parser: check for builtin function redefinitions (fix #7499) (#7507)

vlib/v/parser/fn.v

@@ -229,10 +229,12 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	}
 	mut name := ''
 	if p.tok.kind == .name {
+		pos := p.tok.position()
 		// TODO high order fn
 		name = if language == .js { p.check_js_name() } else { p.check_name() }
 		if language == .v && !p.pref.translated && util.contains_capital(name) && p.mod != 'builtin' {
-			p.error('function names cannot contain uppercase letters, use snake_case instead')
+			p.error_with_pos('function names cannot contain uppercase letters, use snake_case instead',
+				pos)
 			return ast.FnDecl{
 				scope: 0
 			}
@@ -240,7 +242,14 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		type_sym := p.table.get_type_symbol(rec_type)
 		// interfaces are handled in the checker, methods can not be defined on them this way
 		if is_method && (type_sym.has_method(name) && type_sym.kind != .interface_) {
-			p.error('duplicate method `$name`')
+			p.error_with_pos('duplicate method `$name`', pos)
+			return ast.FnDecl{
+				scope: 0
+			}
+		}
+		// cannot redefine buildin function
+		if !is_method && p.mod != 'builtin' && name in builtin_functions {
+			p.error_with_pos('cannot redefine builtin function `$name`', pos)
 			return ast.FnDecl{
 				scope: 0
 			}

vlib/v/parser/parser.v

@@ -14,6 +14,10 @@ import os
 import runtime
 import time
 
+const (
+	builtin_functions = ['print', 'println', 'eprint', 'eprintln', 'isnil', 'panic', 'exit']
+)
+
 pub struct Parser {
 	pref              &pref.Preferences
 mut:

vlib/v/parser/tests/fn_use_builtin_err.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/fn_use_builtin_err.vv:1:4: error: cannot redefine builtin function `print`
+    1 | fn print(strings ...string) {
+      |    ~~~~~
+    2 |     for s in strings {
+    3 |         println(s)

vlib/v/parser/tests/fn_use_builtin_err.vv

@@ -0,0 +1,9 @@
+fn print(strings ...string) {
+	for s in strings {
+		println(s)
+	}
+}
+
+fn main() {
+	print('text')
+}