cmd/go, flag: make undefined flag msg less ambiguous. Fixes #36364

src/cmd/go/internal/cmdflag/flag.go

@@ -31,7 +31,7 @@ type FlagNotDefinedError struct {
 }
 
 func (e FlagNotDefinedError) Error() string {
-	return fmt.Sprintf("flag provided but not defined: -%s", e.Name)
+	return fmt.Sprintf("The flag '-%s' is an unknown flag.", e.Name)
 }
 
 // A NonFlagError indicates an argument that is not a syntactically-valid flag.

src/cmd/go/testdata/script/mod_getmode_vendor.txt

@@ -13,7 +13,7 @@ stdout '^golang.org/x/text v0.0.0.* .*vendor[\\/]golang.org[\\/]x[\\/]text[\\/]l
 ! go list -mod=vendor -m rsc.io/quote@latest
 stderr 'go list -m: rsc.io/quote@latest: cannot query module due to -mod=vendor'
 ! go get -mod=vendor -u
-stderr 'flag provided but not defined: -mod'
+stderr 'The flag ''-mod'' is an unknown flag.'
 
 # Since we don't have a complete module graph, 'go list -m' queries
 # that require the complete graph should fail with a useful error.

src/flag/flag.go

@@ -926,7 +926,7 @@ func (f *FlagSet) parseOne() (bool, error) {
 			f.usage()
 			return false, ErrHelp
 		}
-		return false, f.failf("flag provided but not defined: -%s", name)
+		return false, f.failf("The flag '-%s' is an unknown flag.", name)
 	}
 
 	if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg

src/flag/flag_test.go

@@ -454,7 +454,7 @@ func TestUsageOutput(t *testing.T) {
 	defer func(old []string) { os.Args = old }(os.Args)
 	os.Args = []string{"app", "-i=1", "-unknown"}
 	Parse()
-	const want = "flag provided but not defined: -i\nUsage of app:\n"
+	const want = "The flag '-i' is an unknown flag.\nUsage of app:\n"
 	if got := buf.String(); got != want {
 		t.Errorf("output = %q; want %q", got, want)
 	}