register_theme! accepts a theme that cannot render, and hasmethod cannot be the guard that rejects it

[sotashimozono]

register_theme! is a one-line insert with no validation:

register_theme!(name::Symbol, theme::Theme) = (_THEMES[name] = theme)

A theme that never defines emit_document registers happily and fails at the first render that selects it. The message when it does fail is good — Pinax.emit_document's fallback on ::Theme errors by name:

ERROR: Pinax: theme NoEmit does not implement `emit_document`. Define `Pinax.emit_document(::NoEmit, …)`.

So this is about when, not about diagnosis quality. A user theme loaded from a path (theme="mytheme.jl") is evaluated, type-checked as a Theme, registered, and only then discovered to be unusable — potentially after a long resolve/materialize pass.

The part worth writing down: hasmethod cannot express this check

The obvious guard does not work, and it does not work silently. Measured:

struct NoEmit  <: Pinax.Theme end
struct HasEmit <: Pinax.Theme end
Pinax.emit_document(::HasEmit, doc, out, cache; comments_file="") = "x"

theme	hasmethod(emit_document, Tuple{T,Any,Any,Any})	which(...).sig
NoEmit	true	Tuple{typeof(emit_document), Theme, Any, Any, Any}
HasEmit	true	Tuple{typeof(emit_document), HasEmit, Any, Any, Any}

hasmethod is true for both, because the erroring fallback on ::Theme is a method. A guard written with it would pass every theme and look like it was doing something — the failure mode this repository keeps finding in other people's code, in its own registry.

The distinguishing test is which(...).sig: the fallback's first parameter is Theme itself, a real implementation's is the concrete type.

Fix

If this is worth guarding at all:

function register_theme!(name::Symbol, theme::Theme)
    m = which(emit_document, Tuple{typeof(theme),Any,Any,Any})
    m.sig.parameters[2] === Theme && error(
        "Pinax: $(typeof(theme)) does not implement `emit_document`, so it cannot render. " *
        "Define it before registering.",
    )
    return _THEMES[name] = theme
end

Low priority — nothing is silently wrong today, and the render-time message already names the missing method. Filed mainly so the hasmethod trap is on record before someone reaches for it.