nilctxpassed: spread call f(nil...) to a variadic context.Context param is a false positive — nil variadic slice misreported as [Content truncated due to length]

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Summary

nilctxpassed (the newest un-audited linter; registry is now 45 analyzers) reports a false positive for a variadic spread call such as takesVariadicCtx(nil...). In that call nil is the whole []context.Context slice (a nil slice = zero contexts, which is perfectly valid), not a context.Context element. The linter never inspects call.Ellipsis, so it treats the spread arg as an individual variadic element and emits nil passed as context.Context; use context.Background() or context.TODO() instead — a wrong, non-actionable diagnostic.

nilctxpassed is the only custom linter that does variadic element-wise indexing via sig.Variadic(), and the only one that fails to check call.Ellipsis. The analogous variadic linter appendbytestring gets this right (pkg/linters/appendbytestring/appendbytestring.go:57 gates on call.Ellipsis.IsValid()).

Location

pkg/linters/nilctxpassed/nilctxpassed.go:57-70 (the per-arg variadic branch) together with the nil check at :76.

Root-cause walkthrough

For takesVariadicCtx(nil...) where func takesVariadicCtx(args ...context.Context):

1. call.Args has length 1: the single nil expression, and call.Ellipsis is valid (the ... spread).
2. Loop for i, arg := range call.Args at i=0: sig.Variadic() && params.Len() > 0 && i >= params.Len()-1 is true && true && 0 >= 0 → enters the variadic element branch.
3. paramType is set to the slice element type context.Context (lines 61-65).
4. isContextContext(paramType) → true; isBuiltinNil(pass, arg) → true (the predeclared nil, Uses[nil] is *types.Nil).
5. pass.Report(...) fires.

But the code never distinguishes a spread call from a plain variadic call. In a spread call the final argument corresponds to the slice type []context.Context, not its element — and passing nil as that slice is a valid empty variadic, not a nil context.

Evidence it is unique:

$ grep -rln 'Variadic()' pkg/linters --include=*.go | grep -v _test
pkg/linters/nilctxpassed/nilctxpassed.go        # only element-indexing variadic linter
$ grep -rln '\.Ellipsis' pkg/linters --include=*.go | grep -v _test
pkg/linters/appendbytestring/appendbytestring.go # the only linter that guards on Ellipsis

Testdata (testdata/src/nilctxpassed/nilctxpassed.go:33-35) only exercises the non-spread variadic case takesVariadicCtx(nil) (a legitimately-flagged nil context). There is no spread-call fixture, so the FP is unguarded and untested.

Impact

• False positive on valid code f(nil...) that spreads a nil []context.Context — the message is factually wrong (no context value is passed).
• Latent enforcement blocker: nilctxpassed is otherwise clean and well-behaved (type-resolved context.Context identity via obj.Pkg().Path() == "context", wires internal/nolint, skips test + generated files via filecheck.ShouldSkipFilename). It is a natural CI-enforcement candidate, but this FP would fail CI on any legitimate nil... spread site.
• Severity is low (spread-of-nil to a variadic context param is uncommon), but it is a clear correctness defect in a brand-new linter with zero test coverage for the case.

Recommendation

In the variadic branch, treat a spread call's final argument as the whole slice rather than an element. A nil slice is a valid empty variadic, so skip the element-wise nil-context check:

Intent (before → after):

• Before: for takesVariadicCtx(nil...) the linter enters the element branch and reports a nil context.Context.
• After: when call.Ellipsis.IsValid() and the arg is the variadic slice position, the linter recognizes nil as a nil []context.Context slice and does not report.

Concretely, guard the variadic branch on !call.Ellipsis.IsValid() (mirroring appendbytestring), or continue for the final arg when the call is a spread. The non-spread variadic path (takesVariadicCtx(nil)) must stay flagged.

Validation checklist

• Add testdata fixture takesVariadicCtx(nil...) — asserted not flagged.
• Add testdata fixture takesVariadicCtx(existingSlice...) — asserted not flagged (regression guard for the general spread case).
• Keep existing takesVariadicCtx(nil) (non-spread) — still flagged.
• go test ./pkg/linters/nilctxpassed/... passes.

Effort

XS — one call.Ellipsis.IsValid() guard in run plus 1-2 testdata fixtures.

References: §29472028410

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• expires on Jul 22, 2026, 8:58 PM UTC-08:00