Add a warning for auto-import of types (#57311)

This adds a warning for the auto-import of types cases (#25744) that we
have long considered a bit of a bug, but didn't want to change because
it is too breaking.

The reason to do it now is that the binding rework has made this case
more problematic (see #57290). To summarize, the question is what
happens when the compiler sees `f(x) = ...` and `f` is currently and
implicitly imported binding. There are two options:

1. We add a method to the generic function referred to by `f`, or
2. We create a new generic function `f` in the current module.

Historically, case 1 has the additional complication that this error'd
unless `f` is a type. It is my impression that a lot of existing code
did not have a particularly good understanding of the resolved-ness
dependence of this behavior.

However, because case 1 errors for generic functions, it appears that
existing code generally expects case 2. On the other hand, for types,
there is existing code in both directions (#57290 is an example of case
2; see #57302 for examples of case 1). That said, case 1 is more common
(because types tend to be resolved because they're used in signatures at
toplevel).

Thus, to retain compatibility, the current behavior on master (where
resolvedness is no longer available) is that we always choose case 2 for
functions and case 1 for types. This inconsistency is unfortunate, but I
tried resolving this in either way (making all situations case 1 or all
case 2) and the result was too breaking.

Nevertheless, it is problematic that there is existing code that expects
case 2 beavior for types and we should help users to know what the
correct way to fix it is. The proposed resolution is thus:
1. Retain case 1 behavior for types
2. Make it a warning to use, encouraging people to explicitly import,
since we generally consider the #25744 case a bug.

Example:
```
julia> module Foo
         String(i::Int) = i
       end
WARNING: Type Core.String was auto-`import`ed in `Foo`.
NOTE: This behavior is deprecated and may change in future Julia versions.
NOTE: This behavior may have differed in Julia versions prior to 1.12 depending on binding resolution.
Hint: To retain the current behavior, add an explicit `import Core: String` in Foo.
Hint: To create a new generic function of the same name use `function String end`.
Main.Foo
```

Author: Keno

Diff for: src/codegen.cpp

@@ -1005,7 +1005,7 @@ static const auto jlgenericfunction_func = new JuliaFunction<>{
         auto T_jlvalue = JuliaType::get_jlvalue_ty(C);
         auto T_pjlvalue = PointerType::get(T_jlvalue, 0);
         auto T_prjlvalue = PointerType::get(T_jlvalue, AddressSpace::Tracked);
-        return FunctionType::get(T_prjlvalue, {T_pjlvalue, T_pjlvalue, T_pjlvalue}, false);
+        return FunctionType::get(T_prjlvalue, {T_pjlvalue, T_pjlvalue}, false);
     },
     nullptr,
 };
@@ -6853,8 +6853,6 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaidx_
             jl_value_t *mn = args[0];
             assert(jl_is_symbol(mn) || jl_is_slotnumber(mn) || jl_is_globalref(mn));
 
-            Value *bp = NULL, *name;
-            jl_binding_t *bnd = NULL;
             bool issym = jl_is_symbol(mn);
             bool isglobalref = !issym && jl_is_globalref(mn);
             jl_module_t *mod = ctx.module;
@@ -6863,26 +6861,11 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaidx_
                     mod = jl_globalref_mod(mn);
                     mn = (jl_value_t*)jl_globalref_name(mn);
                 }
-                JL_TRY {
-                    if (jl_symbol_name((jl_sym_t*)mn)[0] == '@')
-                        jl_errorf("macro definition not allowed inside a local scope");
-                    name = literal_pointer_val(ctx, mn);
-                    bnd = jl_get_binding_for_method_def(mod, (jl_sym_t*)mn);
-                }
-                JL_CATCH {
-                    jl_value_t *e = jl_current_exception(jl_current_task);
-                    // errors. boo. :(
-                    JL_GC_PUSH1(&e);
-                    e = jl_as_global_root(e, 1);
-                    JL_GC_POP();
-                    raise_exception(ctx, literal_pointer_val(ctx, e));
-                    return ghostValue(ctx, jl_nothing_type);
-                }
-                bp = julia_binding_gv(ctx, bnd);
                 jl_cgval_t gf = mark_julia_type(
                         ctx,
-                        ctx.builder.CreateCall(prepare_call(jlgenericfunction_func), { bp,
-                            literal_pointer_val(ctx, (jl_value_t*)mod), name
+                        ctx.builder.CreateCall(prepare_call(jlgenericfunction_func), {
+                            literal_pointer_val(ctx, (jl_value_t*)mod),
+                            literal_pointer_val(ctx, (jl_value_t*)mn)
                         }),
                         true,
                         jl_function_type);

