[202x] Enable auto return types

[Chris B (llvm-beanz)]

This PR enables return type deduction for normal functions following C++ 14's use of auto as the return type in normal function prototype syntax.

The HLSL proposal is in progress here:
https://hlsl-tc57.github.io/tc57/proposal/0010

While this proposal is still in Refinement, so this is getting a bit ahead of the standard committee it seems like a highly likely feature that brings a lot of value for a fairly small set of changes.

Resolves #8903

Assisted-by: GitHub Copilot

[Copilot]

🟡 Changes recommended

HLSL-specific deduction restrictions are bypassed, and important test and release-note coverage is missing.

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

Pull request overview

Enables C++14-style auto return-type deduction for HLSL functions across semantic analysis and code generation.

Changes:

• Enables deduction during declaration, return analysis, and function use.
• Adds DXIL and SPIR-V validation.
• Adds diagnostics and legacy-version warning tests.

File summaries

File	Description
tools/clang/lib/Sema/SemaType.cpp	Permits deduced function return types in HLSL.
tools/clang/lib/Sema/SemaStmt.cpp	Deduces types from return statements.
tools/clang/lib/Sema/SemaExpr.cpp	Handles uses of functions with undeduced returns.
tools/clang/lib/Sema/SemaDecl.cpp	Handles declarations, templates, and implicit void.
tools/clang/test/HLSLFileCheckLit/hlsl/auto/auto-return-type.hlsl	Tests standard deduction and DXIL output.
tools/clang/test/HLSLFileCheckLit/hlsl/auto/auto-return-extension-warning.hlsl	Tests legacy-mode warnings.
tools/clang/test/HLSLFileCheckLit/hlsl/auto/auto-return-errors.hlsl	Tests invalid deduction cases.
tools/clang/test/CodeGenSPIRV/fn.auto.return.hlsl	Tests SPIR-V return types.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 4
• Review effort level: Balanced

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[Joshua Batista (bob80905)]

Do we want to disallow auto for entry-point functions?

[Copilot]

🟡 Changes recommended

Dynamic descriptor-heap returns bypass existing auto restrictions, and release-note coverage is missing.

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

Review details

Suppressed comments (2)

tools/clang/lib/Sema/SemaStmt.cpp:3080

• The new return-type path omits the dynamic descriptor-heap check used for auto variables in SemaDecl.cpp:6421-6433 and 9047-9053. Those heap expressions have HLSLDynamicResourceAttr, not HLSLNonAutoDeducibleAttr, so IsTypeDeducibleWithAuto accepts them and a function can deduce an internal dynamic resource/sampler return type even though auto x = ResourceDescriptorHeap[i] is rejected. Apply the same heap-expression check here before generic type classification and add resource/sampler return tests.

    if (getLangOpts().HLSL && !Deduced->isDependentType() &&
        !hlsl::IsTypeDeducibleWithAuto(*this, Deduced)) {
      Diag(RetExpr->getExprLoc(), diag::err_hlsl_auto_undeducible_type)
          << Deduced;

tools/clang/test/HLSLFileCheckLit/hlsl/auto/auto-return-errors.hlsl:29

• The comment announces initializer-list coverage, but no initializer-list return test follows it, so the existing err_auto_fn_return_init_list path is not exercised in HLSL mode. Add the omitted auto function with return { 1, 2, 3 }; and verify cannot deduce return type from initializer list.

// Returning an initializer list as the deduced return value is not allowed.
// (Initializer lists in return statements are also unsupported in HLSL more
// broadly, but the auto-return-type case is still flagged early.)

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

Auto returns currently bypass descriptor-heap restrictions, and release-note and test coverage gaps remain.

