Diagnose non-void functions in Jacobian mode by Elvand-Lie · Pull Request #2001 · vgvassilev/clad

Elvand-Lie · 2026-08-20T16:53:41Z

Description

Clad's Jacobian API generates a void derivative function and exposes outputs through pointer, reference, or array parameters.

Requesting clad::jacobian for a non-void primal function previously proceeded into derivative generation and emitted invalid C++, resulting in a cryptic downstream compiler error.

This PR adds a direct diagnostic in DerivativeBuilder::Derive that rejects non-void primal functions before invalid C++ is generated, explaining that Jacobian mode currently requires a void-returning primal function and that outputs must be provided through pointer, reference, or array parameters.

Current Contract: Strict void-returning primal requirement.
Coverage: Adds tests in test/Jacobian/NonVoidReturnDiagnostic.C covering scalar returns, output pointers with scalar returns (reproducing Jacobian Generates return in void Function #1306), and status-returning functions to document current behavior.
Follow-up Work: Selective status-return support, if desired by maintainers, involves separate return-control-flow and output-classification semantics and can be evaluated as a follow-up feature.

Related Issues

Fixes #1306

github-actions · 2026-08-20T17:02:25Z

clang-tidy review says "All clean, LGTM! 👍"

codecov · 2026-08-20T17:09:20Z

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

github-actions · 2026-08-20T17:23:16Z

clang-tidy review says "All clean, LGTM! 👍"

Elvand-Lie · 2026-08-21T07:43:37Z

Before I finalize the #1306 diagnostic, I would like to confirm the intended Jacobian return-value contract.

The current patch rejects every non-void primal. That handles differentiable scalar returns, for which the generated Jacobian ABI has no return-derivative output slot, but it also rejects status-returning C-style functions such as:

int status_output(double x, double* out) {
  *out = x * x;
  return 0;
}

I propose the following narrower rule:

keep the generated Jacobian function void-returning;
reject differentiable non-void returns with a clear diagnostic;
allow bool, integral, or enum status returns when Jacobian mode already recognizes a differentiable output parameter;
evaluate the status-return expression for side effects and lower it to a bare return, preserving early-return control flow.

This avoids inventing an ABI slot for scalar-return derivatives while preserving common status-code APIs.

Would you prefer this selective status-return policy, or should Jacobian primal functions be strictly void-returning?

Elvand-Lie · 2026-08-21T08:45:22Z

After reviewing the merged vector-forward Jacobian design and its void-returning derivative ABI, I am keeping this PR scoped to the current void-returning primal contract. Selective status-return support would require separate return-control-flow and output-classification semantics, so I will leave that as follow-up feature work unless maintainers prefer otherwise.

github-actions

clang-tidy made some suggestions

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github-actions · 2026-08-21T08:51:30Z

-        return true;
-    return false;
-  }
+static bool hasAttribute(const Decl* D, attr::Kind Kind) {


warning: no header providing "clang::attr::Kind" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <clang/Basic/AttrKinds.h> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

+ClonedFunction DerivativeBuilder::cloneFunction(const clang::FunctionDecl* FD,
+                                                clad::VisitorBase& VB,
+                                                clang::DeclContext* DC,
+                                                clang::SourceLocation& noLoc,


warning: no header providing "clang::SourceLocation" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <clang/Basic/SourceLocation.h> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

+                                                clad::VisitorBase& VB,
+                                                clang::DeclContext* DC,
+                                                clang::SourceLocation& noLoc,
+                                                clang::DeclarationNameInfo name,


warning: no header providing "clang::DeclarationNameInfo" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <clang/AST/DeclarationName.h> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

+  unsigned NamespaceCount = 0;
+  TypeSourceInfo* TSI = m_Context.getTrivialTypeSourceInfo(functionType);
+  if (isa<CXXMethodDecl>(FD)) {
+    CXXRecordDecl* CXXRD = cast<CXXRecordDecl>(DC);


warning: use auto when initializing with a template cast to avoid duplicating the type name [modernize-use-auto]

Suggested change

CXXRecordDecl* CXXRD = cast<CXXRecordDecl>(DC);

auto* CXXRD = cast<CXXRecordDecl>(DC);

github-actions · 2026-08-21T08:51:30Z

+    // even if their original function had different access specifier.
+    returnedFD->setAccess(AS_public);
+  } else {
+    assert(isa<FunctionDecl>(FD) && "Unexpected!");


warning: no header providing "assert" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <cassert> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

