[202x] Enable variadic templates

[Chris B (llvm-beanz)]

This enables C++11 variadic templates in HLSL and exposes ... function parameter packs.

This feature is being refined by TC57 for inclusion in the first draft of the HLSL specification, and while including it here is getting a little ahead of the committee, it's a frequently requested high-value feature that is minimally invasive to support in DXC and is already supported in Clang.

Resolves #8905

Assisted-by: GitHub Copilot

[Copilot]

🟡 Changes recommended

The newly enabled template-template parameter-pack parser path lacks positive and backward-compatibility coverage.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Enables C++11 variadic templates for HLSL 202x while preserving rejection in earlier language versions.

Changes:

• Adds version-aware parsing and semantic support for parameter packs, expansions, and sizeof....
• Adds DXIL, SPIR-V, semantic, and negative coverage.
• Documents the feature in release notes.

File summaries

File	Description
tools/clang/test/SemaHLSL/v202x/templates/variadic-templates.hlsl	Tests core semantics and ASTs.
tools/clang/test/SemaHLSL/v202x/templates/variadic-templates-pre202x.hlsl	Tests HLSL 2021 rejection.
tools/clang/test/SemaHLSL/v202x/templates/variadic-templates-negative.hlsl	Tests invalid constructs.
tools/clang/test/SemaHLSL/v202x/templates/variadic-templates-initlist-scalarize.hlsl	Tests initializer scalarization.
tools/clang/test/SemaHLSL/v202x/templates/variadic-templates-initlist-scalarize-negative.hlsl	Tests scalarization diagnostics.
tools/clang/test/SemaHLSL/v202x/templates/variadic-base-class-unsupported.hlsl	Tests unsupported base packs.
tools/clang/test/HLSLFileCheckLit/hlsl/templates/variadic-initlist-scalarize.hlsl	Verifies DXIL scalarization.
tools/clang/test/HLSLFileCheckLit/hlsl/templates/variadic-builtin-templates.hlsl	Verifies built-in template expansion.
tools/clang/test/HLSLFileCheckLit/hlsl/templates/variadic-202x.hlsl	Verifies basic DXIL generation.
tools/clang/test/CodeGenSPIRV/variadic.templates.initlist-scalarize.hlsl	Verifies SPIR-V scalarization.
tools/clang/test/CodeGenSPIRV/variadic.templates.builtin.hlsl	Verifies SPIR-V built-in templates.
tools/clang/test/CodeGenSPIRV/variadic.templates.basic.hlsl	Verifies basic SPIR-V generation.
tools/clang/lib/Sema/SemaType.cpp	Enables parameter-pack types in 202x.
tools/clang/lib/Sema/SemaTemplateVariadic.cpp	Enables pack expansion semantics.
tools/clang/lib/Parse/ParseTemplate.cpp	Enables variadic template syntax.
tools/clang/lib/Parse/ParseInit.cpp	Enables initializer-list expansions.
tools/clang/lib/Parse/ParseExpr.cpp	Enables expression expansions and sizeof....
tools/clang/lib/Parse/ParseDecl.cpp	Enables function parameter packs.
tools/clang/include/clang/Basic/LangOptions.h	Adds the version feature predicate.
docs/ReleaseNotes.md	Announces HLSL 202x support.

Review details

• Files reviewed: 20/20 changed files
• Comments generated: 2
• Review effort level: Balanced

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[Copilot]

🔵 Needs a closer look

Preprocessor feature reporting is inconsistent with the new language support, and template-template parameter packs lack coverage.

