Fix nil pointer dereference for interface-typed flag values

[mrueg]

reflect.TypeOf returns nil when passed a nil interface value. FlagBase is generic over its value type T, and for GenericFlag that type is the Value interface, so both reflect.TypeOf(f.Value) and reflect.TypeOf(zero) yield nil there. Two call sites dereference the result without checking, panicking on an otherwise perfectly valid flag.

SchemaItemsType()

Panics for any GenericFlag, since it unconditionally calls reflect.TypeOf(zero).Kind():

fl := &cli.GenericFlag{Name: "gen"}
fl.SchemaItemsType() // panic: runtime error: invalid memory address or nil pointer dereference

PostParse()

Panics for a GenericFlag whose value source resolves to an empty string:

os.Setenv("SOME_VAR", "")
fl := &cli.GenericFlag{Name: "gen", Sources: cli.EnvVars("SOME_VAR")}
fl.PreParse()
fl.PostParse() // panic: runtime error: invalid memory address or nil pointer dereference

The val != "" short circuit means reflect.TypeOf(f.Value).Kind() is never evaluated when the source produces a non-empty value, so this only reproduces with an empty environment variable or an empty file. That is why it has gone unnoticed.

Fix

TypeName() and IsMultiValueFlag() already guard against a nil type; this does the same in both of the unguarded spots. PostParse() now derives the kind once up front and reuses it, which also removes a duplicate reflect.TypeOf call.

No public API change — go doc -all output is identical to testdata/godoc-v3.x.txt.

Testing

Adds TestFlagBaseInterfaceValueType covering SchemaItemsType, PostParse with an empty source value, and PostParse with a non-empty source value (to pin the non-panicking path). Both subtests panic on main without the change.

🤖 Generated with Claude Code