checker: prohibit `char.str()`, add tests (#10615)

vlib/v/checker/checker.v

@@ -1839,6 +1839,10 @@ pub fn (mut c Checker) method_call(mut call_expr ast.CallExpr) ast.Type {
 		}
 		call_expr.return_type = info.return_type
 		return info.return_type
+	} else if left_type_sym.kind == .char && left_type.nr_muls() == 0 && method_name == 'str' {
+		c.error('calling `.str()` on type `char` is not allowed, use its address or cast it to an integer instead',
+			call_expr.left.position().extend(call_expr.pos))
+		return ast.void_type
 	}
 	mut method := ast.Fn{}
 	mut has_method := false
@@ -2492,11 +2496,15 @@ pub fn (mut c Checker) fn_call(mut call_expr ast.CallExpr) ast.Type {
 		c.inside_println_arg = true
 		c.expected_type = ast.string_type
 		call_expr.args[0].typ = c.expr(call_expr.args[0].expr)
-		_ := c.check_expr_opt_call(call_expr.args[0].expr, call_expr.args[0].typ)
+		arg := call_expr.args[0]
-		if call_expr.args[0].typ.is_void() {
+		_ := c.check_expr_opt_call(arg.expr, arg.typ)
+		if arg.typ.is_void() {
 			c.error('`$fn_name` can not print void expressions', call_expr.pos)
+		} else if arg.typ == ast.char_type && arg.typ.nr_muls() == 0 {
+			c.error('`$fn_name` cannot print type `char` directly, print its address or cast it to an integer instead',
+				call_expr.pos)
 		}
-		c.fail_if_unreadable(call_expr.args[0].expr, call_expr.args[0].typ, 'argument to print')
+		c.fail_if_unreadable(arg.expr, arg.typ, 'argument to print')
 		c.inside_println_arg = false
 		/*
 		// TODO: optimize `struct T{} fn (t &T) str() string {return 'abc'} mut a := []&T{} a << &T{} println(a[0])`

vlib/v/checker/tests/char_str.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/char_str.vv:1:1: error: calling `.str()` on type `char` is not allowed, use its address or cast it to an integer instead
+    1 | char(91).str()
+      | ~~~~~~~~~~~~~~

vlib/v/checker/tests/char_str.vv

@@ -0,0 +1 @@
+char(91).str()

vlib/v/checker/tests/print_char.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/print_char.vv:1:1: error: `println` cannot print type `char` directly, print its address or cast it to an integer instead
+    1 | println(char(91))
+      | ~~~~~~~~~~~~~~~~~

vlib/v/checker/tests/print_char.vv

@@ -0,0 +1 @@
+println(char(91))