Review details

Suppressed comments (2)

Previously missed (1) — in code that hasn't changed since the last review.

tools/clang/include/clang/Basic/LangOptions.h:194

• The new language-version gate is not reflected in the preprocessor feature probes. HLSL sets CPlusPlus but not CPlusPlus11, so PPMacroExpansion.cpp:1159 still makes __has_feature(cxx_variadic_templates) false in 202x, while line 1252 makes __has_extension(cxx_variadic_templates) true even in pre-202x modes. These are the documented probes for variadic-template support, so version-conditional code receives contradictory results. Please gate both probes on HLSL 202x as well (and consider the corresponding __cpp_variadic_templates macro).

tools/clang/lib/Parse/ParseTemplate.cpp:625

• This branch newly enables template-template parameter packs, but none of the added HLSL tests exercises syntax such as template <template <typename> class... Ts>. Add a 202x acceptance/AST case and a pre-202x rejection case so this distinct parser path cannot regress independently of type and non-type packs.

    if (getLangOpts().HLSLDisallowsVariadicTemplates())
      Diag(EllipsisLoc, diag::err_hlsl_variadic_templates);
    else if (!getLangOpts().HLSL) // HLSL has no C++98-compat warnings

• Files reviewed: 20/20 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

The broad expansion gate unintentionally enables C++17 fold expressions, and one newly enabled parser path lacks coverage.

Review details

Suppressed comments (2)

Previously missed (1) — in code that hasn't changed since the last review.

tools/clang/lib/Sema/SemaTemplateVariadic.cpp:548

• This global gate also enables C++17 fold expressions in HLSL 202x, although this change is scoped to C++11 variadic templates. ParseFoldExpression accepts the syntax (with only an extension warning), and TreeTransform::TransformCXXFoldExpr reaches this function during instantiation, so a function such as template<typename... T> int sum(T... x) { return (x + ...); } is now expanded. The linked TC57 proposal lists fold expressions separately under C++17 rather than as part of this adoption. Please retain an explicit HLSL rejection for fold expressions and add a negative 202x test.

tools/clang/lib/Parse/ParseTemplate.cpp:625

• This branch newly enables template-template parameter packs, but the added tests cover only type and non-type packs. Please add HLSL 202x acceptance/instantiation coverage (and preferably the pre-202x rejection) for syntax such as template <template <typename> class... Templates> so this distinct parser/Sema path is exercised.

    if (getLangOpts().HLSLDisallowsVariadicTemplates())
      Diag(EllipsisLoc, diag::err_hlsl_variadic_templates);
    else if (!getLangOpts().HLSL) // HLSL has no C++98-compat warnings

• Files reviewed: 20/20 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🟡 Changes recommended

Pack expansion can misassign parameter modifiers, enable out-of-scope fold expressions, and report inconsistent feature availability.

Get a fresh assessment by requesting another Copilot review.

Review details

Suppressed comments (2)

Previously missed (2) — in code that hasn't changed since the last review.

tools/clang/include/clang/Basic/LangOptions.h:195

• Please apply this version gate to the preprocessor capability probes as well. HLSL sets CPlusPlus but not CPlusPlus11, so __has_feature(cxx_variadic_templates) remains false in 202x, while __has_extension(cxx_variadic_templates) remains true even in pre-202x modes (PPMacroExpansion.cpp:1159,1252); __cpp_variadic_templates is also still omitted by InitPreprocessor.cpp:477-490. Code cannot reliably detect the newly versioned feature until these probes use the same condition.
  tools/clang/lib/Sema/SemaTemplateVariadic.cpp:550
• This shared expansion gate also enables C++17 fold expressions in HLSL 202x, although the PR is scoped to C++11 variadic templates. ParseFoldExpression accepts (args + ...) with only an extension diagnostic, and TransformCXXFoldExpr reaches this method during instantiation; changing this condition therefore lets the fold instantiate instead of retaining HLSL's rejection. Keep an explicit HLSL rejection for fold expressions and add a 202x negative test.

• Files reviewed: 22/22 changed files
• Comments generated: 1
• Review effort level: Balanced

[Chris B (llvm-beanz)]

A function template can't have multiple different parameter packs for different parameters. It would be ambiguous at the call site to split the parameter packs.