Diff for: src/interpreter.c

@@ -93,8 +93,7 @@ static jl_value_t *eval_methoddef(jl_expr_t *ex, interpreter_state *s)
         if (!jl_is_symbol(fname)) {
             jl_error("method: invalid declaration");
         }
-        jl_binding_t *b = jl_get_binding_for_method_def(modu, fname);
-        return jl_declare_const_gf(b, modu, fname);
+        return jl_declare_const_gf(modu, fname);
     }
 
     jl_value_t *atypes = NULL, *meth = NULL, *fname = NULL;

Diff for: src/julia.h

@@ -1475,6 +1475,13 @@ STATIC_INLINE char *jl_symbol_name_(jl_sym_t *s) JL_NOTSAFEPOINT
 }
 #define jl_symbol_name(s) jl_symbol_name_(s)
 
+STATIC_INLINE const char *jl_module_debug_name(jl_module_t *mod) JL_NOTSAFEPOINT
+{
+    if (!mod)
+        return "<null>";
+    return jl_symbol_name(mod->name);
+}
+
 static inline uint32_t jl_fielddesc_size(int8_t fielddesc_type) JL_NOTSAFEPOINT
 {
     assert(fielddesc_type >= 0 && fielddesc_type <= 2);
@@ -1901,7 +1908,7 @@ JL_DLLEXPORT jl_value_t *jl_get_binding_value(jl_binding_t *b JL_PROPAGATES_ROOT
 JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_const(jl_binding_t *b JL_PROPAGATES_ROOT);
 JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
 JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved_and_const(jl_binding_t *b JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
-JL_DLLEXPORT jl_value_t *jl_declare_const_gf(jl_binding_t *b, jl_module_t *mod, jl_sym_t *name);
+JL_DLLEXPORT jl_value_t *jl_declare_const_gf(jl_module_t *mod, jl_sym_t *name);
 JL_DLLEXPORT jl_method_t *jl_method_def(jl_svec_t *argdata, jl_methtable_t *mt, jl_code_info_t *f, jl_module_t *module);
 JL_DLLEXPORT jl_code_info_t *jl_code_for_staged(jl_method_instance_t *linfo, size_t world, jl_code_instance_t **cache);
 JL_DLLEXPORT jl_code_info_t *jl_copy_code_info(jl_code_info_t *src);
@@ -2051,7 +2058,7 @@ JL_DLLEXPORT jl_value_t *jl_get_binding_type(jl_module_t *m, jl_sym_t *var);
 // get binding for assignment
 JL_DLLEXPORT void jl_check_binding_currently_writable(jl_binding_t *b, jl_module_t *m, jl_sym_t *s);
 JL_DLLEXPORT jl_binding_t *jl_get_binding_wr(jl_module_t *m JL_PROPAGATES_ROOT, jl_sym_t *var);
-JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m JL_PROPAGATES_ROOT, jl_sym_t *var);
+JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m JL_PROPAGATES_ROOT, jl_sym_t *var, size_t new_world);
 JL_DLLEXPORT int jl_boundp(jl_module_t *m, jl_sym_t *var, int allow_import);
 JL_DLLEXPORT int jl_defines_or_exports_p(jl_module_t *m, jl_sym_t *var);
 JL_DLLEXPORT int jl_is_const(jl_module_t *m, jl_sym_t *var);

