parser: fix checking for invalid PrefixExpr (fix #20388) (#20392)

vlib/v/parser/expr.v

@@ -666,7 +666,17 @@ fn (mut p Parser) infix_expr(left ast.Expr) ast.Expr {
 		p.inside_in_array = true
 	}
 
+	right_op_pos := p.tok.pos()
 	right = p.expr(precedence)
+	if op in [.plus, .minus, .mul, .div, .mod, .lt, .eq] && mut right is ast.PrefixExpr {
+		mut right_expr := right.right
+		for mut right_expr is ast.ParExpr {
+			right_expr = right_expr.expr
+		}
+		if right.op in [.plus, .minus, .mul, .div, .mod, .lt, .eq] && right_expr.is_pure_literal() {
+			p.error_with_pos('invalid expression: unexpected token `${op}`', right_op_pos)
+		}
+	}
 	if is_key_in {
 		p.inside_in_array = false
 	}

vlib/v/parser/tests/prefix_err_1.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/prefix_err_1.vv:1:9: error: invalid expression: unexpected token `-`
+    1 | _ = 5 - - -5
+      |         ^

vlib/v/parser/tests/prefix_err_1.vv

@@ -0,0 +1 @@
+_ = 5 - - -5

vlib/v/parser/tests/prefix_err_2.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/prefix_err_2.vv:1:9: error: invalid expression: unexpected token `-`
+    1 | _ = 5 - - (((-5)))
+      |         ^

vlib/v/parser/tests/prefix_err_2.vv

@@ -0,0 +1 @@
+_ = 5 - - (((-5)))