parse/pargs: do not guess on boolean flags with values

parse/pargs/posix.go

@@ -165,6 +165,19 @@ func (p *Parser) next() bool {
 	if !hasValue && p.pos < len(p.Args) {
 		value = p.Args[p.pos]
 		if len(value) > 0 && value[0] != '-' {
+			if p.isBoolFlag(name) {
+				dash := "--"
+				if short {
+					dash = "-"
+				}
+				p.fail(""+
+					"ambiguous boolean flag %[1]s%[2]s value: can't guess whether "+
+					"the %[3]q is the flag value or the non-flag argument "+
+					"(consider using `%[1]s%[2]s=%[3]s` or `%[1]s%[2]s -- %[3]s`)",
+					dash, name, value,
+				)
+				return false
+			}
 			hasValue = true
 			p.pos++
 		}

parse/pargs/posix_test.go

@@ -74,10 +74,10 @@ func TestPosixParse(t *testing.T) {
 			},
 		},
 		{
-			name: "booleans",
+			name: "short booleans",
 			args: []string{
-				"-t", "true",
-				"-f", "false",
+				"-t=true",
+				"-f=false",
 			},
 			flags: map[string]bool{
 				"t": true,
@@ -88,6 +88,45 @@ func TestPosixParse(t *testing.T) {
 				{"f", "false"},
 			},
 		},
+		{
+			name: "short ambiguous booleans",
+			args: []string{
+				"-t", "true",
+				"-f", "false",
+			},
+			flags: map[string]bool{
+				"t": true,
+				"f": true,
+			},
+			err: true,
+		},
+		{
+			name: "long booleans",
+			args: []string{
+				"--foo=true",
+				"--bar=false",
+			},
+			flags: map[string]bool{
+				"foo": true,
+				"bar": true,
+			},
+			expPairs: [][2]string{
+				{"foo", "true"},
+				{"bar", "false"},
+			},
+		},
+		{
+			name: "long ambiguous booleans",
+			args: []string{
+				"--foo", "true",
+				"--bar", "false",
+			},
+			flags: map[string]bool{
+				"foo": true,
+				"bar": true,
+			},
+			err: true,
+		},
 		{
 			name: "non-boolean without argument",
 			args: []string{