database/sql: support scanning into user defined string types

User defined numeric types such as "type Int int64" have
been able to be scanned into without a custom scanner by
using the reflect scan code path used to convert between
various numeric types. Add in a path for string types
for symmetry and least surprise.

Fixes #18101

Change-Id: I00553bcf021ffe6d95047eca0067ee94b54ff501
Reviewed-on: https://go-review.googlesource.com/39031
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/convert.go

@@ -270,6 +270,11 @@ func convertAssign(dest, src interface{}) error {
 		return nil
 	}
 
+	// The following conversions use a string value as an intermediate representation
+	// to convert between various numeric types.
+	//
+	// This also allows scanning into user defined types such as "type Int int64".
+	// For symmetry, also check for string destination types.
 	switch dv.Kind() {
 	case reflect.Ptr:
 		if src == nil {
@@ -306,6 +311,15 @@ func convertAssign(dest, src interface{}) error {
 		}
 		dv.SetFloat(f64)
 		return nil
+	case reflect.String:
+		switch v := src.(type) {
+		case string:
+			dv.SetString(v)
+			return nil
+		case []byte:
+			dv.SetString(string(v))
+			return nil
+		}
 	}
 
 	return fmt.Errorf("unsupported Scan, storing driver.Value type %T into type %T", src, dest)

src/database/sql/convert_test.go

@@ -17,9 +17,11 @@ import (
 var someTime = time.Unix(123, 0)
 var answer int64 = 42
 
-type userDefined float64
-
-type userDefinedSlice []int
+type (
+	userDefined       float64
+	userDefinedSlice  []int
+	userDefinedString string
+)
 
 type conversionTest struct {
 	s, d interface{} // source and destination
@@ -39,6 +41,7 @@ type conversionTest struct {
 	wantptr    *int64 // if non-nil, *d's pointed value must be equal to *wantptr
 	wantnil    bool   // if true, *d must be *int64(nil)
 	wantusrdef userDefined
+	wantusrstr userDefinedString
 }
 
 // Target variables for scanning into.
@@ -171,6 +174,7 @@ var conversionTests = []conversionTest{
 	{s: int64(123), d: new(userDefined), wantusrdef: 123},
 	{s: "1.5", d: new(userDefined), wantusrdef: 1.5},
 	{s: []byte{1, 2, 3}, d: new(userDefinedSlice), wanterr: `unsupported Scan, storing driver.Value type []uint8 into type *sql.userDefinedSlice`},
+	{s: "str", d: new(userDefinedString), wantusrstr: "str"},
 
 	// Other errors
 	{s: complex(1, 2), d: &scanstr, wanterr: `unsupported Scan, storing driver.Value type complex128 into type *string`},
@@ -260,6 +264,9 @@ func TestConversions(t *testing.T) {
 		if ct.wantusrdef != 0 && ct.wantusrdef != *ct.d.(*userDefined) {
 			errf("want userDefined %f, got %f", ct.wantusrdef, *ct.d.(*userDefined))
 		}
+		if len(ct.wantusrstr) != 0 && ct.wantusrstr != *ct.d.(*userDefinedString) {
+			errf("want userDefined %q, got %q", ct.wantusrstr, *ct.d.(*userDefinedString))
+		}
 	}
 }
 

src/database/sql/sql_test.go

@@ -3155,6 +3155,24 @@ func TestPing(t *testing.T) {
 	}
 }
 
+// Issue 18101.
+func TestTypedString(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	type Str string
+	var scanned Str
+
+	err := db.QueryRow("SELECT|people|name|name=?", "Alice").Scan(&scanned)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := Str("Alice")
+	if scanned != expected {
+		t.Errorf("expected %+v, got %+v", expected, scanned)
+	}
+}
+
 func BenchmarkConcurrentDBExec(b *testing.B) {
 	b.ReportAllocs()
 	ct := new(concurrentDBExecTest)