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Swapping `def x` and `def y` of implicit2.aya causes NPE

See original GitHub issue

implicit2.aya:

open data Unit : Set | unit

def y : Pi {a : Unit} -> Unit => \{a} => a
def x : Pi {a : Unit} -> Unit => y
def z : Pi {a : Unit} -> Unit => x

It does not typecheck as expected.

But a slightly different version causes NPE

open data Unit : Set | unit

def x : Pi {a : Unit} -> Unit => y
def y : Pi {a : Unit} -> Unit => \{a} => a
def z : Pi {a : Unit} -> Unit => x

stacktrace:

    java.lang.NullPointerException
        at java.base/java.util.Objects.requireNonNull(Objects.java:208)
        at org.aya.core.def.Def.defLevels(Def.java:39)
        at org.aya.tyck.ExprTycker.levelStuffs(ExprTycker.java:277)
        at org.aya.tyck.ExprTycker.defCall(ExprTycker.java:265)
        at org.aya.tyck.ExprTycker.inferRef(ExprTycker.java:231)
        at org.aya.tyck.ExprTycker.doVisitRef(ExprTycker.java:223)
        at org.aya.tyck.ExprTycker.visitRef(ExprTycker.java:212)
        at org.aya.tyck.ExprTycker.visitRef(ExprTycker.java:57)
        at org.aya.concrete.Expr$RefExpr.doAccept(Expr.java:244)
        at org.aya.concrete.Expr.accept(Expr.java:41)
        at org.aya.tyck.ExprTycker.checkNoZonk(ExprTycker.java:125)
        at org.aya.tyck.ExprTycker.lambda$checkNoZonk$2(ExprTycker.java:123)
        at org.aya.tyck.LocalCtx.with(LocalCtx.java:62)
        at org.aya.tyck.LocalCtx.with(LocalCtx.java:47)
        at org.aya.tyck.ExprTycker.checkNoZonk(ExprTycker.java:122)
        at org.aya.tyck.ExprTycker.checkExpr(ExprTycker.java:135)
        at org.aya.tyck.StmtTycker.lambda$visitFn$8(StmtTycker.java:184)
        at kala.control.Either$Left.map(Either.java:166)
        at org.aya.tyck.StmtTycker.visitFn(StmtTycker.java:183)
        at org.aya.tyck.StmtTycker.visitFn(StmtTycker.java:43)
        at org.aya.concrete.stmt.Decl$FnDecl.doAccept(Decl.java:324)
        at org.aya.concrete.stmt.Decl.accept(Decl.java:70)
        at org.aya.concrete.stmt.Decl.tyck(Decl.java:61)
        at org.aya.concrete.resolve.module.FileModuleLoader.tyckModule(FileModuleLoader.java:99)
        at org.aya.cli.single.SingleFileCompiler.compile(SingleFileCompiler.java:51)
        at org.aya.test.TestRunner.runFile(TestRunner.java:66)
        at org.aya.test.TestRunner.lambda$runDir$2(TestRunner.java:55)
        at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
        at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
        at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
        at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197)
        at java.base/java.util.Iterator.forEachRemaining(Iterator.java:133)
        at java.base/java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1845)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
        at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
        at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
        at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
        at org.aya.test.TestRunner.runDir(TestRunner.java:55)
        at org.aya.test.TestRunner.runAllAyaTests(TestRunner.java:44)