don't use compact floats in msgpack

There is a bug in the compact floats implementation, which could cause
some integer values stored in a float to encode as an incorrect value.
The result of the expression used to check for compact integer encoding
is `float64(int64(n)) == n`, which is undefined when conversion is
not exact.

cty/msgpack/marshal.go

@@ -32,7 +32,10 @@ func Marshal(val cty.Value, ty cty.Type) ([]byte, error) {
 	var buf bytes.Buffer
 	enc := msgpack.NewEncoder(&buf)
 	enc.UseCompactInts(true)
-	enc.UseCompactFloats(true)
+
+	// UseCompactFloats can fail on some platforms due to undefined behavior of
+	// float conversions
+	enc.UseCompactFloats(false)
 
 	err := marshal(val, ty, path, enc)
 	if err != nil {

cty/msgpack/roundtrip_test.go

@@ -84,6 +84,18 @@ func TestRoundTrip(t *testing.T) {
 			bigNumberVal,
 			cty.Number,
 		},
+		{
+			cty.MustParseNumberVal("9223372036854775807"),
+			cty.Number,
+		},
+		{
+			cty.MustParseNumberVal("9223372036854775808"),
+			cty.Number,
+		},
+		{
+			cty.MustParseNumberVal("9223372036854775809"),
+			cty.Number,
+		},
 		{
 			awkwardFractionVal,
 			cty.Number,