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Summary

Fixes #3908 — implicit conversion from DateTime to DateTimeOffset generates a call to a non-existent op_Implicit function in the runtime library, causing:

SyntaxError: The requested module '…/DateOffset.js' does not provide an export named 'op_Implicit'

Root Cause

When F# compiles let dto: DateTimeOffset = someDateTime, it emits a call to op_Implicit. In Replacements.fs, the dateTimeOffset function had no case for "op_Implicit", so it fell through to the catch-all handler:

| meth ->
    let meth = Naming.removeGetSetPrefix meth |> Naming.lowerFirst
    Helper.LibCall(com, moduleName, meth, t, args, ...)

This generated import { op_Implicit } from "…/DateOffset.js" — but op_Implicit was never exported from that module.

Fix

Add an explicit "op_Implicit" case that redirects to the existing fromDate function, which already handles DateTime → DateTimeOffset conversion (it's used by the .ctor path when a DateTime is passed):

| "op_Implicit" ->
    Helper.LibCall(com, moduleName, "fromDate", t, args, i.SignatureArgTypes, ?loc = r)
    |> Some

Changed file: src/Fable.Transforms/Replacements.fs — 3-line addition in the dateTimeOffset handler.

Test

Added a test in tests/Js/Main/DateTimeTests.fs that assigns a DateTime to a DateTimeOffset binding and verifies the UtcDateTime round-trips correctly.

Closes #3908

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