fmt: handle os.Error values

Handling os.Error is no different than handling fmt.Stringer
here, so the code is redundant now, but it will be necessary
once error goes in.

Adding it now will make gofix fix it.

R=r
CC=golang-dev
https://golang.org/cl/5331045

src/pkg/fmt/doc.go

@@ -89,10 +89,14 @@
 	If an operand implements interface Formatter, that interface
 	can be used for fine control of formatting.
 
-	If an operand implements method String() string that method
+	Next, if an operand implements the error interface, the Error method
 	will be used to convert the object to a string, which will then
-	be formatted as required by the verb (if any). To avoid
+	be formatted as required by the verb (if any).
-	recursion in cases such as
+
+	Finally, if an operand implements method String() string that method
+	will be used to convert the object to a string, which will then
+	be formatted as required by the verb (if any).
+	To avoid recursion in cases such as
 		type X int
 		func (x X) String() string { return Sprintf("%d", x) }
 	cast the value before recurring:

src/pkg/fmt/print.go

@@ -630,12 +630,23 @@ func (p *pp) handleMethods(verb rune, plus, goSyntax bool, depth int) (wasString
 			return
 		}
 	} else {
-		// Is it a Stringer?
+		// Is it an error or Stringer?
-		if stringer, ok := p.field.(Stringer); ok {
+		// The duplication in the bodies is necessary:
+		// setting wasString and handled and deferring catchPanic
+		// must happen before calling the method.
+		switch v := p.field.(type) {
+		case os.Error:
 			wasString = false
 			handled = true
 			defer p.catchPanic(p.field, verb)
-			p.printField(stringer.String(), verb, plus, false, depth)
+			p.printField(v.String(), verb, plus, false, depth)
+			return
+
+		case Stringer:
+			wasString = false
+			handled = true
+			defer p.catchPanic(p.field, verb)
+			p.printField(v.String(), verb, plus, false, depth)
 			return
 		}
 	}