checker: check duplicate consts

vlib/v/checker/checker.v

@@ -29,6 +29,7 @@ mut:
 	fn_return_type table.Type // current function's return type
 	const_decl     string
 	const_deps     []string
+	const_names    []string
 	pref           &pref.Preferences // Preferences shared from V struct
 	in_for_count   int // if checker is currently in an for loop
 	// checked_ident  string // to avoid infinit checker loops
@@ -1271,6 +1272,10 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			mut field_names := []string{}
 			mut field_order := []int{}
 			for i, field in it.fields {
+				if field.name in c.const_names {
+					c.error('field name `$field.name` duplicate', field.pos)
+				}
+				c.const_names << field.name
 				field_names << field.name
 				field_order << i
 			}

vlib/v/checker/tests/const_field_name_duplicate_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/const_field_name_duplicate_err.v:3:2: error: field name `aaa` duplicate
+    1| const (
+    2|     aaa = 1
+    3|     aaa = 2
+           ~~~
+    4| )
+    5| fn main() {

vlib/v/checker/tests/const_field_name_duplicate_err.vv

@@ -0,0 +1,7 @@
+const (
+	aaa = 1
+	aaa = 2
+)
+fn main() {
+	println(aaa)
+}

vlib/v/checker/tests/multi_const_field_name_duplicate_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/multi_const_field_name_duplicate_err.v:2:8: error: field name `aaa` duplicate
+    1| const (aaa = 1)
+    2| const (aaa = 2)
+              ~~~
+    3| fn main() {
+    4|     println(aaa)

vlib/v/checker/tests/multi_const_field_name_duplicate_err.vv

@@ -0,0 +1,5 @@
+const (aaa = 1)
+const (aaa = 2)
+fn main() {
+	println(aaa)
+}

vlib/v/parser/parser.v

@@ -1002,6 +1002,7 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 		if p.tok.kind == .comment {
 			p.comment()
 		}
+		pos := p.tok.position()
 		name := p.prepend_mod(p.check_name())
 		// name := p.check_name()
 		// println('!!const: $name')
@@ -1010,7 +1011,7 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 		field := ast.ConstField{
 			name: name
 			expr: expr
-			pos: p.tok.position()
+			pos: pos
 		}
 		fields << field
 		p.global_scope.register(field.name, field)