Compiler gives incorrect error for variable usage in xml interpolated expressions

[gimantha]

Consider the following code

import ballerina/io;

public function main() {
    int|decimal|float|string|boolean firstExpr = 34;
    xml<xml:Element> secondExpr = xml `<Book>"${firstExpr}"</Book>`;
}

This gives the following error incompatible types: expected '(int|float|decimal|string|boolean|xml)', found '(int|decimal|float|string|boolean)'

Infact, the firstExpr is allowed in xml as it is simpleType

[poorna2152]

Created spec issue for clarification