checker: allow using methods as vars when expecting a ctx arg (#14414)

vlib/v/ast/ast.v

@@ -263,6 +263,7 @@ pub mut:
 	gkind_field         GenericKindField // `T.name` => ast.GenericKindField.name, `T.typ` => ast.GenericKindField.typ, or .unknown
 	scope               &Scope
 	from_embed_types    []Type // holds the type of the embed that the method is called from
+	has_hidden_receiver bool
 }
 
 // root_ident returns the origin ident where the selector started.

vlib/v/checker/checker.v

@@ -1851,6 +1851,14 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 		return field.typ
 	}
 	if mut method := c.table.find_method(sym, field_name) {
+		if c.expected_type != 0 && c.expected_type != ast.none_type {
+			fn_type := ast.new_type(c.table.find_or_register_fn_type(c.mod, method, false,
+				true))
+			// if the expected type includes the receiver, don't hide it behind a closure
+			if c.check_types(fn_type, c.expected_type) {
+				return fn_type
+			}
+		}
 		receiver := method.params[0].typ
 		if receiver.nr_muls() > 0 {
 			if !c.inside_unsafe {
@@ -1867,6 +1875,7 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 			}
 		}
 		method.params = method.params[1..]
+		node.has_hidden_receiver = true
 		fn_type := ast.new_type(c.table.find_or_register_fn_type(c.mod, method, false,
 			true))
 		return fn_type

vlib/v/gen/c/cgen.v

@@ -3347,6 +3347,10 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 			}
 		}
 		if !has_embeds {
+			if !node.has_hidden_receiver {
+				g.write('${g.typ(node.expr_type.idx())}_$m.name')
+				return
+			}
 			receiver := m.params[0]
 			expr_styp := g.typ(node.expr_type.idx())
 			data_styp := g.typ(receiver.typ.idx())

vlib/v/tests/methods_as_fields_test.v

@@ -95,3 +95,25 @@ fn test_methods_as_fields_ref() {
 	new := set_val(5)
 	assert old == new && old == 1
 }
+
+struct GG_Ctx {
+	frame_fn fn (voidptr) int
+}
+
+[heap]
+struct App {
+	msg string = 'hello'
+}
+
+fn (app &App) frame() int {
+	return app.msg.len
+}
+
+fn test_ctx_arg_expected() {
+	mut app := &App{}
+	mut ctx := &GG_Ctx{
+		frame_fn: app.frame
+	}
+	assert typeof(ctx.frame_fn).str() == 'fn (voidptr) int'
+	assert ctx.frame_fn(app) == 5
+}