feat(examples): added check for overflow/underflow

examples/gno.land/p/demo/grc/grc20/banker.gno

@@ -1,6 +1,8 @@
 package grc20
 
 import (
+	"errors"
+	"math"
 	"std"
 	"strconv"
 
@@ -55,7 +57,11 @@ func (b *Banker) Mint(address std.Address, amount uint64) error {
 		return ErrInvalidAddress
 	}
 
-	// TODO: check for overflow
+	// check for overflow
+	if diff := math.MaxUint64 - b.totalSupply; diff < amount {
+		err := ufmt.Sprintf("you can't mint more than %d tokens", diff)
+		return errors.New(err)
+	}
 
 	b.totalSupply += amount
 	currentBalance := b.BalanceOf(address)

examples/gno.land/p/demo/grc/grc20/types.gno

@@ -12,6 +12,7 @@ var (
 	ErrInsufficientAllowance = errors.New("insufficient allowance")
 	ErrInvalidAddress        = errors.New("invalid address")
 	ErrCannotTransferToSelf  = errors.New("cannot send transfer to self")
+	ErrUnderflow             = errors.New("the amount is higher than the total supply")
 )
 
 type Token interface {

examples/gno.land/p/demo/grc/grc20/z_0_filetest.gno

@@ -0,0 +1,21 @@
+package main
+
+import (
+	banker "gno.land/p/demo/grc/grc20"
+	"gno.land/p/demo/testutils"
+)
+
+const uint64_max = 1<<64 - 1
+
+func main() {
+	b := banker.NewBanker("Dummy", "DUMMY", 6)
+	var owner = testutils.TestAddress("owner")
+	// Mint uint64_max - 1000 tokens for owner
+	b.Mint(owner, uint64_max-1000)
+	// Try to mint 1001 tokens for owner, should fail because it exceeds the limit
+	println(b.Mint(owner, 1001).Error())
+
+}
+
+// Output:
+// you can't mint more than 1000 tokens