fix: recursively convert slices of structs when decoding to map

[AdamMagued]

When decoding a struct to a map with deep mapping enabled (either via the deep tag option or DecoderConfig.Deep), fields that are slices of pointers to structs (such as []*MyStruct) were not being converted to []map[string]any. Instead, the original []*MyStruct type was retained in the target map.

Root Cause

In decodeMapFromStruct, v.Type().Elem().Kind() was checked directly without dereferencing pointer types:

switch v.Type().Elem().Kind() {
case reflect.Struct:
    childType = reflect.TypeOf(map[string]any{})
default:
    childType = v.Type().Elem()
}

For slices of struct pointers, v.Type().Elem().Kind() is reflect.Ptr, which fell through to the default branch and preserved the pointer type rather than selecting map[string]any.

Solution

Dereference pointer indirection on v.Type().Elem() when determining childType so that both direct structs and struct pointer elements are handled recursively.

Testing

Added TestDecode_structPtrArrayDeepMap in mapstructure_test.go verifying conversion of []*SourceChild and *[]*SourceChild to []map[string]any via:

• deep struct tag option
• DecoderConfig.Deep = true
• Slices containing nil pointer elements

Fixes #120