parser: check enum method duplicated

vlib/v/ast/ast.v

@@ -666,6 +666,7 @@ pub:
 	file                  string
 	file_mode             Language
 	pos                   token.Pos
+	name_pos              token.Pos
 	return_type_pos       token.Pos
 pub mut:
 	return_type        Type

vlib/v/parser/fn.v

@@ -347,7 +347,15 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 				}
 			}
 			if is_duplicate {
-				p.error_with_pos('duplicate method `${name}`', name_pos)
+				if type_sym.kind == .enum_ {
+					if enum_fn := type_sym.find_method(name) {
+						name_pos = enum_fn.name_pos
+					}
+					p.error_with_pos('duplicate method `${name}`, `${name}` is an enum type built-in method',
+						name_pos)
+				} else {
+					p.error_with_pos('duplicate method `${name}`', name_pos)
+				}
 				return ast.FnDecl{
 					scope: unsafe { nil }
 				}
@@ -539,6 +547,7 @@ run them via `v file.v` instead',
 			mod:      p.mod
 			file:     p.file_path
 			pos:      start_pos
+			name_pos: name_pos
 			language: language
 		})
 	} else {
@@ -591,6 +600,7 @@ run them via `v file.v` instead',
 			mod:      p.mod
 			file:     p.file_path
 			pos:      start_pos
+			name_pos: name_pos
 			language: language
 		})
 	}

vlib/v/parser/tests/enum_method_duplicated_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/enum_method_duplicated_err.vv:13:14: error: duplicate method `has`, `has` is an enum type built-in method
+   11 | }
+   12 | 
+   13 | fn (f Flags) has(f2 Flags) bool {
+      |              ~~~
+   14 |     return f & f2 > 0
+   15 | }

vlib/v/parser/tests/enum_method_duplicated_err.vv

@@ -0,0 +1,17 @@
+@[flag]
+enum Flags {
+	bit0
+	bit1
+	bit2
+	bit3
+	bit4
+	bit5
+	bit6
+	bit7
+}
+
+fn (f Flags) has(f2 Flags) bool {
+	return f & f2 > 0
+}
+
+fn main() {}