Fix regression where supertype in field expression would not resolve

A change between 7 and 8 to use exact-matching for document types was
implemented a tad too aggressively, as it should still be possible to
match against supertypes in a document selection field expression.

Let type `B` inherit `A`, and `A` have a field `foo`, and matching a
document of type `B`:

Prior to this fix:
 * `B.foo` _would_ match (implicit supertype field resolving)
 * `A.foo` would _not_ match due to exact type mismatch

After this fix, both will match as generally expected.

2 files changed, 18 insertions, 7 deletions

diff --git a/document/src/tests/documentselectparsertest.cpp b/document/src/tests/documentselectparsertest.cpp
index ce4b69419a3..5e14a080f27 100644
--- a/document/src/tests/documentselectparsertest.cpp
+++ b/document/src/tests/documentselectparsertest.cpp
@@ -673,8 +673,9 @@ TEST_F(DocumentSelectParserTest, operators_1)
     // Inherited doctypes
     PARSE("testdoctype2", *_doc[4], True);
     PARSE("testdoctype2", *_doc[3], False);
-    PARSE("testdoctype1", *_doc[4], False); // testdoctype2 inherits testdoctype1, but we use exact matching for types
-    PARSE("testdoctype1.headerval = 10", *_doc[4], Invalid);
+    PARSE("testdoctype1", *_doc[4], False); // testdoctype2 inherits testdoctype1, but we use exact matching for "standalone" doctype matches.
+    PARSE("testdoctype1.headerval = 10", *_doc[4], True); // But _field lookups_ use is-a type matching semantics.
+    PARSE("testdoctype2.headerval = 10", *_doc[4], True); // Exact type match with parent field also works transparently
 }
 
 TEST_F(DocumentSelectParserTest, operators_2)
diff --git a/document/src/vespa/document/select/valuenodes.cpp b/document/src/vespa/document/select/valuenodes.cpp
index 8102a944ff0..b3052cc07e2 100644
--- a/document/src/vespa/document/select/valuenodes.cpp
+++ b/document/src/vespa/document/select/valuenodes.cpp
@@ -20,10 +20,20 @@ LOG_SETUP(".document.select.valuenode");
 namespace document::select {
 
 namespace {
-    bool documentTypeEqualsName(const DocumentType& type, vespalib::stringref name)
-    {
-        return (type.getName() == name);
+
+[[nodiscard]] bool document_type_is_a(const DocumentType& type, vespalib::stringref name) {
+    if (type.getName() == name) {
+        return true;
+    }
+    // No exact match; try to recursively match name against any types inherited from.
+    for (const auto* parent : type.getInheritedTypes()) {
+        if (document_type_is_a(*parent, name)) {
+            return true;
+        }
     }
+    return false;
+}
+
 }
 
 InvalidValueNode::InvalidValueNode(vespalib::stringref name)
@@ -409,7 +419,7 @@ FieldValueNode::getValue(const Context& context) const
 
     const Document& doc = *context._doc;
 
-    if (!documentTypeEqualsName(doc.getType(), _doctype)) {
+    if (!document_type_is_a(doc.getType(), _doctype)) {
         return std::make_unique<InvalidValue>();
     }
     // Imported fields can only be meaningfully evaluated inside Proton, so we
@@ -473,7 +483,7 @@ FieldValueNode::traceValue(const Context &context, std::ostream& out) const
         return defaultTrace(getValue(context), out);
     }
     const Document &doc(*context._doc);
-    if (!documentTypeEqualsName(doc.getType(), _doctype)) {
+    if (!document_type_is_a(doc.getType(), _doctype)) {
         out << "Document is of type " << doc.getType() << " which isn't a "
             << _doctype << " document, thus resolving invalid.\n";
         return std::make_unique<InvalidValue>();