database/sql/driver: remove string exclusion

The exclusion of string from IsScanValue prevents driver authors from
writing their drivers in such a way that would allow users to
distinguish between strings and byte arrays returned from a database.
Such drivers are possible today, but require their authors to deviate
from the guidance provided by the standard library.

This exclusion has been in place since the birth of this package in
357f2cb,
but the fakedb implementation shipped in the same commit violates the
exclusion!

Strictly speaking this is a breaking change, but it increases the set
of permissible Scan types, and should not cause breakage in practice.

No test changes are necessary because fakedb already exercises this.

Fixes #6497.

Change-Id: I69dbd3a59d90464bcae8c852d7ec6c97bfd120f8
Reviewed-on: https://go-review.googlesource.com/19439
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/driver/driver.go

@@ -17,7 +17,7 @@ import "errors"
 //   float64
 //   bool
 //   []byte
-//   string   [*] everywhere except from Rows.Next.
+//   string
 //   time.Time
 type Value interface{}
 
@@ -165,10 +165,6 @@ type Rows interface {
 	// the provided slice. The provided slice will be the same
 	// size as the Columns() are wide.
 	//
-	// The dest slice may be populated only with
-	// a driver Value type, but excluding string.
-	// All string values must be converted to []byte.
-	//
 	// Next should return io.EOF when there are no more rows.
 	Next(dest []Value) error
 }

src/database/sql/driver/types.go

@@ -172,28 +172,21 @@ func (n NotNull) ConvertValue(v interface{}) (Value, error) {
 }
 
 // IsValue reports whether v is a valid Value parameter type.
-// Unlike IsScanValue, IsValue permits the string type.
 func IsValue(v interface{}) bool {
-	if IsScanValue(v) {
+	if v == nil {
 		return true
 	}
-	if _, ok := v.(string); ok {
+	switch v.(type) {
+	case []byte, bool, float64, int64, string, time.Time:
 		return true
 	}
 	return false
 }
 
-// IsScanValue reports whether v is a valid Value scan type.
-// Unlike IsValue, IsScanValue does not permit the string type.
+// IsScanValue is equivalent to IsValue.
+// It exists for compatibility.
 func IsScanValue(v interface{}) bool {
-	if v == nil {
-		return true
-	}
-	switch v.(type) {
-	case int64, float64, []byte, bool, time.Time:
-		return true
-	}
-	return false
+	return IsValue(v)
 }
 
 // DefaultParameterConverter is the default implementation of