checker: check that right `is` type exists

vlib/v/ast/ast.v

@@ -24,6 +24,7 @@ pub type ScopeObject = ConstField | GlobalDecl | Var
 pub struct Type {
 pub:
 	typ table.Type
+	pos token.Position
 }
 
 pub struct Block {

vlib/v/checker/checker.v

@@ -198,6 +198,11 @@ pub fn (c mut Checker) infix_expr(infix_expr mut ast.InfixExpr) table.Type {
 	infix_expr.left_type = left_type
 	c.expected_type = left_type
 	if infix_expr.op == .key_is {
+		type_expr := infix_expr.right as ast.Type
+		typ_sym := c.table.get_type_symbol(type_expr.typ)
+		if typ_sym.kind == .placeholder {
+			c.error('is: type `${typ_sym.name}` does not exist', type_expr.pos)
+		}
 		return table.bool_type
 	}
 	right_type := c.expr(infix_expr.right)

vlib/v/checker/tests/inout/is_type_not_exist.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/inout/is_type_not_exist.v:8:10: error: is: type `SomethingThatDontExist` does not exist
+    6|
+    7| fn fn_with_sum_type_param(i Integer) {
+    8|     if i is SomethingThatDontExist {
+                   ~~~~~~~~~~~~~~~~~~~~~~
+    9|         println('It should fail !')
+   10|     }

vlib/v/checker/tests/inout/is_type_not_exist.vv

@@ -0,0 +1,11 @@
+type Integer = i8 | i16 | int | i64
+
+fn main() {
+	fn_with_sum_type_param(1)
+}
+
+fn fn_with_sum_type_param(i Integer) {
+	if i is SomethingThatDontExist {
+		println('It should fail !')
+	}
+}

vlib/v/parser/parser.v

@@ -551,8 +551,11 @@ pub fn (var p Parser) name_expr() ast.Expr {
 	var node := ast.Expr{}
 	if p.inside_is {
 		p.inside_is = false
+		// get type position before moving to next
+		type_pos := p.tok.position()
 		return ast.Type{
 			typ: p.parse_type()
+			pos: type_pos
 		}
 	}
 	is_c := p.tok.lit == 'C'