Review details

Suppressed comments (4)

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

tools/clang/lib/Sema/SemaStmt.cpp:3081

• This check misses the separate descriptor-heap rule used for auto variables. .Resource/.Sampler carry HLSLDynamicResourceAttr, not HLSLNonAutoDeducibleAttr, so IsTypeDeducibleWithAuto returns true and an auto function can expose the otherwise-forbidden placeholder type with return ResourceDescriptorHeap[i] (or SamplerDescriptorHeap[i]). Apply the equivalent dynamic-heap expression check here, ideally via a shared helper, and add return-type cases alongside auto-no-descriptor-heap.hlsl.

tools/clang/lib/Sema/SemaType.cpp:2665

• This is a user-visible HLSL language feature, so the repository's release-note policy requires an entry in docs/ReleaseNotes.md under Upcoming Release / HLSL Language. Please add that entry, or identify the related PR that will provide shared release-note coverage.

  // HLSL Change Begin - HLSL supports 'auto' as a function declarator return
  // type with C++14-style deduction; skip this check for functions.
  if (ContainsPlaceholderType &&
      (!(SemaRef.getLangOpts().CPlusPlus11 || SemaRef.getLangOpts().HLSL) ||
       !D.isFunctionDeclarator())) {

tools/clang/test/HLSLFileCheckLit/hlsl/auto/auto-return-errors.hlsl:29

• This comment describes an initializer-list diagnostic, but the file contains no initializer-list return case after it, so that behavior is not actually tested. Add an auto function returning a braced initializer with the expected diagnostic (or remove the claim if it is intentionally out of scope).

// Returning an initializer list as the deduced return value is not allowed.
// (Initializer lists in return statements are also unsupported in HLSL more
// broadly, but the auto-return-type case is still flagged early.)

tools/clang/lib/Sema/SemaDecl.cpp:7490

• The new dependent-context path is not exercised by the added tests: auto-template.hlsl only has templates with explicit return types. Add an HLSL template such as template<typename T> auto Identity(T value) { return value; } and instantiate it for multiple types; also cover deferred rejection of a non-auto-deducible type so the instantiation-time check is verified.

    if ((getLangOpts().CPlusPlus14 || getLangOpts().HLSL) && // HLSL Change
        (NewFD->isDependentContext() ||
         (isFriend && CurContext->isDependentContext())) &&
        NewFD->getReturnType()->isUndeducedType()) {

• Files reviewed: 9/9 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🟡 Changes recommended

Release-note coverage, template-instantiation tests, and updated attribute documentation are still needed.

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

Review details

Suppressed comments (2)

tools/clang/lib/Sema/SemaType.cpp:3758

• This is a user-visible HLSL language feature, but the PR does not add the release note required for new language features by CONTRIBUTING.md:115-134. Please add a concise entry under docs/ReleaseNotes.md → Upcoming Release → HLSL Language (or point to the related PR that will provide the shared entry).

        // HLSL Change Begin - HLSL supports C++14-style deduced return types.
        if (D.getDeclSpec().containsPlaceholderType() &&
            !FTI.hasTrailingReturnType() && chunkIndex == 0 &&
            !(S.getLangOpts().CPlusPlus14 || S.getLangOpts().HLSL)) {

tools/clang/lib/Sema/SemaDecl.cpp:7490

• This explicitly enables deduced returns for dependent functions, but none of the added tests exercises template<typename T> auto f(T); the existing auto-template.hlsl only uses auto for local variables in templates. Please cover successful instantiation (ideally for multiple types) and an instantiation whose return type is non-auto-deducible, since this branch defers deduction and validation until instantiation.

    if ((getLangOpts().CPlusPlus14 || getLangOpts().HLSL) && // HLSL Change
        (NewFD->isDependentContext() ||
         (isFriend && CurContext->isDependentContext())) &&
        NewFD->getReturnType()->isUndeducedType()) {

• Files reviewed: 10/10 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

The user-visible language feature needs a release note and template return-deduction coverage.

Review details

Suppressed comments (2)

tools/clang/lib/Sema/SemaType.cpp:2665

• This is a user-visible HLSL language feature, but the PR does not add the release note required by CONTRIBUTING.md for significant language changes. Please add a one-sentence entry under docs/ReleaseNotes.md → Upcoming Release → HLSL Language, or point to the related PR that will provide the shared release-note coverage.

