fmt: make %F a synonym for %f

Rationale:
        It already is for scanning.
        It is accepted for complexes already, but doesn't work.
        It's analogous to %G and %E.
        C accepts it too, and we try to be roughly compatible.
Fixes #7518.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77580044

src/pkg/fmt/doc.go

@@ -37,6 +37,7 @@
 		%e	scientific notation, e.g. -1234.456e+78
 		%E	scientific notation, e.g. -1234.456E+78
 		%f	decimal point but no exponent, e.g. 123.456
+		%F	synonym for %f
 		%g	whichever of %e or %f produces more compact output
 		%G	whichever of %E or %f produces more compact output
 	String and slice of bytes:

src/pkg/fmt/fmt_test.go

@@ -220,6 +220,8 @@ var fmtTests = []struct {
 	{"%+.3e", 0.0, "+0.000e+00"},
 	{"%+.3e", 1.0, "+1.000e+00"},
 	{"%+.3f", -1.0, "-1.000"},
+	{"%+.3F", -1.0, "-1.000"},
+	{"%+.3F", float32(-1.0), "-1.000"},
 	{"%+07.2f", 1.0, "+001.00"},
 	{"%+07.2f", -1.0, "-001.00"},
 	{"% .3E", -1.0, "-1.000E+00"},
@@ -241,6 +243,8 @@ var fmtTests = []struct {
 	{"%+.3g", 1 + 2i, "(+1+2i)"},
 	{"%.3e", 0i, "(0.000e+00+0.000e+00i)"},
 	{"%.3f", 0i, "(0.000+0.000i)"},
+	{"%.3F", 0i, "(0.000+0.000i)"},
+	{"%.3F", complex64(0i), "(0.000+0.000i)"},
 	{"%.3g", 0i, "(0+0i)"},
 	{"%.3e", 1 + 2i, "(1.000e+00+2.000e+00i)"},
 	{"%.3f", 1 + 2i, "(1.000+2.000i)"},

src/pkg/fmt/format.go

@@ -447,7 +447,7 @@ func (f *fmt) fmt_c64(v complex64, verb rune) {
 			f.fmt_e32(r)
 		case 'E':
 			f.fmt_E32(r)
-		case 'f':
+		case 'f', 'F':
 			f.fmt_f32(r)
 		case 'g':
 			f.fmt_g32(r)
@@ -477,7 +477,7 @@ func (f *fmt) fmt_c128(v complex128, verb rune) {
 			f.fmt_e64(r)
 		case 'E':
 			f.fmt_E64(r)
-		case 'f':
+		case 'f', 'F':
 			f.fmt_f64(r)
 		case 'g':
 			f.fmt_g64(r)

src/pkg/fmt/print.go

@@ -447,7 +447,7 @@ func (p *pp) fmtFloat32(v float32, verb rune) {
 		p.fmt.fmt_e32(v)
 	case 'E':
 		p.fmt.fmt_E32(v)
-	case 'f':
+	case 'f', 'F':
 		p.fmt.fmt_f32(v)
 	case 'g', 'v':
 		p.fmt.fmt_g32(v)
@@ -466,7 +466,7 @@ func (p *pp) fmtFloat64(v float64, verb rune) {
 		p.fmt.fmt_e64(v)
 	case 'E':
 		p.fmt.fmt_E64(v)
-	case 'f':
+	case 'f', 'F':
 		p.fmt.fmt_f64(v)
 	case 'g', 'v':
 		p.fmt.fmt_g64(v)