module: Tweak backdate-warning-turned error (#58651)

This makes two changes to the backdate-warning-turned-error (#58266):
1. Fix a bug where the error would only trigger the first time. We do
only want to print once per process, but of course, we do want to error
every time if enabled.
2. If we are in speculative execution context (generators and
speculatively run functions during inference), always use the
UndefVarError. Effects from these functions are not supposed to be
observable, and it's very confusing if the printed warning goes away
when depwarns are enabled. This is marginally more breaking, but the
burden is on generated function authors (which already have to be
world-age aware and are somewhat more regularly broken) and is
consistent with other things that are stronger errors in pure context.

Fixes #58648

(cherry picked from commit d2cc061)

Author: Keno

src/module.c

@@ -846,8 +846,6 @@ JL_DLLEXPORT jl_module_t *jl_get_module_of_binding(jl_module_t *m, jl_sym_t *var
 
 static NOINLINE void print_backdate_admonition(jl_binding_t *b) JL_NOTSAFEPOINT
 {
-    if (jl_options.depwarn == JL_OPTIONS_DEPWARN_ERROR)
-        jl_undefined_var_error(b->globalref->name, (jl_value_t*)b->globalref->mod);
     jl_safe_printf(
         "WARNING: Detected access to binding `%s.%s` in a world prior to its definition world.\n"
         "  Julia 1.12 has introduced more strict world age semantics for global bindings.\n"
@@ -860,9 +858,13 @@ static NOINLINE void print_backdate_admonition(jl_binding_t *b) JL_NOTSAFEPOINT
 
 static inline void check_backdated_binding(jl_binding_t *b, enum jl_partition_kind kind) JL_NOTSAFEPOINT
 {
-    if (__unlikely(kind == PARTITION_KIND_BACKDATED_CONST) &&
-        !(jl_atomic_fetch_or_relaxed(&b->flags, BINDING_FLAG_DID_PRINT_BACKDATE_ADMONITION) & BINDING_FLAG_DID_PRINT_BACKDATE_ADMONITION)) {
-        print_backdate_admonition(b);
+    if (__unlikely(kind == PARTITION_KIND_BACKDATED_CONST)) {
+        // We don't want functions that inference executes speculatively to print this warning, so turn those into
+        // an error for inference purposes.
+        if (jl_current_task->ptls->in_pure_callback || jl_options.depwarn == JL_OPTIONS_DEPWARN_ERROR)
+            jl_undefined_var_error(b->globalref->name, (jl_value_t*)b->globalref->mod);
+        if (!(jl_atomic_fetch_or_relaxed(&b->flags, BINDING_FLAG_DID_PRINT_BACKDATE_ADMONITION) & BINDING_FLAG_DID_PRINT_BACKDATE_ADMONITION))
+            print_backdate_admonition(b);
     }
 }