checker: fix static method used as map/filter argument (#27328)

vlib/v/checker/checker.v

@@ -8372,17 +8372,27 @@ fn (mut c Checker) static_fn_value_from_enum_val(mut node ast.EnumVal, _ string,
 	return node.typ
 }
 
-fn (mut c Checker) enum_val_as_static_fn(mut node ast.EnumVal, typ_sym ast.TypeSymbol, fsym ast.TypeSymbol) ast.Type {
-	fn_name := '${typ_sym.name}__static__${node.val}'
-	if func := c.table.find_fn(fn_name) {
-		return c.static_fn_value_from_enum_val(mut node, fn_name, func)
+// static_method_of_enum_val resolves the static method that an `ast.EnumVal`-shaped
+// expression (`Type.method`, syntactically identical to an enum value `Color.red`)
+// refers to. It also checks the final/unaliased symbol, so static methods reached
+// through a supported type alias (`type Alias = Struct; Alias.new`) are found by
+// their real fkey instead of the alias name.
+fn (c &Checker) static_method_of_enum_val(node ast.EnumVal, typ_sym ast.TypeSymbol, fsym ast.TypeSymbol) ?ast.Fn {
+	if func := c.table.find_fn('${typ_sym.name}__static__${node.val}') {
+		return func
 	}
 	if fsym.name != typ_sym.name {
-		alias_fn_name := '${fsym.name}__static__${node.val}'
-		if func := c.table.find_fn(alias_fn_name) {
-			return c.static_fn_value_from_enum_val(mut node, alias_fn_name, func)
+		if func := c.table.find_fn('${fsym.name}__static__${node.val}') {
+			return func
 		}
 	}
+	return none
+}
+
+fn (mut c Checker) enum_val_as_static_fn(mut node ast.EnumVal, typ_sym ast.TypeSymbol, fsym ast.TypeSymbol) ast.Type {
+	if func := c.static_method_of_enum_val(node, typ_sym, fsym) {
+		return c.static_fn_value_from_enum_val(mut node, func.name, func)
+	}
 	return ast.void_type
 }
 

vlib/v/checker/fn.v

@@ -4930,6 +4930,34 @@ fn (mut c Checker) array_builtin_method_call(mut node ast.CallExpr, left_type as
 		}
 		arg_type = c.check_expr_option_or_result_call(arg.expr, expr_type)
 	}
+	// `arr.map(Type.from)`, `arr.filter(Type.ok)` etc.: a static method used as a
+	// first-class value is parsed as an `ast.EnumVal` (it is syntactically identical
+	// to an enum value, e.g. `Color.red`). The parser can only emit a function
+	// `ast.Ident` for it when the static method is already registered, which fails
+	// for cross-module (and forward) references, since imported modules are parsed
+	// after the call site. Once the checker has resolved the value to a function
+	// type, rewrite it into a function `ast.Ident`, so the backends call it and
+	// markused keeps the static method, exactly like any other function value.
+	if node.kind in [.map, .filter, .any, .all, .count] && node.args.len > 0 {
+		arg_expr := node.args[0].expr
+		if arg_expr is ast.EnumVal && c.table.sym(arg_expr.typ).kind == .function {
+			enum_typ := ast.new_type(c.table.find_type_idx(arg_expr.enum_name))
+			typ_sym := c.table.sym(enum_typ)
+			fsym := c.table.final_sym(enum_typ)
+			if func := c.static_method_of_enum_val(arg_expr, typ_sym, fsym) {
+				node.args[0].expr = ast.Ident{
+					name:  func.name
+					mod:   c.mod
+					kind:  .function
+					info:  ast.IdentFn{
+						typ: ast.new_type(c.table.find_or_register_fn_type(func, false, true))
+					}
+					pos:   arg_expr.pos
+					scope: node.scope
+				}
+			}
+		}
+	}
 	if node.kind == .map {
 		// eprintln('>>>>>>> map node.args[0].expr: ${node.args[0].expr}, left_type: ${left_type} | elem_typ: ${elem_typ} | arg_type: ${arg_type}')
 		// check fn

vlib/v/tests/fns/static_method_as_map_arg_test.v

@@ -0,0 +1,50 @@
+// Regression test for a static method used as a first-class value in the
+// builtin array methods (`map`/`filter`/...). It is parsed as an `ast.EnumVal`
+// (same syntax as an enum value, e.g. `Color.red`); when the static method is
+// not yet registered while parsing the call site (forward reference here, or a
+// cross-module reference), the parser cannot emit a function `ast.Ident`, so the
+// checker has to rewrite the resolved value into one. See issue #27328.
+
+fn test_static_method_as_map_arg() {
+	nums := [1, 2, 3]
+	foos := nums.map(Foo.from)
+	assert foos.map(it.x) == [1, 2, 3]
+}
+
+fn test_static_method_as_map_arg_explicit_call() {
+	nums := [1, 2, 3]
+	foos := nums.map(Foo.from(it))
+	assert foos.map(it.x) == [1, 2, 3]
+}
+
+fn test_static_method_as_filter_arg() {
+	nums := [1, 2, 3, 4]
+	odds := nums.filter(is_odd)
+	assert odds == [1, 3]
+}
+
+fn test_aliased_static_method_as_map_arg() {
+	// the static method is reached through a type alias, so it must be resolved
+	// by its real fkey (`Foo__static__from`), not the alias name
+	nums := [1, 2, 3]
+	foos := nums.map(FooAlias.from)
+	assert foos.map(it.x) == [1, 2, 3]
+}
+
+type FooAlias = Foo
+
+// Defined *after* the call sites on purpose, to exercise the forward-reference
+// path (the function is not in the table yet when the calls above are parsed).
+struct Foo {
+	x int
+}
+
+fn Foo.from(n int) Foo {
+	return Foo{
+		x: n
+	}
+}
+
+fn is_odd(n int) bool {
+	return n % 2 == 1
+}