bpart: Give a warning when accessing a backdated const binding

This implements the strategy proposed in #57102 (comment).
Example:
```
julia> function foo(i)
           eval(:(const x = $i))
           x
       end
foo (generic function with 1 method)

julia> foo(1)
WARNING: Detected access to binding Main.x in a world prior to its definition world.
  Julia 1.12 has introduced more strict world age semantics for global bindings.
  !!! This code may malfunction under Revise.
  !!! This code will error in future versions of Julia.
Hint: Add an appropriate `invokelatest` around the access to this binding.
1
```

The warning is triggered once per binding to avoid spamming for repeated access.

Compiler/src/Compiler.jl

@@ -49,7 +49,8 @@ using Core: ABIOverride, Builtin, CodeInstance, IntrinsicFunction, MethodInstanc
 
 using Base
 using Base: @_foldable_meta, @_gc_preserve_begin, @_gc_preserve_end, @nospecializeinfer,
-    BINDING_KIND_GLOBAL, BINDING_KIND_UNDEF_CONST, Base, BitVector, Bottom, Callable, DataTypeFieldDesc,
+    BINDING_KIND_GLOBAL, BINDING_KIND_UNDEF_CONST, BINDING_KIND_BACKDATED_CONST,
+    Base, BitVector, Bottom, Callable, DataTypeFieldDesc,
     EffectsOverride, Filter, Generator, IteratorSize, JLOptions, NUM_EFFECTS_OVERRIDES,
     OneTo, Ordering, RefValue, SizeUnknown, _NAMEDTUPLE_NAME,
     _array_for, _bits_findnext, _methods_by_ftype, _uniontypes, all, allocatedinline, any,

Compiler/src/abstractinterpretation.jl

@@ -3524,6 +3524,11 @@ function abstract_eval_partition_load(interp::AbstractInterpreter, partition::Co
     end
 
     if is_defined_const_binding(kind)
+        if kind == BINDING_KIND_BACKDATED_CONST
+            # Infer this as guard. We do not want a later const definition to retroactively improve
+            # inference results in an earlier world.
+            return RTEffects(Any, UndefVarError, generic_getglobal_effects)
+        end
         rt = Const(partition_restriction(partition))
         return RTEffects(rt, Union{}, Effects(EFFECTS_TOTAL, inaccessiblememonly=is_mutation_free_argtype(rt) ? ALWAYS_TRUE : ALWAYS_FALSE))
     end

Compiler/src/ssair/slot2ssa.jl

@@ -137,10 +137,6 @@ function fixemup!(@specialize(slot_filter), @specialize(rename_slot), ir::IRCode
                 return nothing
             end
             op[] = x
-        elseif isa(val, GlobalRef) && !(isdefined(val.mod, val.name) && isconst(val.mod, val.name))
-            typ = typ_for_val(val, ci, ir, idx, Any[])
-            new_inst = NewInstruction(val, typ)
-            op[] = NewSSAValue(insert_node!(ir, idx, new_inst).id - length(ir.stmts))
         elseif isexpr(val, :static_parameter)
             ty = typ_for_val(val, ci, ir, idx, Any[])
             if isa(ty, Const)

Compiler/src/ssair/verify.jl

@@ -121,7 +121,7 @@ function verify_ir(ir::IRCode, print::Bool=true,
         if mi !== nothing
             push!(error_args, "\n", "  Method instance: ", mi)
         end
-        error(error_args...)
+        invokelatest(error, error_args...)
     end
     # Verify CFG graph. Must be well formed to construct domtree
     if !(length(ir.cfg.blocks) - 1 <= length(ir.cfg.index) <= length(ir.cfg.blocks))
@@ -380,6 +380,15 @@ function verify_ir(ir::IRCode, print::Bool=true,
                         # undefined GlobalRef is OK in isdefined
                         continue
                     end
+                elseif stmt.head === :throw_undef_if_not
+                    if length(stmt.args) > 3
+                        @verify_error "malformed throw_undef_if_not"
+                        raise_error()
+                    end
+                    if stmt.args[1] isa GlobalRef
+                        # undefined GlobalRef is OK in throw_undef_if_not
+                        continue
+                    end
                 elseif stmt.head === :gc_preserve_end
                     # We allow gc_preserve_end tokens to span across try/catch
                     # blocks, which isn't allowed for regular SSA values, so