Diff for: src/julia_internal.h

@@ -876,6 +876,7 @@ STATIC_INLINE size_t module_usings_max(jl_module_t *m) JL_NOTSAFEPOINT {
     return m->usings.max/3;
 }
 
+JL_DLLEXPORT jl_sym_t *jl_module_name(jl_module_t *m) JL_NOTSAFEPOINT;
 jl_value_t *jl_eval_global_var(jl_module_t *m JL_PROPAGATES_ROOT, jl_sym_t *e);
 jl_value_t *jl_interpret_opaque_closure(jl_opaque_closure_t *clos, jl_value_t **args, size_t nargs);
 jl_value_t *jl_interpret_toplevel_thunk(jl_module_t *m, jl_code_info_t *src);

Diff for: src/method.c

@@ -1050,10 +1050,11 @@ JL_DLLEXPORT void jl_check_gf(jl_value_t *gf, jl_sym_t *name)
         jl_errorf("cannot define function %s; it already has a value", jl_symbol_name(name));
 }
 
-JL_DLLEXPORT jl_value_t *jl_declare_const_gf(jl_binding_t *b, jl_module_t *mod, jl_sym_t *name)
+JL_DLLEXPORT jl_value_t *jl_declare_const_gf(jl_module_t *mod, jl_sym_t *name)
 {
     JL_LOCK(&world_counter_lock);
     size_t new_world = jl_atomic_load_relaxed(&jl_world_counter) + 1;
+    jl_binding_t *b = jl_get_binding_for_method_def(mod, name, new_world);
     jl_binding_partition_t *bpart = jl_get_binding_partition(b, new_world);
     jl_ptr_kind_union_t pku = jl_atomic_load_relaxed(&bpart->restriction);
     jl_value_t *gf = NULL;

Diff for: src/module.c

@@ -588,10 +588,10 @@ JL_DLLEXPORT jl_value_t *jl_reresolve_binding_value_seqcst(jl_binding_t *b)
 
 // get binding for adding a method
 // like jl_get_binding_wr, but has different error paths and messages
-JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_t *var)
+JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_t *var, size_t new_world)
 {
     jl_binding_t *b = jl_get_module_binding(m, var, 1);
-    jl_binding_partition_t *bpart = jl_get_binding_partition(b, jl_current_task->world_age);
+    jl_binding_partition_t *bpart = jl_get_binding_partition(b, new_world);
     jl_ptr_kind_union_t pku = jl_atomic_load_relaxed(&bpart->restriction);
     enum jl_partition_kind kind = decode_restriction_kind(pku);
     if (kind == BINDING_KIND_GLOBAL || kind == BINDING_KIND_DECLARED || jl_bkind_is_some_constant(decode_restriction_kind(pku)))
@@ -601,7 +601,7 @@ JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_
         return b;
     }
     jl_binding_t *ownerb = b;
-    pku = jl_walk_binding_inplace(&ownerb, &bpart, jl_current_task->world_age);
+    pku = jl_walk_binding_inplace(&ownerb, &bpart, new_world);
     jl_value_t *f = NULL;
     if (jl_bkind_is_some_constant(decode_restriction_kind(pku)))
         f = decode_restriction_value(pku);
@@ -613,7 +613,7 @@ JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_
         jl_module_t *from = jl_binding_dbgmodule(b, m, var);
         // we must have implicitly imported this with using, so call jl_binding_dbgmodule to try to get the name of the module we got this from
         jl_errorf("invalid method definition in %s: exported function %s.%s does not exist",
-                    jl_symbol_name(m->name), from ? jl_symbol_name(from->name) : "<null>", jl_symbol_name(var));
+                    jl_module_debug_name(m), jl_module_debug_name(from), jl_symbol_name(var));
     }
     int istype = f && jl_is_type(f);
     if (!istype) {
@@ -626,13 +626,25 @@ JL_DLLEXPORT jl_binding_t *jl_get_binding_for_method_def(jl_module_t *m, jl_sym_
             //       or we might want to drop this error entirely
             jl_module_t *from = jl_binding_dbgmodule(b, m, var);
             jl_errorf("invalid method definition in %s: function %s.%s must be explicitly imported to be extended",
-                        jl_symbol_name(m->name), from ? jl_symbol_name(from->name) : "<null>", jl_symbol_name(var));
+                        jl_module_debug_name(m), jl_module_debug_name(from), jl_symbol_name(var));
         }
     }
-    else if (strcmp(jl_symbol_name(var), "=>") == 0 && (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_EXPLICIT)) {
+    else if (kind != BINDING_KIND_IMPORTED) {
+        int should_error = strcmp(jl_symbol_name(var), "=>") == 0;
         jl_module_t *from = jl_binding_dbgmodule(b, m, var);
-        jl_errorf("invalid method definition in %s: function %s.%s must be explicitly imported to be extended",
-                    jl_symbol_name(m->name), from ? jl_symbol_name(from->name) : "<null>", jl_symbol_name(var));
+        if (should_error)
+            jl_errorf("invalid method definition in %s: function %s.%s must be explicitly imported to be extended",
+                        jl_module_debug_name(m), jl_module_debug_name(from), jl_symbol_name(var));
+        else
+            jl_printf(JL_STDERR, "WARNING: Constructor for type \"%s\" was extended in `%s` without explicit qualification or import.\n"
+                                 "  NOTE: Assumed \"%s\" refers to `%s.%s`. This behavior is deprecated and may differ in future versions.`\n"
+                                 "  NOTE: This behavior may have differed in Julia versions prior to 1.12.\n"
+                                 "  Hint: If you intended to create a new generic function of the same name, use `function %s end`.\n"
+                                 "  Hint: To silence the warning, qualify `%s` as `%s.%s` or explicitly `import %s: %s`\n",
+                jl_symbol_name(var), jl_module_debug_name(m),
+                jl_symbol_name(var), jl_module_debug_name(from), jl_symbol_name(var),
+                jl_symbol_name(var), jl_symbol_name(var), jl_module_debug_name(from), jl_symbol_name(var),
+                jl_module_debug_name(from), jl_symbol_name(var));
     }
     return ownerb;
 }