      (!(SemaRef.getLangOpts().CPlusPlus11 || SemaRef.getLangOpts().HLSL) ||
       !D.isFunctionDeclarator())) {

tools/clang/lib/Sema/SemaDecl.cpp:7490

• This branch explicitly enables deduced returns in dependent function contexts, but the new tests cover only non-template functions; the existing auto-template.hlsl uses explicit return types. Please add coverage for an instantiated template<typename T> auto function (including at least two concrete return types), so regressions in the template-instantiation deduction path are detected.

    if ((getLangOpts().CPlusPlus14 || getLangOpts().HLSL) && // HLSL Change
        (NewFD->isDependentContext() ||
         (isFriend && CurContext->isDependentContext())) &&
        NewFD->getReturnType()->isUndeducedType()) {

• Files reviewed: 11/11 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🔵 Needs a closer look

Release-note coverage and tests for the newly enabled dependent-function path are missing.

Review details

Suppressed comments (2)

tools/clang/lib/Sema/SemaDecl.cpp:7490

• This branch also enables deduced return types for dependent functions, but none of the added tests declares a function template with an auto return type; the existing auto-template.hlsl only uses auto for local variables. Please cover at least successful instantiations with distinct deduced types and an instantiation whose return type is non-auto-deducible, so this newly enabled path and its deferred check cannot regress.

    if ((getLangOpts().CPlusPlus14 || getLangOpts().HLSL) && // HLSL Change
        (NewFD->isDependentContext() ||
         (isFriend && CurContext->isDependentContext())) &&
        NewFD->getReturnType()->isUndeducedType()) {

tools/clang/lib/Sema/SemaType.cpp:2665

• This enables a user-visible HLSL language feature, but the PR does not update docs/ReleaseNotes.md. The repository policy in CONTRIBUTING.md:117-128 explicitly lists new language features as significant compiler changes requiring a release note. Please add a single-sentence entry under Upcoming Release → HLSL Language, or point to the related PR that will provide shared release-note coverage.

      (!(SemaRef.getLangOpts().CPlusPlus11 || SemaRef.getLangOpts().HLSL) ||
       !D.isFunctionDeclarator())) {

• Files reviewed: 12/12 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

The semantic changes are focused and covered by positive, negative, compatibility-warning, DXIL, and SPIR-V tests.

Review details

• Files reviewed: 12/12 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

Copilot review overview

🔵 Needs a closer look

The newly enabled dependent-context path lacks coverage for auto-returning function templates.

Review effort: Balanced
Findings: 1 · 3

Open (4)

• This dependent-context handling specifically enables deduced return types for function templates,…
• Adding this marker to descriptor-heap wrapper records makes the attribute documentation stale:…
• This is a user-visible HLSL language feature, so the repository release-note policy requires an…
• This is a user-visible HLSL language feature, so the repository's release-note policy requires an…

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is focused and covers successful deduction, diagnostics, templates, compatibility modes, DXIL, and SPIR-V.

Review effort: Balanced
Findings: 1 · 2

Open (3)

• This dependent-context handling specifically enables deduced return types for function templates,…
• This is a user-visible HLSL language feature, so the repository release-note policy requires an…
• This is a user-visible HLSL language feature, so the repository's release-note policy requires an…

Resolved since last review (1)

• Adding this marker to descriptor-heap wrapper records makes the attribute documentation stale:…

[Chris B (llvm-beanz)]

Do we want to disallow auto for entry-point functions?

I'm not sure it really matters. At the end of the day auto is compile-time resolved in the frontend to a concrete type. If that turns into a type that shouldn't be returned from an entry function the user will get an error, otherwise it will work.

It isn't necessarily useful to have an auto return type for an entry function since entry functions can't be templates (and that's really where this is useful), but it should "work".

[Joshua Batista (bob80905)]

OK, otherwise, it looks good to me. I'll leave it up to you if you want to add entry point functions that return auto for pos/neg testing.