Compiler/test/invalidation.jl

@@ -55,7 +55,7 @@ let mi = Base.method_instance(basic_caller, (Float64,))
 end
 
 # this redefinition below should invalidate the cache
-const BASIC_CALLER_WORLD = Base.get_world_counter()
+const BASIC_CALLER_WORLD = Base.get_world_counter()+1
 basic_callee(x) = x, x
 @test !isdefined(Base.method_instance(basic_callee, (Float64,)), :cache)
 let mi = Base.method_instance(basic_caller, (Float64,))

base/Base_compiler.jl

@@ -257,6 +257,10 @@ using .Order
 include("coreir.jl")
 include("invalidation.jl")
 
+# Because lowering inserts direct references, it is mandatory for this binding
+# to exist before we start inferring code.
+function string end
+
 # For OS specific stuff
 # We need to strcat things here, before strings are really defined
 function strcat(x::String, y::String)

base/boot.jl

@@ -383,9 +383,10 @@ struct StackOverflowError  <: Exception end
 struct UndefRefError       <: Exception end
 struct UndefVarError <: Exception
     var::Symbol
+    world::UInt64
     scope # a Module or Symbol or other object describing the context where this variable was looked for (e.g. Main or :local or :static_parameter)
-    UndefVarError(var::Symbol) = new(var)
-    UndefVarError(var::Symbol, @nospecialize scope) = new(var, scope)
+    UndefVarError(var::Symbol) = new(var, ccall(:jl_get_tls_world_age, UInt, ()))
+    UndefVarError(var::Symbol, @nospecialize scope) = new(var, ccall(:jl_get_tls_world_age, UInt, ()), scope)
 end
 struct ConcurrencyViolationError <: Exception
     msg::AbstractString
@@ -717,7 +718,8 @@ macro __doc__(x)
 end
 
 isbasicdoc(@nospecialize x) = (isa(x, Expr) && x.head === :.) || isa(x, Union{QuoteNode, Symbol})
-iscallexpr(ex::Expr) = (isa(ex, Expr) && ex.head === :where) ? iscallexpr(ex.args[1]) : (isa(ex, Expr) && ex.head === :call)
+firstarg(arg1, args...) = arg1
+iscallexpr(ex::Expr) = (isa(ex, Expr) && ex.head === :where) ? iscallexpr(firstarg(ex.args...)) : (isa(ex, Expr) && ex.head === :call)
 iscallexpr(ex) = false
 function ignoredoc(source, mod, str, expr)
     (isbasicdoc(expr) || iscallexpr(expr)) && return Expr(:escape, nothing)

base/docs/bindings.jl

@@ -16,8 +16,8 @@ end
 
 bindingexpr(x) = Expr(:call, Binding, splitexpr(x)...)
 
-defined(b::Binding) = isdefined(b.mod, b.var)
-resolve(b::Binding) = getfield(b.mod, b.var)
+defined(b::Binding) = invokelatest(isdefined, b.mod, b.var)
+resolve(b::Binding) = invokelatest(getfield, b.mod, b.var)
 
 function splitexpr(x::Expr)
     isexpr(x, :macrocall) ? splitexpr(x.args[1]) :

base/runtime_internals.jl

@@ -229,8 +229,9 @@ const BINDING_KIND_FAILED       = 0x6
 const BINDING_KIND_DECLARED     = 0x7
 const BINDING_KIND_GUARD        = 0x8
 const BINDING_KIND_UNDEF_CONST  = 0x9
+const BINDING_KIND_BACKDATED_CONST = 0xa
 
-is_defined_const_binding(kind::UInt8) = (kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT)
+is_defined_const_binding(kind::UInt8) = (kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT || kind == BINDING_KIND_BACKDATED_CONST)
 is_some_const_binding(kind::UInt8) = (is_defined_const_binding(kind) || kind == BINDING_KIND_UNDEF_CONST)
 is_some_imported(kind::UInt8) = (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_EXPLICIT || kind == BINDING_KIND_IMPORTED)
 is_some_guard(kind::UInt8) = (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED || kind == BINDING_KIND_UNDEF_CONST)

base/show.jl

@@ -35,7 +35,7 @@ function _isself(ft::DataType)
     isdefined(ftname, :mt) || return false
     name = ftname.mt.name
     mod = parentmodule(ft)  # NOTE: not necessarily the same as ft.name.mt.module
-    return isdefined(mod, name) && ft == typeof(getfield(mod, name))
+    return invokelatest(isdefinedglobal, mod, name) && ft == typeof(invokelatest(getglobal, mod, name))
 end
 
