src/backend/nodes/equalfuncs.c

CONFIRMED defensive-unreachable. _equalRPRPattern (T_RPRPattern, tag 387) is only dispatched from equalfuncs.switch.c:889. RPRPattern is the COMPILED pattern node built exclusively in the optimizer (createplan.c:2906 via buildRPRPattern -> rpr.c makeRPRPattern) and stored only as a field of the WindowAgg plan node (plannodes.h:1393) and the WindowAggState exec node (execnodes.h:2654).
It never appears in a raw-parse or rewritten query tree. equal() is auto-invoked by the COPY/WRITE_READ_PARSE_PLAN_TREES debug machinery in tcop/postgres.c ONLY on raw parse trees (lines 638/655) and rewritten query trees (lines 849/878).
The two plan-tree equal() checks at postgres.c:935 and 958 sit inside #ifdef NOT_USED ("equal() currently does not have routines to compare Plan nodes") and are never compiled;
NOT_USED is not defined anywhere.
I attempted to refute via the one real equal()-on-rpPattern call site: planner.c:6283 equal(wc->rpPattern, ...) -- but wc is a WindowClause whose rpPattern field is RPRPatternNode* (parsenodes.h:1767, the PARSE node T_RPRPatternNode tag 88), so that call dispatches to _equalRPRPatternNode, NOT _equalRPRPattern.
The other equal() near RPR code (nodeWindowAgg.c:2917) compares a WindowFunc. _equalRPRPattern exists solely because the struct carries pg_node_attr(custom_copy_equal, ...) (plannodes.h:1285), which forces the gen_node_support machinery to emit a switch case and require a hand-written equal counterpart even though the copy half is what is actually exercised (custom_copy_equal cannot request copy-only).
No SQL/regression input can construct a standalone T_RPRPattern node and feed it to equal();
line 154 is genuinely unreachable.

Function-level recommendation.
Since equal() is never dispatched for RPRPattern (Plan-only container with no_equal), the custom_copy_equal attribute over-specifies what is needed: only custom copy is actually exercised.
Two viable cleanups:
(1) Change the pg_node_attr on RPRPattern (plannodes.h ~line 1285) from custom_copy_equal to custom_copy plus no_equal, which stops emitting/registering the dead _equalRPRPattern and removes the unreachable T_RPRPattern case from equalfuncs.switch.c — eliminating all 94-counted dead lines in this function rather than just line 173. (2) If the hand-written _equalRPRPattern is intentionally kept for API completeness/future direct use, exclude it from modified-line coverage accounting (it is provably unreachable from SQL).
Do NOT attempt to add a SQL test or a runtime Assert here — neither can drive execution through line 173.