Diff for: stdlib/SharedArrays/src/SharedArrays.jl

@@ -9,6 +9,7 @@ using Mmap, Distributed, Random
 
 import Base: length, size, elsize, ndims, IndexStyle, reshape, convert, deepcopy_internal,
              show, getindex, setindex!, fill!, similar, reduce, map!, copyto!, cconvert
+import Base: Array
 import Random
 using Serialization
 using Serialization: serialize_cycle_header, serialize_type, writetag, UNDEFREF_TAG, serialize, deserialize

Diff for: test/arrayops.jl

@@ -2948,7 +2948,7 @@ end
 
 Base.ArithmeticStyle(::Type{F21666{T}}) where {T} = T()
 Base.:+(x::F, y::F) where {F <: F21666} = F(x.x + y.x)
-Float64(x::F21666) = Float64(x.x)
+Base.Float64(x::F21666) = Float64(x.x)
 @testset "Exactness of cumsum # 21666" begin
     # test that cumsum uses more stable algorithm
     # for types with unknown/rounding arithmetic

Diff for: test/errorshow.jl

@@ -438,7 +438,7 @@ let err_str
     @test occursin("For element-wise subtraction, use broadcasting with dot syntax: array .- scalar", err_str)
 end
 
-
+import Core: String
 method_defs_lineno = @__LINE__() + 1
 String() = throw(ErrorException("1"))
 (::String)() = throw(ErrorException("2"))

Diff for: test/intrinsics.jl

@@ -66,7 +66,7 @@ end
 # test functionality of non-power-of-2 primitive type constants
 primitive type Int24 24 end
 Int24(x::Int) = Core.Intrinsics.trunc_int(Int24, x)
-Int(x::Int24) = Core.Intrinsics.zext_int(Int, x)
+Base.Int(x::Int24) = Core.Intrinsics.zext_int(Int, x)
 let x, y, f
     x = Int24(Int(0x12345678)) # create something (via truncation)
     @test Int(0x345678) === Int(x)