The type issue seems to affect version 5.x as well. I've updated the test case to use Hibernate 5.0.2 and it threw almost the same error, please see below. The updated test case is attached to the see, it now contains the two tests described in the original issue (e.g. the one with a join and without).

I think the query without the join is a valid JPA 2.1 JPQL query. If you look at the spec in section 4.6.17.5 the argument to the type operator can also be a "single_valued_object_path_expression"

{{entity_type_expression ::=
type_discriminator |
entity_type_literal |
input_parameter
type_discriminator ::=
TYPE(general_identification_variable |
single_valued_object_path_expression |
input_parameter )
}}

Exception thrown by Hibernate:
{{Caused by: org.hibernate.QueryException: could not resolve property: class of: hibernatetypeissue.Leaf [select l from hibernatetypeissue.Leaf l where TYPE(l.tree) = PineTree]
at org.hibernate.QueryException.generateQueryException(QueryException.java:120)
at org.hibernate.QueryException.wrapWithQueryString(QueryException.java:103)
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:218)
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:142)
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:115)
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:76)
at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:150)
at org.hibernate.internal.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:298)
at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:236)
at org.hibernate.internal.SessionImpl.createQuery(SessionImpl.java:1825)
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:291)
... 34 more
Caused by: org.hibernate.QueryException: could not resolve property: class of: hibernatetypeissue.Leaf
at org.hibernate.persister.entity.AbstractPropertyMapping.propertyException(AbstractPropertyMapping.java:62)
at org.hibernate.persister.entity.AbstractPropertyMapping.toType(AbstractPropertyMapping.java:56)
at org.hibernate.persister.entity.AbstractEntityPersister.getSubclassPropertyTableNumber(AbstractEntityPersister.java:1769)
at org.hibernate.persister.entity.BasicEntityPropertyMapping.toColumns(BasicEntityPropertyMapping.java:46)
at org.hibernate.persister.entity.AbstractEntityPersister.toColumns(AbstractEntityPersister.java:1744)
at org.hibernate.persister.entity.AbstractEntityPersister$3.getSqlFragment(AbstractEntityPersister.java:1821)
at org.hibernate.hql.internal.ast.tree.FromElement$TypeDiscriminatorMetadataImpl.getSqlFragment(FromElement.java:472)
at org.hibernate.hql.internal.ast.tree.MethodNode.typeDiscriminator(MethodNode.java:111)
at org.hibernate.hql.internal.ast.tree.MethodNode.resolve(MethodNode.java:80)
at org.hibernate.hql.internal.ast.HqlSqlWalker.processFunction(HqlSqlWalker.java:1074)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.functionCall(HqlSqlBaseWalker.java:2717)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.expr(HqlSqlBaseWalker.java:1342)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.exprOrSubquery(HqlSqlBaseWalker.java:4686)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.comparisonExpr(HqlSqlBaseWalker.java:4156)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.logicalExpr(HqlSqlBaseWalker.java:2104)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.whereClause(HqlSqlBaseWalker.java:796)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:597)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:301)
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:249)
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:262)
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:190)
... 42 more
}}