fmt: print values for map keys with non-reflexive equality

Previously fmt would first obtain a list of map keys
and then look up the value for each key. Since NaNs can
be map keys but cannot be fetched directly, the lookup would
fail and return a zero reflect.Value, which formats as <nil>.

golang.org/cl/33572 added a map iterator to the reflect package
that is used in this CL to retrieve the key and value from
the map and prints the correct value even for keys that are not
equal to themselves.

Fixes #14427

Change-Id: I9e1522959760b3de8b7ecf7a6e67cd603339632a
Reviewed-on: https://go-review.googlesource.com/129777
Reviewed-by: Alan Donovan <adonovan@google.com>

src/fmt/fmt_test.go

@@ -861,13 +861,8 @@ var fmtTests = []struct {
 	// Extra argument errors should format without flags set.
 	{"%010.2", "12345", "%!(NOVERB)%!(EXTRA string=12345)"},
 
-	// The "<nil>" show up because maps are printed by
-	// first obtaining a list of keys and then looking up
-	// each key. Since NaNs can be map keys but cannot
-	// be fetched directly, the lookup fails and returns a
-	// zero reflect.Value, which formats as <nil>.
-	// This test is just to check that it shows the two NaNs at all.
-	{"%v", map[float64]int{NaN: 1, NaN: 2}, "map[NaN:<nil> NaN:<nil>]"},
+	// Test that maps with non-reflexive keys print all keys and values.
+	{"%v", map[float64]int{NaN: 1, NaN: 1}, "map[NaN:1 NaN:1]"},
 
 	// Comparison of padding rules with C printf.
 	/*
@@ -1033,7 +1028,7 @@ var fmtTests = []struct {
 	{"%☠", &[]interface{}{I(1), G(2)}, "&[%!☠(fmt_test.I=1) %!☠(fmt_test.G=2)]"},
 	{"%☠", SI{&[]interface{}{I(1), G(2)}}, "{%!☠(*[]interface {}=&[1 2])}"},
 	{"%☠", reflect.Value{}, "<invalid reflect.Value>"},
-	{"%☠", map[float64]int{NaN: 1}, "map[%!☠(float64=NaN):%!☠(<nil>)]"},
+	{"%☠", map[float64]int{NaN: 1}, "map[%!☠(float64=NaN):%!☠(int=1)]"},
 }
 
 // zeroFill generates zero-filled strings of the specified width. The length

src/fmt/print.go

@@ -743,8 +743,8 @@ func (p *pp) printValue(value reflect.Value, verb rune, depth int) {
 		} else {
 			p.buf.WriteString(mapString)
 		}
-		keys := f.MapKeys()
-		for i, key := range keys {
+		iter := f.MapRange()
+		for i := 0; iter.Next(); i++ {
 			if i > 0 {
 				if p.fmt.sharpV {
 					p.buf.WriteString(commaSpaceString)
@@ -752,9 +752,9 @@ func (p *pp) printValue(value reflect.Value, verb rune, depth int) {
 					p.buf.WriteByte(' ')
 				}
 			}
-			p.printValue(key, verb, depth+1)
+			p.printValue(iter.Key(), verb, depth+1)
 			p.buf.WriteByte(':')
-			p.printValue(f.MapIndex(key), verb, depth+1)
+			p.printValue(iter.Value(), verb, depth+1)
 		}
 		if p.fmt.sharpV {
 			p.buf.WriteByte('}')