internal/encoding/yaml: add underlying error to "cannot decode V as T"

This can be useful context that would otherwise be lost.
Some of these errors may be a bit repetitive, such as repeating
the input YAML string, but that's fine.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ia95c25566288a4f1b7403a7b1fec1fe1e13ae92e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1195045
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

internal/encoding/yaml/decode.go

@@ -542,8 +542,10 @@ func (d *decoder) scalar(yn *yaml.Node) (ast.Expr, error) {
 		// We make the assumption that any valid YAML integer literal will be a valid
 		// CUE integer literal as well, with the only exception of octal numbers above.
 		// Note that `!!int 123.456` is not allowed.
-		if err := literal.ParseNum(value, &info); err != nil || !info.IsInt() {
-			return nil, d.posErrorf(yn, "cannot decode %q as %s", value, yn.ShortTag())
+		if err := literal.ParseNum(value, &info); err != nil {
+			return nil, d.posErrorf(yn, "cannot decode %q as %s: %v", value, tag, err)
+		} else if !info.IsInt() {
+			return nil, d.posErrorf(yn, "cannot decode %q as %s: not a literal number", value, tag)
 		}
 		return d.makeNum(yn, value, token.INT), nil
 
@@ -563,7 +565,7 @@ func (d *decoder) scalar(yn *yaml.Node) (ast.Expr, error) {
 			// CUE float literal as well, with the only exception of Inf/NaN above.
 			// Note that `!!float 123` is allowed.
 			if err := literal.ParseNum(value, &info); err != nil {
-				return nil, d.posErrorf(yn, "cannot decode %q as %s", value, yn.ShortTag())
+				return nil, d.posErrorf(yn, "cannot decode %q as %s: %v", value, tag, err)
 			}
 			// If the decoded YAML scalar was explicitly or implicitly a float,
 			// and the scalar literal looks like an integer,

internal/encoding/yaml/decode_test.go

@@ -902,9 +902,9 @@ func TestUnmarshalNaN(t *testing.T) {
 var unmarshalErrorTests = []struct {
 	data, error string
 }{
-	{"\nv: !!float 'error'", `test.yaml:2: cannot decode "error" as !!float`},
-	{"\nv: !!int 'error'", `test.yaml:2: cannot decode "error" as !!int`},
-	{"\nv: !!int 123.456", `test.yaml:2: cannot decode "123.456" as !!int`},
+	{"\nv: !!float 'error'", `test.yaml:2: cannot decode "error" as !!float: illegal number start "error"`},
+	{"\nv: !!int 'error'", `test.yaml:2: cannot decode "error" as !!int: illegal number start "error"`},
+	{"\nv: !!int 123.456", `test.yaml:2: cannot decode "123.456" as !!int: not a literal number`},
 	{"v: [A,", "test.yaml:1: did not find expected node content"},
 	{"v:\n- [A,", "test.yaml:2: did not find expected node content"},
 	{"a:\n- b: *,", "test.yaml:2: did not find expected alphabetic or numeric character"},