fix: handle untyped constant expressions in append() (gnolang#1177)

This is a particular case of Go append builtin where the array type is
inferred from the first argument. We detect an untyped const passed as
argument, then we convert it to the array type prior to proceed further.
It fixes further consistency checks and value generation.

Fixes gnolang#1136, fixes gnolang#1149, fixes gnolang#1341.

<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Morgan <[email protected]>
Co-authored-by: Morgan <[email protected]>

examples/gno.land/r/gnoland/faucet/z2_filetest.gno

@@ -16,11 +16,11 @@ func main() {
 	)
 	std.TestSetOrigCaller(adminaddr)
 	err := faucet.AdminAddController(controlleraddr1)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	err = faucet.AdminAddController(controlleraddr2)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	println(faucet.Render(""))

examples/gno.land/r/gnoland/faucet/z3_filetest.gno

@@ -18,21 +18,21 @@ func main() {
 	)
 	std.TestSetOrigCaller(adminaddr)
 	err := faucet.AdminAddController(controlleraddr1)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	err = faucet.AdminAddController(controlleraddr2)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	std.TestSetOrigCaller(controlleraddr1)
 	err = faucet.Transfer(testaddr1, 1000000)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	std.TestSetOrigCaller(controlleraddr2)
 	err = faucet.Transfer(testaddr1, 2000000)
-	if err != nil {
+	if err != "" {
 		panic(err)
 	}
 	println(faucet.Render(""))

gnovm/pkg/gnolang/preprocess.go

@@ -1012,6 +1012,29 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 								args1 = Preprocess(nil, last, args1).(Expr)
 								n.Args[1] = args1
 							}
+						} else {
+							var tx *constTypeExpr // array type expr, lazily initialized
+							// Another special case for append: adding untyped constants.
+							// They must be converted to the array type for consistency.
+							for i, arg := range n.Args[1:] {
+								if _, ok := arg.(*ConstExpr); !ok {
+									// Consider only constant expressions.
+									continue
+								}
+								if t1 := evalStaticTypeOf(store, last, arg); t1 != nil && !isUntyped(t1) {
+									// Consider only untyped values (including nil).
+									continue
+								}
+
+								if tx == nil {
+									// Get the array type from the first argument.
+									s0 := evalStaticTypeOf(store, last, n.Args[0])
+									tx = constType(arg, s0.Elem())
+								}
+								// Convert to the array type.
+								arg1 := Call(tx, arg)
+								n.Args[i+1] = Preprocess(nil, last, arg1).(Expr)
+							}
 						}
 					} else if fv.PkgPath == uversePkgPath && fv.Name == "copy" {
 						if len(n.Args) == 2 {

gnovm/pkg/gnolang/values_conversions.go

@@ -77,6 +77,10 @@ GNO_CASE:
 	}
 	// special case for undefined/nil source
 	if tv.IsUndefined() {
+		switch t.Kind() {
+		case BoolKind, StringKind, IntKind, Int8Kind, Int16Kind, Int32Kind, Int64Kind, UintKind, Uint8Kind, Uint16Kind, Uint32Kind, Uint64Kind, Float32Kind, Float64Kind, BigintKind, BigdecKind:
+			panic(fmt.Sprintf("cannot convert %v to %v", tv, t))
+		}
 		tv.T = t
 		return
 	}

gnovm/tests/files/append5.gno

@@ -7,4 +7,4 @@ func main() {
 }
 
 // Output:
-// X
+// X

gnovm/tests/files/append6.gno

@@ -0,0 +1,12 @@
+package main
+
+func main() {
+	const x = 118999
+	y := 11
+	p := []int{}
+	p = append(p, x, y)
+	println(p[0] + p[1])
+}
+
+// Output:
+// 119010

gnovm/tests/files/append7.gno

@@ -0,0 +1,10 @@
+package main
+
+func main() {
+	var errors []error
+	errors = append(errors, nil, nil)
+	println(len(errors))
+}
+
+// Output:
+// 2

gnovm/tests/files/convert4.gno

@@ -0,0 +1,7 @@
+package main
+
+func main() {
+	println(int(nil))
+}
+
+// error: cannot convert (undefined) to int

gnovm/tests/files/convert5.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	var ints []int
+	ints = append(ints, nil, nil)
+	println(ints)
+}
+
+// error: cannot convert (undefined) to int