Repro to support proposal to support parsing out `<nil>`

database_sql-convert.go.diff

diff --git a/src/database/sql/convert.go b/src/database/sql/convert.go
index b79ec3f..bb384a9 100644
--- a/src/database/sql/convert.go
+++ b/src/database/sql/convert.go
@@ -402,11 +402,15 @@ func convertAssign(dest, src interface{}) error {
 		dv.SetUint(u64)
 		return nil
 	case reflect.Float32, reflect.Float64:
-		s := asString(src)
-		f64, err := strconv.ParseFloat(s, dv.Type().Bits())
-		if err != nil {
-			err = strconvErr(err)
-			return fmt.Errorf("converting driver.Value type %T (%q) to a %s: %v", src, s, dv.Kind(), err)
+		var f64 float64
+		if src != nil {
+			var err error
+			s := asString(src)
+			f64, err = strconv.ParseFloat(s, dv.Type().Bits())
+			if err != nil {
+				err = strconvErr(err)
+				return fmt.Errorf("converting driver.Value type %T (%q) to a %s: %v", src, s, dv.Kind(), err)
+			}
 		}
 		dv.SetFloat(f64)
 		return nil

db-float-nil-repro.go

package main

import (
	"database/sql"
	"flag"
	"log"

	// MySQL driver
	_ "github.com/go-sql-driver/mysql"
)

type trade struct {
	DBID uint64
	Buy  float64
	Sell float64
	Exch string
}

func main() {
	var dbURL string
	flag.StringVar(&dbURL, "db-url", "root:@/trades", "the URL to connect to MySQL")
	flag.Parse()
	db, err := sql.Open("mysql", dbURL)
	if err != nil {
		log.Fatalf("openDB: %v", err)
	}
	setup := []string{
		"create database if not exists repro",
		"use repro",
		`create table if not exists repro(
			 id integer not null AUTO_INCREMENT,
			 buy float(53),
			 sell float(53),
			 exch varchar(128),
			primary key(id)
		);`,
		"insert into repro (buy, exch) values(10, 'gophexch')",
		"insert into repro (sell, exch) values(-97.6, 'goos')",
	}

	for i, line := range setup {
		if _, err := db.Exec(line); err != nil {
			log.Fatalf("line: #%d err: %v", i, err)
		}
	}

	rows, err := db.Query("select buy, sell, exch from repro")
	if err != nil {
		log.Fatalf("row err: %v", err)
	}
	for rows.Next() {
		curTrade := new(trade)
		err := rows.Scan(&curTrade.Buy, &curTrade.Sell, &curTrade.Exch)
		if err != nil {
			log.Printf("error: %v\n", err)
		}
	}
}

without-patch.md

$ go run sample.go 
2017/11/02 02:37:13 error: sql: Scan error on column index 1: converting driver.Value type <nil> ("<nil>") to a float64: invalid syntax
2017/11/02 02:37:13 error: sql: Scan error on column index 0: converting driver.Value type <nil> ("<nil>") to a float64: invalid syntax