sprintferrdot: verb-set check still wrong both directions after issue 40434 closed

[github-actions]

Problem

sprintferrdot reports .Error() calls as "redundant" whenever the format verb is s or v, but the verb classification is wrong in both directions:

• False positive: %#v is recorded as verb v (the # flag is silently skipped, not tracked), so fmt.Sprintf("%#v", err.Error()) is flagged as redundant. It is not: %#v on an error value invokes GoStringer/prints the Go-syntax struct representation (not .Error()), so removing the explicit .Error() call changes the output from a quoted message string to a struct dump.
• False negative: %q, %x, %X also invoke the operand's Error() method per the fmt package's special-case rules (same family as %s/%v), but these verbs are excluded from the check (sprintferrdot.go:75), so genuinely redundant .Error() calls under these verbs are never flagged.

This is the exact bug described in #40434 ("sprintferrdot precision: verb handling {s,v} is wrong in both directions — %#v false positive, %q/%x/%X false negative"), which is closed, but the fix was never landed: the current code is unchanged in this respect and the test fixture has no case for #v, %q, %x, or %X.

Evidence

• pkg/linters/sprintferrdot/sprintferrdot.go:142 — the flag-skipping loop consumes # (along with -, +, , 0) without recording it, so %#v and %v produce the identical verb rune v.
• pkg/linters/sprintferrdot/sprintferrdot.go:75 — if verbs[i] != 's' && verbs[i] != 'v' { continue } excludes q/x/X, even though those verbs also trigger the error-interface special case documented in the fmt package docs ("If an operand implements the error interface, the Error method will be invoked..." — applies to %v, %s, %q, %x, %X; %#v instead uses GoStringer and does not invoke Error()).
• pkg/linters/sprintferrdot/testdata/src/sprintferrdot/sprintferrdot.go has no #v/%q/%x/%X fixture, so neither direction of the bug is covered by analysistest.

Impact

• Applying the suggested fix (dropping .Error()) for a %#v call site is a behavior-changing false positive: the printed output changes from the error message to a Go-syntax struct dump.
• Real redundant .Error() calls under %q/%x/%X are silently missed (false negative), so the linter under-reports on a real, if less common, class of the same bug it exists to catch.

Recommendation

1. Track the # flag while scanning each verb (e.g. return a small struct {verb rune; sharp bool} instead of a bare []rune), and skip (don't flag) any verb with sharp && verb == 'v'.
2. Extend the accepted-verb set at sprintferrdot.go:75 to include q, x, X alongside s, v.
3. Add fixture cases to testdata/src/sprintferrdot/sprintferrdot.go: fmt.Sprintf("%#v", err.Error()) (no diagnostic expected) and fmt.Sprintf("%q", err.Error()) / %x / %X (diagnostic expected), then re-run analysistest.

Validation checklist

• %#v with .Error() no longer flagged
• %q, %x, %X with .Error() are flagged
• existing %s/%v cases still flagged, %w/bare-error cases still clean
• analysistest golden fixtures updated and passing

Effort

Small — self-contained change to parseSimpleFormatVerbs and the verb-acceptance check, plus fixture additions. No SuggestedFix is emitted by this linter, so no autofix-correctness risk.

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