parser: error if operators are used as function names (#7532)

vlib/v/parser/fn.v

@@ -257,6 +257,10 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	}
 	if p.tok.kind in [.plus, .minus, .mul, .div, .mod] {
 		name = p.tok.kind.str() // op_to_fn_name()
+		if rec_type == table.void_type {
+			p.error_with_pos('cannot use operator overloading with normal functions',
+				p.tok.position())
+		}
 		p.next()
 	}
 	// <T>

vlib/v/parser/tests/operator_normal_fn.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/operator_normal_fn.vv:1:4: error: cannot use operator overloading with normal functions
+    1 | fn +(x int) int {
+      |    ^
+    2 |     return 5 + x
+    3 | }

vlib/v/parser/tests/operator_normal_fn.vv

@@ -0,0 +1,3 @@
+fn +(x int) int {
+	return 5 + x
+}