checker: use a more readable error message for non matching fn args

vlib/v/checker/checker.v

@@ -1194,6 +1194,11 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			}
 			if typ_sym.kind == .array_fixed {
 			}
+			if typ_sym.kind == .function && arg_typ_sym.kind == .function {
+				candidate_fn_name := if typ_sym.name.starts_with('anon_') { 'anonymous function' } else { 'fn `$typ_sym.name`' }
+				c.error('cannot use $candidate_fn_name as function type `$arg_typ_sym.str()` in argument ${i+1} to `$fn_name`',
+					call_expr.pos)
+			}
 			c.error('cannot use type `$typ_sym.str()` as type `$arg_typ_sym.str()` in argument ${i+1} to `$fn_name`',
 				call_expr.pos)
 		}

vlib/v/checker/tests/non_matching_functional_args.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/non_matching_functional_args.v:27:2: error: cannot use anonymous function as function type `MyFn` in argument 1 to `sum` 
+   25 | 
+   26 | fn main() {
+   27 |     sum(fn (mut t Table) {
+      |     ~~~~~~~~~~~~~~~~~~~~~~
+   28 |         t.rename()
+   29 |         println(t.name)
+vlib/v/checker/tests/non_matching_functional_args.v:31:2: error: cannot use fn `xxx` as function type `MyFn` in argument 1 to `sum` 
+   29 |         println(t.name)
+   30 |     })
+   31 |     sum(xxx)
+      |     ~~~~~~~~
+   32 |     sum(yyy)
+   33 | }

vlib/v/checker/tests/non_matching_functional_args.vv

@@ -0,0 +1,33 @@
+struct Table {
+pub mut:
+	name string
+}
+
+type MyFn = fn (zzzz Table)
+
+fn (mut t Table) rename() {
+	t.name = 'abc'
+}
+
+fn yyy(t Table) {
+	println(t.name)
+}
+
+fn xxx(mut t Table) {
+	t.rename()
+	println(t.name)
+}
+
+fn sum(myfn MyFn) {
+	mut t := Table{}
+	myfn(t)
+}
+
+fn main() {
+	sum(fn (mut t Table) {
+		t.rename()
+		println(t.name)
+	})
+	sum(xxx)
+	sum(yyy)
+}