RFR: add use site null checks

[Maurizio Cimadamore]

On Thu, 15 Jan 2026 05:46:52 GMT, Vicente Romero <vromero at openjdk.org> wrote:

> Adding use site null checks for method invocations. They are added by default but if option: `noUseSiteNullChecks` is passed, the compiler won't generate them

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/NullChecksWriter.java line 204:

> 202:             }
> 203:             Type formalArgType = declaredArgTypes.head;
> 204:             if (msym.isVarArgs() && formalArgType == varArgsArg) {

I think this check can misfire. Some types are shared -- e.g. syms.stringType, so you might get `==` in "wrong" places. That said, here `varArgsArg` is an array type, and for that we might always get a fresh instance... so it might work.

It also looks like there might be a potential problem here -- this code runs _after_ varargs translation has occurred. Meaning that for varargs, we only see a javac-created array and we can check the variadic elements.

But again, the plan here would be to disallow `C!...` so this is ok.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1910#discussion_r2693697618