Indy string concat changes order of operations

Summary

This change causes the operands of string concatenation expressions to be evaluated and converted to strings in the correct order when using the invokedynamic-based strategies introduced in JEP 280.

Problem

JLS 15.18.1 requires string conversion to be performed on the operands of a string concatenation expression, and JLS 15.7.1 requires the operands to be fully evaluated in left-to-right order. The current behavior of the invokedynamic string concatenation strategies is to evaluate all operands, and then separately convert them all to strings. The correct behavior is to evaluate each argument and eagerly convert it to a string, in left-to-right order.

Solution

The solution is for the invokedynamic-based strategies to convert the operands to strings as they are evaluated, and before they are passed to the invokedynamic call site, to preserve the required evaluation order.

For some well-known types whose string representation is known to be idempotent (including primitive types, their boxes, and java.lang.String) the eager string conversion may be skipped.

Specification

No specification changes are required.

The proposed change to restore the behavior of the implementation to match the specification can be found in https://git.openjdk.java.net/jdk/pull/5844.