RFR: 8261205: AssertionError: Cannot add metadata to an intersection type

[Vicente Romero]

Please review this PR which is adding a field to JCVariableDecl, the need I see for this is that when we declare a local variable using `var` as in:


import java.lang.annotation.ElementType;
import java.lang.annotation.Target;

@Target({ElementType.TYPE_USE, ElementType.LOCAL_VARIABLE})
@interface A {}

class Test {
    void t() {
        @A var c = g(1, 1L);
    }

    <X> X g(X a, X b) {
        return a;
    }
}

the only clue the compiler uses to know that the local variable was declared using `var` is that field JCVariableDecl::vartype is null, but this field is set to an inferred type at some point and from that point on there is no way to know for sure that the original AST had a `var`. Unfortunately there is code that keeps asking for that implicit type declaration afterwards and thus making uninformed decisions as in: TypeAnnotations.TypeAnnotationPositions::visitVarDef, where the compiler is implementing a portion of section: `9.7.4 Where Annotations May Appear` of the spec. In particular how to deal with annotations applied to local variables declared with `var`. This case is interesting because the same annotation works as a declaration annotation and as a type annotation. If it were only a type annotation then the compiler would have issued an error before getting to TypeAnnotations.TypeAnnotationPositions::visitVarDef, precisely at Check::validateAnnotation, see that Check::getAppl
 icableTargets has a special case for local variables declared using `var`. But again given that the annotation in the example above is applicable as a type and as a declaration annotation, it will seep up to the metadata info associated to the local variable symbol, and that is fine. What is not OK is that as the compiler has no clue that the AST was originally declared using `var` that it tries to type annotate the type of the local variable.

This patch is also renaming a method in `JCVariableDecl`, previous name was `isImplicitlyTyped` which was basically checking if the `vartype` field was null and was renamed to `nullVarType` and a new one was added named: `declaredUsingVar` which won't change its returned value during the lifetime of a JCVariableDecl AST.

TIA

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Commit messages:
 - 8261205: AssertionError: Cannot add metadata to an intersection type

Changes: https://git.openjdk.java.net/jdk/pull/4095/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4095&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261205
  Stats: 71 lines in 5 files changed: 55 ins; 3 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4095.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4095/head:pull/4095

PR: https://git.openjdk.java.net/jdk/pull/4095