[jdk17u-dev] RFR: 8306854: javac with '-source 7' should honor default methods in implemented Java 8 interfaces

[Volker Simonis]

Reported by Richard Hernandez (riher at amazon.com).

Consider the following example:


interface A {
  void foo();
}



interface B extends A {
  @Override default void foo() { }
}



class C implements B { }


If we compile `A.java` and `B.java` with `javac -source 8` and `C.java` with `-source 7` we will get the following error:

$ javac -source 8 A.java
$ javac -source 8 B.java
$ javac -source 7 C.java
C.java:1: error: C is not abstract and does not override abstract method foo() in A
class C implements B { }


I think this is wrong, because `foo()` is implemented as a default method in `B`.

The following, slightly simpler example works perfectly fine, although it also depends on a default method in an implemented interface:


interface D {
  default void bar() { }
}



class E implements D { }



$ javac -source 8 D.java 
$ javac -source 7 E.java 


In the second example, `javac` happily accepts the default implementation of `bar()` from interface `D`.

Interestingly, `ecj` (the [Eclipse Compiler for Java](https://ftp.fau.de/eclipse/eclipse/downloads/drops4/R-4.27-202303020300/ecj-4.27.jar)) compiles both examples without any errors:


$ java -jar ecj-4.27.jar -cp . -source 8 A.java 
$ java -jar ecj-4.27.jar -cp . -source 8 B.java 
$ java -jar ecj-4.27.jar -cp . -source 7 C.java 


I think the problem is in [`com.sun.tools.javac.code.Types::firstUnimplementedAbstractImpl()`](https://github.com/openjdk/jdk17u/blob/22cdf79dce8b2ec7c68e3ba71550d70e4eeb2c48/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L3110-L3129), specifically in line 3123:


3123:   if (allowDefaultMethods) {
3124:       MethodSymbol prov = interfaceCandidates(impl.type, absmeth).head;
3125:       if (prov != null && prov.overrides(absmeth, impl, this, true)) {
3126:           implmeth = prov;
3127:       }
3128   }


Here the check for imlementations of `A::foo()` (`absmeth` in the code snippet above) for class `C` (`impl` in the code snippet above) will only be performed if `allowDefaultMethods` is set to true, but `allowDefaultMethods` is globally set to false by `-source 7`.

Instead of relying on the *global* `-source` setting, I think `firstUnimplementedAbstractImpl()` should rather rely on the class file version of the corresponing class (i.e. `B` in this case) and if that class file version is >= 8, it should consider its default method implementations. I even think that the check for `allowDefaultMethods` could be completely removed, because if the class dependency `B` wasn't compiled before, it will be compiled from source and fail for `-source 7` because it contains a default method. If on the other hand, the class dependency `B` was already compiled to a class file with `-source 8` it is OK to use it's default method.

**Notice** that the current fix only affects `javac` if called with `-source 7`. It has no no impact on all other invocations of `javac`.

I know that support for `-source 7` was removed in JDK 20 (by JDK-8173605), but this issue appears in all JDKs from 8 to 19. I still think that fixing it in 8, 11 & 17 might be useful because there are use cases where real world projects migrate to 8, 11 or 17 but still need to compile some source files with `-source -7` (at least temporary, until all their dependencies like bytecode manipulation libraries have been updated).

-------------

Commit messages:
 - 8306854: javac with '-source 7' should honor default methods in implemented Java 8 interfaces

Changes: https://git.openjdk.org/jdk17u-dev/pull/1296/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306854
  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1296.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1296/head:pull/1296

PR: https://git.openjdk.org/jdk17u-dev/pull/1296