make _V_mret_ unique by putting p.token_idx in it

vlib/compiler/parser.v

@@ -1471,7 +1471,7 @@ fn (p mut Parser) var_decl() {
 	if is_decl_assign && var_names.len == 1 && var_names[0] == '_' {
 		p.error_with_token_index('use `=` instead of `:=`', var_token_idxs.last())
 	}
-	p.var_decl_name = if var_names.len > 1 { '_V_mret_'+var_names.join('_') } else { var_names[0] }
+	p.var_decl_name = if var_names.len > 1 { '_V_mret_${p.token_idx}_'+var_names.join('_') } else { var_names[0] }
 	t := p.gen_var_decl(p.var_decl_name, is_static)
 	if t == 'void' {
 		_, fn_name := p.is_expr_fn_call(p.token_idx-3)

vlib/compiler/tests/repeated_multiret_values_test.v

@@ -0,0 +1,15 @@
+// verify fix for #2913
+
+fn some_multiret_fn(a int, b int) (int, int) {
+	return a+1, b+1
+}
+
+fn test_repeated_multiple_multiret() {
+	a, b := some_multiret_fn(1,2)
+	assert a == 2
+	assert b == 3
+
+	c, d := some_multiret_fn(3,4)
+	assert c == 4
+	assert d == 5
+}

vlib/compiler/tests/reusable_mut_multiret_values_test.v

@@ -0,0 +1,27 @@
+// verify fix for #2913
+
+fn some_multiret_fn(a int, b int) (int, int) {
+	return a+1, b+1
+}
+
+fn test_reuse_multiple_multiret() {
+	mut c, mut d := some_multiret_fn(4,10)
+
+	mut a, mut b := some_multiret_fn(c,d)
+	assert a == c+1
+	assert b == d+1
+
+	for i in 1..10 {
+		c += i
+		d += i
+		a, b = some_multiret_fn(c,d)
+		assert a == c+1
+		assert b == d+1
+
+		c += i+1
+		d += i+1
+		a, b = some_multiret_fn(c,d)
+		assert a == c+1
+		assert b == d+1
+	}
+}