limitation

The fix displays pointer-to-member template arguments as integer offsets (e.g., Data<0>, Data<4>) rather than symbolic names (e.g., Data<&S::x>). A follow-up patch will improve the display.

Test Plan

Added lldb/test/API/lang/cpp/non-type-template-param-member-ptr/ with a test that verifies both Data<&S::x> and Data<&S::y> are resolvable as distinct specializations.

Debugger Evidence

Collected at two DW_TAG_template_value_parameter DIEs during ParseTemplateDIE, both with name="PtrToMember" and type_class=256 (eTypeClassMemberPointer): DIE 0xcb: uval64=8; DIE 0x1e9 : uval64=24

These correspond to two members of Fiber:

DW_TAG_member "listHook_"        DW_AT_data_member_location(0x08)  ← uval64=8
DW_TAG_member "globalListHook_"  DW_AT_data_member_location(0x18)  ← uval64=24

Clang's ground truth AST correctly produces TemplateArgument decl (kind=Declaration) with distinct FieldDecl references for each specialization.

22:| | |-ClassTemplateSpecializationDecl 0x36d785b7cc0 <line:43:1, line:56:1> line:44:8 struct mhtraits definition instantiated_from 0x36d785b4238 implicit_instantiation
23-| | | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
24-| | | | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr
25-| | | | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
26-| | | | |-MoveConstructor exists simple trivial
27-| | | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
28-| | | | |-MoveAssignment exists simple trivial needs_implicit
29-| | | | `-Destructor simple irrelevant trivial
30-| | | |-TemplateArgument type 'intrusive::Fiber'
31-| | | | `-RecordType 0x36d785b3fb0 'intrusive::Fiber' canonical
32-| | | |   `-CXXRecord 0x36d78344850 'Fiber'
33-| | | |-TemplateArgument type 'intrusive::list_member_hook<>'
34-| | | | `-RecordType 0x36d785b3c20 'intrusive::list_member_hook<>' canonical
35-| | | |   `-ClassTemplateSpecialization 0x36d78344b48 'list_member_hook'
36-| | | |-TemplateArgument decl '&intrusive::Fiber::listHook_'
37-| | | | `-Field 0x36d785b3e70 'listHook_' 'list_member_hook<>':'intrusive::list_member_hook<>'
38-| | | |-CXXRecordDecl 0x36d785b93a0 <col:1, col:8> col:8 implicit struct mhtraits
39-| | | |-TypeAliasDecl 0x36d785b9470 <line:45:3, col:22> col:9 referenced value_type 'intrusive::Fiber'
40-| | | | `-SubstTemplateTypeParmType 0x36d785b9430 'intrusive::Fiber' sugar typename depth 0 index 0 T
41-| | | |   |-ClassTemplateSpecialization 0x36d785b7cc0 'mhtraits'
42-| | | |   `-RecordType 0x36d785b3fb0 'intrusive::Fiber' canonical
--
78:| | `-ClassTemplateSpecializationDecl 0x36d785b8210 <line:43:1, line:56:1> line:44:8 struct mhtraits definition instantiated_from 0x36d785b4238 implicit_instantiation
79-| |   |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
80-| |   | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr
81-| |   | |-CopyConstructor simple trivial has_const_param implicit_has_const_param
82-| |   | |-MoveConstructor exists simple trivial
83-| |   | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
84-| |   | |-MoveAssignment exists simple trivial needs_implicit
85-| |   | `-Destructor simple irrelevant trivial
86-| |   |-TemplateArgument type 'intrusive::Fiber'
87-| |   | `-RecordType 0x36d785b3fb0 'intrusive::Fiber' canonical
88-| |   |   `-CXXRecord 0x36d78344850 'Fiber'
89-| |   |-TemplateArgument type 'intrusive::list_member_hook<>'
90-| |   | `-RecordType 0x36d785b3c20 'intrusive::list_member_hook<>' canonical
91-| |   |   `-ClassTemplateSpecialization 0x36d78344b48 'list_member_hook'
92-| |   |-TemplateArgument decl '&intrusive::Fiber::globalListHook_'
93-| |   | `-Field 0x36d785b3f60 'globalListHook_' 'list_member_hook<>':'intrusive::list_member_hook<>'
94-| |   |-CXXRecordDecl 0x36d785be080 <col:1, col:8> col:8 implicit struct mhtraits
95-| |   |-TypeAliasDecl 0x36d785be150 <line:45:3, col:22> col:9 referenced value_type 'intrusive::Fiber'
96-| |   | `-SubstTemplateTypeParmType 0x36d785be110 'intrusive::Fiber' sugar typename depth 0 index 0 T
97-| |   |   |-ClassTemplateSpecialization 0x36d785b8210 'mhtraits'
98-| |   |   `-RecordType 0x36d785b3fb0 'intrusive::Fiber' canonical