 function show(io::IO, ::MIME"text/plain", f::Function)
@@ -542,8 +542,8 @@ function show_function(io::IO, f::Function, compact::Bool, fallback::Function)
         fallback(io, f)
     elseif compact
         print(io, mt.name)
-    elseif isdefined(mt, :module) && isdefined(mt.module, mt.name) &&
-        getfield(mt.module, mt.name) === f
+    elseif isdefined(mt, :module) && isdefinedglobal(mt.module, mt.name) &&
+            getglobal(mt.module, mt.name) === f
         # this used to call the removed internal function `is_exported_from_stdlib`, which effectively
         # just checked for exports from Core and Base.
         mod = get(io, :module, UsesCoreAndBaseOnly)
@@ -1025,15 +1025,15 @@ function isvisible(sym::Symbol, parent::Module, from::Module)
     from_owner = ccall(:jl_binding_owner, Ptr{Cvoid}, (Any, Any), from, sym)
     return owner !== C_NULL && from_owner === owner &&
         !isdeprecated(parent, sym) &&
-        isdefined(from, sym) # if we're going to return true, force binding resolution
+        isdefinedglobal(from, sym) # if we're going to return true, force binding resolution
 end
 
 function is_global_function(tn::Core.TypeName, globname::Union{Symbol,Nothing})
     if globname !== nothing
         globname_str = string(globname::Symbol)
         if ('#' ∉ globname_str && '@' ∉ globname_str && isdefined(tn, :module) &&
-                isbindingresolved(tn.module, globname) && isdefined(tn.module, globname) &&
-                isconcretetype(tn.wrapper) && isa(getfield(tn.module, globname), tn.wrapper))
+                isbindingresolved(tn.module, globname) && isdefinedglobal(tn.module, globname) &&
+                isconcretetype(tn.wrapper) && isa(getglobal(tn.module, globname), tn.wrapper))
             return true
         end
     end
@@ -3364,7 +3364,10 @@ function print_partition(io::IO, partition::Core.BindingPartition)
     end
     print(io, " - ")
     kind = binding_kind(partition)
-    if is_defined_const_binding(kind)
+    if kind == BINDING_KIND_BACKDATED_CONST
+        print(io, "backdated constant binding to ")
+        print(io, partition_restriction(partition))
+    elseif is_defined_const_binding(kind)
         print(io, "constant binding to ")
         print(io, partition_restriction(partition))
     elseif kind == BINDING_KIND_UNDEF_CONST

doc/src/manual/methods.md

@@ -587,12 +587,13 @@ In the example above, we see that the "current world" (in which the method `newf
 is one greater than the task-local "runtime world" that was fixed when the execution of `tryeval` started.
 
