Revert "fmt: fix incorrect format of whole-number floats when using %#v"

Numbers without decimals are valid Go representations of whole-number
floats. That is, "var x float64 = 5" is valid Go. Avoid breakage in
tests that expect a certain output from %#v by reverting to it.

To guarantee the right type is generated by a print use %T(%#v) instead.

Added a test to lock in this behavior.

This reverts commit 7c7cecc184.

Fixes #27634
Updates #26363

Change-Id: I544c400a0903777dd216452a7e86dfe60b0b0283
Reviewed-on: https://go-review.googlesource.com/c/142597
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/fmt/fmt_test.go

@@ -686,11 +686,10 @@ var fmtTests = []struct {
 	{"%#v", 1.2345678, "1.2345678"},
 	{"%#v", float32(1.2345678), "1.2345678"},
 
-	// Whole number floats should have a single trailing zero added, but not
-	// for exponent notation.
-	{"%#v", 1.0, "1.0"},
+	// Whole number floats are printed without decimals. See Issue 27634.
+	{"%#v", 1.0, "1"},
 	{"%#v", 1000000.0, "1e+06"},
-	{"%#v", float32(1.0), "1.0"},
+	{"%#v", float32(1.0), "1"},
 	{"%#v", float32(1000000.0), "1e+06"},
 
 	// Only print []byte and []uint8 as type []byte if they appear at the top level.

src/fmt/format.go

@@ -481,19 +481,15 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 		return
 	}
 	// The sharp flag forces printing a decimal point for non-binary formats
-	// and retains trailing zeros, which we may need to restore. For the sharpV
-	// flag, we ensure a single trailing zero is present if the output is not
-	// in exponent notation.
-	if f.sharpV || (f.sharp && verb != 'b') {
+	// and retains trailing zeros, which we may need to restore.
+	if f.sharp && verb != 'b' {
 		digits := 0
-		if !f.sharpV {
-			switch verb {
-			case 'g', 'G':
-				digits = prec
-				// If no precision is set explicitly use a precision of 6.
-				if digits == -1 {
-					digits = 6
-				}
+		switch verb {
+		case 'v', 'g', 'G':
+			digits = prec
+			// If no precision is set explicitly use a precision of 6.
+			if digits == -1 {
+				digits = 6
 			}
 		}
 
@@ -502,32 +498,25 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 		var tailBuf [5]byte
 		tail := tailBuf[:0]
 
-		var hasDecimalPoint, hasExponent bool
+		hasDecimalPoint := false
 		// Starting from i = 1 to skip sign at num[0].
 		for i := 1; i < len(num); i++ {
 			switch num[i] {
 			case '.':
 				hasDecimalPoint = true
 			case 'e', 'E':
-				hasExponent = true
 				tail = append(tail, num[i:]...)
 				num = num[:i]
 			default:
 				digits--
 			}
 		}
-		if f.sharpV {
-			if !hasDecimalPoint && !hasExponent {
-				num = append(num, '.', '0')
-			}
-		} else {
-			if !hasDecimalPoint {
-				num = append(num, '.')
-			}
-			for digits > 0 {
-				num = append(num, '0')
-				digits--
-			}
+		if !hasDecimalPoint {
+			num = append(num, '.')
+		}
+		for digits > 0 {
+			num = append(num, '0')
+			digits--
 		}
 		num = append(num, tail...)
 	}

test/switch5.go

@@ -24,8 +24,8 @@ func f0(x int) {
 func f1(x float32) {
 	switch x {
 	case 5:
-	case 5: // ERROR "duplicate case 5 .value 5\.0. in switch"
-	case 5.0: // ERROR "duplicate case 5 .value 5\.0. in switch"
+	case 5: // ERROR "duplicate case 5 in switch"
+	case 5.0: // ERROR "duplicate case 5 in switch"
 	}
 }
 
@@ -44,9 +44,9 @@ func f3(e interface{}) {
 	case 0: // ERROR "duplicate case 0 in switch"
 	case int64(0):
 	case float32(10):
-	case float32(10): // ERROR "duplicate case float32\(10\) .value 10\.0. in switch"
+	case float32(10): // ERROR "duplicate case float32\(10\) .value 10. in switch"
 	case float64(10):
-	case float64(10): // ERROR "duplicate case float64\(10\) .value 10\.0. in switch"
+	case float64(10): // ERROR "duplicate case float64\(10\) .value 10. in switch"
 	}
 }