checker: disallow top level decl builtin fn (#17857)

vlib/v/checker/fn.v

@@ -275,6 +275,11 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.error('fn `init` cannot have a return type', node.pos)
 		}
 	}
+
+	if node.language == .v && node.mod == 'main' && node.name.after_char(`.`) in reserved_type_names
+		&& !node.is_method && !c.is_builtin_mod {
+		c.error('top level declaration cannot shadow builtin type', node.pos)
+	}
 	if node.return_type != ast.Type(0) {
 		c.ensure_type_exists(node.return_type, node.return_type_pos) or { return }
 		if node.language == .v && node.is_method && node.name == 'str' {

vlib/v/checker/tests/top_level_fn_builtin_decl_err.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/top_level_fn_builtin_decl_err.vv:3:1: error: top level declaration cannot shadow builtin type
+    1 |
+    2 | [inline]
+    3 | fn char(ch byte) fn (string) !(byte, string) {
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    4 |     return fn [ch] (input string) !(byte, string) {
+    5 |         return if input[0] == ch {
+vlib/v/checker/tests/top_level_fn_builtin_decl_err.vv:19:18: error: unknown function: a_char
+   17 |
+   18 |     for i, input in inputs {
+   19 |         got, remain := a_char(input)!
+      |                        ~~~~~~~~~~~~~
+   20 |
+   21 |         assert got == 'a'[0]
+vlib/v/checker/tests/top_level_fn_builtin_decl_err.vv:21:10: error: invalid variable `got`
+   19 |         got, remain := a_char(input)!
+   20 |
+   21 |         assert got == 'a'[0]
+      |                ~~~
+   22 |         assert remain == remains[i]
+   23 |     }
+vlib/v/checker/tests/top_level_fn_builtin_decl_err.vv:22:10: error: invalid variable `remain`
+   20 |
+   21 |         assert got == 'a'[0]
+   22 |         assert remain == remains[i]
+      |                ~~~~~~
+   23 |     }
+   24 | }

vlib/v/checker/tests/top_level_fn_builtin_decl_err.vv

@@ -0,0 +1,24 @@
+
+[inline]
+fn char(ch byte) fn (string) !(byte, string) {
+	return fn [ch] (input string) !(byte, string) {
+		return if input[0] == ch {
+			ch, input[1..]
+		} else {
+			error('Some error')
+		}
+	}
+}
+
+fn main() {
+	a_char := int('a'[0])
+	inputs := ['a', 'abc']
+	remains := ['', 'bc']
+
+	for i, input in inputs {
+		got, remain := a_char(input)!
+
+		assert got == 'a'[0]
+		assert remain == remains[i]
+	}
+}