Full Paste

$ llvm-dwarfdump --debug-info=0x000000cb repro.dwp -c -p
repro.dwp:	file format elf64-x86-64
.debug_info.dwo contents:
0x00000014: DW_TAG_compile_unit
              DW_AT_producer	("clang version 22.1.20 ...)
              DW_AT_language	(DW_LANG_C_plus_plus_14)
              DW_AT_name	("repro.cpp")
              DW_AT_dwo_name	("repro-repro.dwo")
0x0000001a:   DW_TAG_namespace
                DW_AT_name	("intrusive")
0x000000b9:     DW_TAG_structure_type
                  DW_AT_calling_convention	(DW_CC_pass_by_value)
                  DW_AT_name	("mhtraits<intrusive::Fiber, intrusive::list_member_hook<(intrusive::link_mode_type)2>, &intrusive::Fiber::listHook_>")
                  DW_AT_byte_size	(0x01)
                  DW_AT_decl_file	(0x00)
                  DW_AT_decl_line	(44)
0x000000cb:       DW_TAG_template_value_parameter
                    DW_AT_type	(0x0000022d "intrusive::list_member_hook<(intrusive::link_mode_type)2> intrusive::Fiber::*")
                    DW_AT_name	("PtrToMember")
                    DW_AT_const_value	(8)

lldb prints:

p name
(const char *) 0x000055f44554c4ca "PtrToMember"
p uval64
(uint64_t) 8
p (unsigned)clang_type.GetTypeClass()
(unsigned int) 256

$ llvm-dwarfdump --debug-info=0x000001e9 repro.dwp -c -p
repro.dwp:	file format elf64-x86-64
.debug_info.dwo contents:
0x00000014: DW_TAG_compile_unit
              DW_AT_producer	("clang version 22.1.20")
              DW_AT_language	(DW_LANG_C_plus_plus_14)
              DW_AT_name	("repro.cpp")
              DW_AT_dwo_name	("repro-repro.dwo")
0x0000001a:   DW_TAG_namespace
                DW_AT_name	("intrusive")
0x000001d7:     DW_TAG_structure_type
                  DW_AT_calling_convention	(DW_CC_pass_by_value)
                  DW_AT_name	("mhtraits<intrusive::Fiber, intrusive::list_member_hook<(intrusive::link_mode_type)2>, &intrusive::Fiber::globalListHook_>")
                  DW_AT_byte_size	(0x01)
                  DW_AT_decl_file	(0x00)
                  DW_AT_decl_line	(44)
0x000001e9:       DW_TAG_template_value_parameter
                    DW_AT_type	(0x0000022d "intrusive::list_member_hook<(intrusive::link_mode_type)2> intrusive::Fiber::*")
                    DW_AT_name	("PtrToMember")
                    DW_AT_const_value	(24)

lldb prints:

 p name
(const char *) 0x000055f44554c4ca "PtrToMember"
p uval64
(uint64_t) 24
p (unsigned)clang_type.GetTypeClass()
(unsigned int) 256

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🐧 Linux x64 Test Results

• 33356 tests passed
• 508 tests skipped

✅ The build succeeded and all tests passed.