+    assert(isa<FunctionDecl>(FD) && "Unexpected!");
+    NamespaceCount = VB.RebuildEnclosingNamespaces(DC);
+
+    auto TrailingRequiresClause =


warning: 'auto TrailingRequiresClause' can be declared as 'auto *TrailingRequiresClause' [llvm-qualified-auto]

Suggested change

auto TrailingRequiresClause =

auto *TrailingRequiresClause =

github-actions · 2026-08-21T08:51:30Z

+      OverloadExpr* ovl = find.Expression;
+
+      if (isa<UnresolvedLookupExpr>(ovl)) {
+        ExprResult result;


warning: no header providing "clang::ExprResult" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <clang/Sema/Ownership.h> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

-      SS.Extend(m_Context, NSD, noLoc, noLoc);
+LookupResult DerivativeBuilder::LookupCustomDerivativeOrNumericalDiff(
+    const std::string& Name, const clang::DeclContext* originalFnDC,
+    CXXScopeSpec& SS, bool forCustomDerv /*=true*/,


warning: no header providing "clang::CXXScopeSpec" is directly included [misc-include-cleaner]

lib/Differentiator/DerivativeBuilder.cpp:52:

- #include <cstddef> + #include <clang/Sema/DeclSpec.h> + #include <cstddef>

github-actions · 2026-08-21T08:51:30Z

+    bool namespaceShouldExist /*=true*/) {
+
+  IdentifierInfo* II = &m_Context.Idents.get(Name);
+  DeclarationName name(II);


warning: no header providing "clang::DeclarationName" is directly included [misc-include-cleaner]

DeclarationName name(II); ^

github-actions · 2026-08-21T08:51:30Z

+      const Expr* Callee = CE->getCallee()->IgnoreParenCasts();
+      if (const auto* DRE = dyn_cast<DeclRefExpr>(Callee))
+        originalFnDC =
+            const_cast<DeclContext*>(DRE->getFoundDecl()->getDeclContext());


warning: do not use const_cast to remove const qualifier [cppcoreguidelines-pro-type-const-cast]

const_cast<DeclContext*>(DRE->getFoundDecl()->getDeclContext()); ^

github-actions · 2026-08-21T09:17:04Z

clang-tidy review says "All clean, LGTM! 👍"

github-actions · 2026-08-21T09:25:12Z

clang-tidy review says "All clean, LGTM! 👍"

Clad's Jacobian API generates a void derivative function and exposes outputs through pointer, reference, or array parameters. Requesting clad::jacobian for a non-void primal previously proceeded into derivative generation and emitted invalid C++, resulting in a cryptic downstream compiler error. Reject unsupported non-void primal functions in DerivativeBuilder::Derive with a direct diagnostic explaining that Jacobian mode currently requires a void-returning primal and how outputs must be provided. Add regression coverage for scalar returns, output pointers with scalar returns (reproducing Issue vgvassilev#1306), and status-returning functions to document the strict void-returning contract. Signed-off-by: Tempris Admin <elvandlie@gmail.com> Fixes vgvassilev#1306

github-actions · 2026-08-21T10:09:12Z

clang-tidy review says "All clean, LGTM! 👍"

Elvand-Lie force-pushed the fix/jacobian-void-return-scalar-fn branch from 81f3d63 to a6d8847 Compare August 20, 2026 17:17

Elvand-Lie marked this pull request as draft August 20, 2026 17:37

Elvand-Lie force-pushed the fix/jacobian-void-return-scalar-fn branch from a6d8847 to 870f7ba Compare August 21, 2026 08:44

github-actions Bot reviewed Aug 21, 2026

View reviewed changes

Elvand-Lie force-pushed the fix/jacobian-void-return-scalar-fn branch from 870f7ba to 75e544e Compare August 21, 2026 09:11

Elvand-Lie force-pushed the fix/jacobian-void-return-scalar-fn branch from 75e544e to 4520092 Compare August 21, 2026 09:18

Elvand-Lie force-pushed the fix/jacobian-void-return-scalar-fn branch from 4520092 to 23412ba Compare August 21, 2026 10:01

Elvand-Lie marked this pull request as ready for review August 22, 2026 11:28

Merge branch 'master' into fix/jacobian-void-return-scalar-fn

899e565

	CXXRecordDecl* CXXRD = cast<CXXRecordDecl>(DC);
	auto* CXXRD = cast<CXXRecordDecl>(DC);

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Elvand-Lie commented Aug 20, 2026 •

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