 Sometimes it is necessary to get around this (for example, if you are implementing the above REPL).
-Fortunately, there is an easy solution: call the function using [`Base.invokelatest`](@ref):
+Fortunately, there is an easy solution: call the function using [`Base.invokelatest`](@ref) or
+the macro version [`Base.@invokelatest`](@ref):
 
 ```jldoctest
 julia> function tryeval2()
            @eval newfun2() = 2
-           Base.invokelatest(newfun2)
+           @invokelatest newfun2()
        end
 tryeval2 (generic function with 1 method)
 

src/codegen.cpp

@@ -3467,7 +3467,8 @@ static jl_cgval_t emit_globalref(jl_codectx_t &ctx, jl_module_t *mod, jl_sym_t *
                 break;
             pku = jl_atomic_load_acquire(&bpart->restriction);
         }
-        if (bpart && jl_bkind_is_some_constant(decode_restriction_kind(pku))) {
+        enum jl_partition_kind kind = decode_restriction_kind(pku);
+        if (bpart && (jl_bkind_is_some_constant(kind) && kind != BINDING_KIND_BACKDATED_CONST)) {
             jl_value_t *constval = decode_restriction_value(pku);
             if (!constval) {
                 undef_var_error_ifnot(ctx, ConstantInt::get(getInt1Ty(ctx.builder.getContext()), 0), name, (jl_value_t*)mod);

src/julia.h

@@ -647,7 +647,10 @@ enum jl_partition_kind {
     // Undef Constant: This binding partition is a constant declared using `const`, but
     // without a value.
     //  ->restriction is NULL
-    BINDING_KIND_UNDEF_CONST  = 0x9
+    BINDING_KIND_UNDEF_CONST  = 0x9,
+    // Backated constant. A constant that was backdated for compatibility. In all other
+    // ways equivalent to BINDING_KIND_CONST, but prints a warning on access
+    BINDING_KIND_BACKDATED_CONST = 0xa,
 };
 
 #ifdef _P64
@@ -693,7 +696,7 @@ typedef struct _jl_binding_t {
     jl_globalref_t *globalref;  // cached GlobalRef for this binding
     _Atomic(jl_value_t*) value;
     _Atomic(jl_binding_partition_t*) partitions;
-    uint8_t declared:1;
+    uint8_t did_print_backdate_admonition:1;
     uint8_t exportp:1; // `public foo` sets `publicp`, `export foo` sets both `publicp` and `exportp`
     uint8_t publicp:1; // exportp without publicp is not allowed.
     uint8_t deprecated:2; // 0=not deprecated, 1=renamed, 2=moved to another package
@@ -2025,10 +2028,6 @@ JL_DLLEXPORT void jl_module_public(jl_module_t *from, jl_sym_t *s, int exported)
 JL_DLLEXPORT int jl_is_imported(jl_module_t *m, jl_sym_t *s);
 JL_DLLEXPORT int jl_module_exports_p(jl_module_t *m, jl_sym_t *var);
 JL_DLLEXPORT void jl_add_standard_imports(jl_module_t *m);
-STATIC_INLINE jl_function_t *jl_get_function(jl_module_t *m, const char *name)
-{
-    return (jl_function_t*)jl_get_global(m, jl_symbol(name));
-}
 
 // eq hash tables
 JL_DLLEXPORT jl_genericmemory_t *jl_eqtable_put(jl_genericmemory_t *h JL_ROOTING_ARGUMENT, jl_value_t *key, jl_value_t *val JL_ROOTED_ARGUMENT, int *inserted);
@@ -2587,9 +2586,18 @@ typedef struct {
 } jl_nullable_float32_t;
 
 #define jl_root_task (jl_current_task->ptls->root_task)
-
 JL_DLLEXPORT jl_task_t *jl_get_current_task(void) JL_GLOBALLY_ROOTED JL_NOTSAFEPOINT;
 
+STATIC_INLINE jl_function_t *jl_get_function(jl_module_t *m, const char *name)
+{
+    jl_task_t *ct = jl_get_current_task();
+    size_t last_world = ct->world_age;
+    ct->world_age = jl_get_world_counter();
+    jl_value_t *r = jl_get_global(m, jl_symbol(name));
+    ct->world_age = last_world;
+    return (jl_function_t*)r;
+}
+
 // TODO: we need to pin the task while using this (set pure bit)
 JL_DLLEXPORT jl_jmp_buf *jl_get_safe_restore(void) JL_NOTSAFEPOINT;
 JL_DLLEXPORT void jl_set_safe_restore(jl_jmp_buf *) JL_NOTSAFEPOINT;

src/julia_internal.h

@@ -933,6 +933,10 @@ EXTERN_INLINE_DECLARE enum jl_partition_kind decode_restriction_kind(jl_ptr_kind
         if (bits == BINDING_KIND_CONST) {
             return BINDING_KIND_UNDEF_CONST;
         }
+    } else {
+        if (bits == BINDING_KIND_DECLARED) {
+            return BINDING_KIND_BACKDATED_CONST;
+        }
     }
 
     return (enum jl_partition_kind)bits;
@@ -956,12 +960,14 @@ STATIC_INLINE jl_ptr_kind_union_t encode_restriction(jl_value_t *val, enum jl_pa
 #ifdef _P64
     if (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED || kind == BINDING_KIND_UNDEF_CONST)
         assert(val == NULL);
-    else if (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_CONST)
+    else if (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_CONST || kind == BINDING_KIND_BACKDATED_CONST)
         assert(val != NULL);
     if (kind == BINDING_KIND_GUARD)
         kind = BINDING_KIND_IMPLICIT;
     else if (kind == BINDING_KIND_UNDEF_CONST)
         kind = BINDING_KIND_CONST;
+    else if (kind == BINDING_KIND_BACKDATED_CONST)
+        kind = BINDING_KIND_DECLARED;
     assert((((uintptr_t)val) & 0x7) == 0);
     return ((jl_ptr_kind_union_t)val) | kind;
 #else
@@ -975,11 +981,11 @@ STATIC_INLINE int jl_bkind_is_some_import(enum jl_partition_kind kind) JL_NOTSAF
 }
 
 STATIC_INLINE int jl_bkind_is_some_constant(enum jl_partition_kind kind) JL_NOTSAFEPOINT {
-    return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT || kind == BINDING_KIND_UNDEF_CONST;
+    return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT || kind == BINDING_KIND_UNDEF_CONST || kind == BINDING_KIND_BACKDATED_CONST;
 }
 
 STATIC_INLINE int jl_bkind_is_defined_constant(enum jl_partition_kind kind) JL_NOTSAFEPOINT {
-    return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT;
+    return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT || kind == BINDING_KIND_BACKDATED_CONST;
 }
 
 STATIC_INLINE int jl_bkind_is_some_guard(enum jl_partition_kind kind) JL_NOTSAFEPOINT {