Decode: ensure signed exponents don't start with an underscore (#699)

decode.go

@@ -465,6 +465,10 @@ func checkAndRemoveUnderscoresFloats(b []byte) ([]byte, error) {
 				return nil, newDecodeError(b[i+1:i+2], "cannot have underscore before exponent")
 			}
 			before = false
+		case '+', '-':
+			// signed exponents
+			cleaned = append(cleaned, c)
+			before = false
 		case 'e', 'E':
 			if i < len(b)-1 && b[i+1] == '_' {
 				return nil, newDecodeError(b[i+1:i+2], "cannot have underscore after exponent")

unmarshaler_test.go

@@ -2317,6 +2317,14 @@ func TestUnmarshalDecodeErrors(t *testing.T) {
 			desc: "number with negative sign and leading underscore",
 			data: `a = -_0`,
 		},
+		{
+			desc: "exponent with plus sign and leading underscore",
+			data: `a = 0e+_0`,
+		},
+		{
+			desc: "exponent with negative sign and leading underscore",
+			data: `a = 0e-_0`,
+		},
 		{
 			desc: "int with wrong base",
 			data: `a = 0f2`,