decode: error on _ before exponent in floats (#647)

Fixes #646

decode.go

@@ -405,6 +405,9 @@ func checkAndRemoveUnderscoresFloats(b []byte) ([]byte, error) {
 			if !before {
 				return nil, newDecodeError(b[i-1:i+1], "number must have at least one digit between underscores")
 			}
+			if i < len(b)-1 && (b[i+1] == 'e' || b[i+1] == 'E') {
+				return nil, newDecodeError(b[i+1:i+2], "cannot have underscore before exponent")
+			}
 			before = false
 		case 'e', 'E':
 			if i < len(b)-1 && b[i+1] == '_' {

unmarshaler_test.go

@@ -204,6 +204,16 @@ func TestUnmarshal_Floats(t *testing.T) {
 				assert.True(t, math.IsNaN(v))
 			},
 		},
+		{
+			desc:  "underscore after integer part",
+			input: `1_e2`,
+			err:   true,
+		},
+		{
+			desc:  "underscore after integer part",
+			input: `1.0_e2`,
+			err:   true,
+		},
 	}
 
 	type doc struct {
@@ -215,12 +225,16 @@ func TestUnmarshal_Floats(t *testing.T) {
 		t.Run(e.desc, func(t *testing.T) {
 			doc := doc{}
 			err := toml.Unmarshal([]byte(`A = `+e.input), &doc)
+			if e.err {
+				require.Error(t, err)
+			} else {
 				require.NoError(t, err)
 				if e.testFn != nil {
 					e.testFn(t, doc.A)
 				} else {
 					assert.Equal(t, e.expected, doc.A)
 				}
+			}
 		})
 	}
 }