parser: fix self-referenced error

vlib/v/checker/tests/assign_expr_undefined_err_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_expr_undefined_err_a.v:2:7: error: undefined: `a`
+    1 | fn main() {
+    2 |     a := a
+      |          ^
+    3 |     println(a)
+    4 | }

vlib/v/checker/tests/assign_expr_undefined_err_b.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/assign_expr_unresolved_variables_err_b.v:2:10: error: unresolved variables `a`
+vlib/v/checker/tests/assign_expr_undefined_err_b.v:2:10: error: undefined: `a`
     1 | fn main() {
     2 |     a, b := a, b
       |             ^

vlib/v/checker/tests/assign_expr_undefined_err_c.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/assign_expr_unresolved_variables_err_c.v:2:10: error: unresolved variables `a`
+vlib/v/checker/tests/assign_expr_undefined_err_c.v:2:10: error: undefined: `a`
     1 | fn main() {
     2 |     a, b := a + 1, b * 3
       |             ^

vlib/v/checker/tests/assign_expr_undefined_err_d.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_expr_undefined_err_d.v:2:9: error: undefined: `s`
+    1 | fn main() {
+    2 |     s := '$s'
+      |            ^
+    3 |     println(s)
+    4 | }

vlib/v/checker/tests/assign_expr_undefined_err_d.vv

@@ -0,0 +1,4 @@
+fn main() {
+	s := '$s'
+	println(s)
+}

vlib/v/checker/tests/assign_expr_unresolved_variables_err_a.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/assign_expr_unresolved_variables_err_a.v:2:7: error: unresolved variables `a`
-    1 | fn main() {
-    2 |     a := a
-      |          ^
-    3 |     println(a)
-    4 | }

vlib/v/parser/assign.v

@@ -9,18 +9,23 @@ fn (mut p Parser) assign_stmt() ast.Stmt {
 	return p.partial_assign_stmt([])
 }
 
-fn (mut p Parser) check_unresolved_variables(idents []ast.Ident, expr ast.Expr) {
+fn (mut p Parser) check_undefined_variables(idents []ast.Ident, expr ast.Expr) {
 	match expr {
 		ast.Ident {
 			for ident in idents {
 				if ident.name == it.name {
-					p.error_with_pos('unresolved variables `$it.name`', it.pos)
+					p.error_with_pos('undefined: `$it.name`', it.pos)
 				}
 			}
 		}
 		ast.InfixExpr {
-			p.check_unresolved_variables(idents, it.left)
-			p.check_unresolved_variables(idents, it.right)
+			p.check_undefined_variables(idents, it.left)
+			p.check_undefined_variables(idents, it.right)
+		}
+		ast.StringInterLiteral {
+			for expr_ in it.exprs {
+				p.check_undefined_variables(idents, expr_)
+			}
 		}
 		else {}
 	}
@@ -44,7 +49,7 @@ fn (mut p Parser) partial_assign_stmt(known_lhs []ast.Ident) ast.Stmt {
 	if is_decl {
 		// a, b := a + 1, b
 		for expr in exprs {
-			p.check_unresolved_variables(idents, expr)
+			p.check_undefined_variables(idents, expr)
 		}
 	}
 	for i, ident in idents {