[rupayon123]

A single required_if literal misses dependencies whose Go type is a defined scalar. For example, type Status string with Status("member") allows an empty field under required_if:Mode,member, while the equivalent built-in string correctly requires it.

Reuse IsEqual after the existing literal conversion succeeds. This keeps the existing conversion and empty-value rules and aligns the single-literal path with the value comparison already used elsewhere.

Validated against current master b148b170 on Go 1.26.0 / macOS arm64:

• Eleven failing regressions and seven controls become 18 passing cases.
• Full coverage and race suites each pass 865 leaf tests, with no skips; vet, build, formatting, tidy and all 11 required benchmarks pass.
• Public API comparisons preserve 4,968 built-in outcomes and eliminate 300 defined-type mismatches across 3,240 paired observations.
• Configured lint reports the same 65 baseline diagnostics, and Staticcheck the same seven; the documented vet fallback passes. This is not a lint-clean claim.

Other supported runtime/OS combinations and hosted CI remain unverified locally.