def X := with multi-key layout body fails to parse; identical body without := works

[hierophantos]

Repro (A/B, byte-identical bodies — verified at 2b1b383d)

ns repro

def r1 :=
  :eu {:host "eu.example.com" :port 443}
  :us {:host "us.example.com" :port 443}
;; => ERROR: Could not infer type

def r2
  :eu {:host "eu.example.com" :port 443}
  :us {:host "us.example.com" :port 443}
;; => r2 : {:eu {:host String :port Int} :us {:host String :port Int}} defined.

The only difference is the := on the def head. The implicit-map form
(def r2, no :=) elaborates the identical layout body to a fully-typed
nested row; the := form fails.

Trigger shape

Fiddly, which itself suggests a defect rather than a documented restriction:

• def X := + a layout body with >= 2 top-level keys fails
• def X := + >= 2 dash items where one carries a multi-line @[...] fails
• single-line := bodies (def X := {:a 1}) work
• the same multi-key layout body under def X (no :=) works

Diagnosis notes

• The diagnostic — Could not infer type — names the wrong subsystem; this is
  a parse/layout seam (the body's keys are apparently not being folded into a
  map literal under the := head), not a typing failure.
• Adjacent history: the implicit-map def WS path was touched in the
  2026-07-18 hand-testing arc (ff31d237, process-string-ws parity), so this
  seam has moved recently.
• Found while running the Path Selection spec's Appendix fixtures
  (4 of its def X := forms hit this). Workaround used in corpus files:
  the def X implicit-map form.

Filed from the CIU T6 Path Selection D5 critique (2026-07-28); also recorded
in docs/tracking/DEFERRED.md.