Optional field access on a union of record and map

[MaryamZi]

Description:
The spec says for "Optional field access expression"

Let T be the static type of expression, let T' be the intersection of T and 
basic type mapping, let K be the singleton type containing just the 
string field-name and let M be the member type of K in T'. The compile-time 
requirements on the optional-field-access-expr depend on whether the 
type descriptor describing T is lax:

- if it is lax, then the only compile-time requirement is that M is non-empty;
- if it is not lax, then, in addition, T must be a subtype of the union of () and 
the mapping basic type, and the type descriptor for T must include field-name 
as an individual-type-descriptor (if the type descriptor is a union, then this 
requirement must be satisfied by at least one member of the union).

For point 2, currently we allow optional field access only on a union of records, if at lease one record has a field by the identifier. We don't allow the same, if at least one member is a map. Should we allow this?

Steps to reproduce:

import ballerina/io;

type Foo record {
    string s;
};

public function main(string... args) {
    Foo f = { s: "hello" };
    map<string>|Foo u = f;
    string? s = u?.s;
    io:println(s);
}

Affected Versions:
1.0.0-beta-SNAPSHOT - 15th August build

[MaryamZi]

See ballerina-platform/ballerina-spec#475.