fmt: allow any type in a format's width argument

The construction
	fmt.Printf("%*d", n, 4)
reads the argument n as a width specifier to use when printing 4.
Until now, only strict int type was accepted here and it couldn't
be fixed because the fix, using reflection, broke escape analysis
and added an extra allocation in every Printf call, even those that
do not use this feature.

The compiler has been fixed, although I am not sure when exactly,
so let's fix Printf and then write

Fixes #10732.

Change-Id: I79cf0c4fadd876265aa39d3cb62867247b36ab65
Reviewed-on: https://go-review.googlesource.com/14491
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 21 insertions, 2 deletions

diff --git a/src/fmt/print.go b/src/fmt/print.go
index 8d3e97c3ab..ebfa13e4d3 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -1024,11 +1024,30 @@ BigSwitch:
 	return wasString
 }
 
-// intFromArg gets the argNumth element of a. On return, isInt reports whether the argument has type int.
+// intFromArg gets the argNumth element of a. On return, isInt reports whether the argument has integer type.
 func intFromArg(a []interface{}, argNum int) (num int, isInt bool, newArgNum int) {
 	newArgNum = argNum
 	if argNum < len(a) {
-		num, isInt = a[argNum].(int)
+		num, isInt = a[argNum].(int) // Almost always OK.
+		if !isInt {
+			// Work harder.
+			switch v := reflect.ValueOf(a[argNum]); v.Kind() {
+			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+				n := v.Int()
+				if int64(int(n)) == n {
+					num = int(n)
+					isInt = true
+				}
+			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+				n := v.Uint()
+				if int64(n) >= 0 && uint64(int(n)) == n {
+					num = int(n)
+					isInt = true
+				}
+			default:
+				// Already 0, false.
+			}
+		}
 		newArgNum = argNum + 1
 		if tooLarge(num) {
 			num = 0