JDK-8179483: Return type inference in if

[Johannes Kuhn]

So, I tried to extract the jdk.classfile API and compile it using Java 
18 with preview features enabled. Using Eclipse.

While doing that, I encountered some compilation problems - one is the 
use of something similar like this:

     <T> T optionValue(Classfile.Option.Key key) {...}
     ...
     if (reader.optionValue(...)) {...}

According to https://bugs.openjdk.org/browse/JDK-8179483 ('if' 
conditions are treated like assignments by inference) this abuses a bug 
in javac - and is not valid java.

The places where I found a use like this are:

https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/constantpool/ConstantPoolBuilder.java#L76
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/BoundAttribute.java#L150
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/CodeImpl.java#L153
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/CodeImpl.java#L208
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/DirectCodeBuilder.java#L111
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/DirectCodeBuilder.java#L209
https://github.com/openjdk/jdk-sandbox/blob/classfile-api-branch/src/java.base/share/classes/jdk/classfile/impl/DirectCodeBuilder.java#L277

As far as I am concerned, there are two possible fixes:

* Make `if` an assignment context - but then why is `for (String s : 
EXPR) {}` not an assignment context for `Iterable<String>`?
* Change the code in jdk.classfile so it adds type hints in those cases.

I am not qualified enough to judge which one is the better fix.

- Johannes

PS.: FYI: There seems also be a bug in ECJ w.r.t. handling of class 
hierarchies of sealed classes. I still try to create a small reproducer.