WKT2:2019 import/export: handle DATUM (at top level object) with PRIMEM

This is a peculiarity of the WKT grammar. Despite ISO 19111 saying that the
prime meridian is a component of the datum, in WKT, they are placed at the
same level, for backward compatibility with earlier WKT versions. So handle
exporting and importing that. The fix is only for situation where DATUM is
the top level object (was working fine otherwise), which is a uncommon use
case. And to limit the amount of issue, on export emit the prime meridian only
if it is not Greenwich.

2 files changed, 44 insertions, 9 deletions

diff --git a/src/iso19111/datum.cpp b/src/iso19111/datum.cpp
index 83f615e9..fbd4fd2d 100644
--- a/src/iso19111/datum.cpp
+++ b/src/iso19111/datum.cpp
@@ -1300,6 +1300,13 @@ void GeodeticReferenceFrame::_exportToWKT(
     }
     // the PRIMEM is exported as a child of the CRS
     formatter->endNode();
+
+    if (formatter->isAtTopLevel()) {
+        const auto &l_primeMeridian(primeMeridian());
+        if (l_primeMeridian->nameStr() != "Greenwich") {
+            l_primeMeridian->_exportToWKT(formatter);
+        }
+    }
 }
 //! @endcond
 
diff --git a/src/iso19111/io.cpp b/src/iso19111/io.cpp
index 16ab22f7..c464b724 100644
--- a/src/iso19111/io.cpp
+++ b/src/iso19111/io.cpp
@@ -388,6 +388,12 @@ void WKTFormatter::leave() {
 
 // ---------------------------------------------------------------------------
 
+bool WKTFormatter::isAtTopLevel() const {
+    return d->level_ == 0 && d->indentLevel_ == 0;
+}
+
+// ---------------------------------------------------------------------------
+
 void WKTFormatter::startNode(const std::string &keyword, bool hasId) {
     if (!d->stackHasChild_.empty()) {
         d->startNewChild();
@@ -4733,13 +4739,7 @@ BaseObjectNNPtr WKTParser::Private::build(const WKTNodeNNPtr &node) {
         return util::nn_static_pointer_cast<BaseObject>(NN_NO_CHECK(crs));
     }
 
-    if (ci_equal(name, WKTConstants::DATUM) ||
-        ci_equal(name, WKTConstants::GEODETICDATUM) ||
-        ci_equal(name, WKTConstants::TRF)) {
-        return util::nn_static_pointer_cast<BaseObject>(
-            buildGeodeticReferenceFrame(node, PrimeMeridian::GREENWICH,
-                                        null_node));
-    }
+    // Datum handled by caller code WKTParser::createFromWKT()
 
     if (ci_equal(name, WKTConstants::ENSEMBLE)) {
         return util::nn_static_pointer_cast<BaseObject>(buildDatumEnsemble(
@@ -6441,8 +6441,36 @@ BaseObjectNNPtr createFromUserInput(const std::string &text, PJ_CONTEXT *ctx) {
  * @throw ParsingException
  */
 BaseObjectNNPtr WKTParser::createFromWKT(const std::string &wkt) {
-    WKTNodeNNPtr root = WKTNode::createFrom(wkt);
-    auto obj = d->build(root);
+    const auto build = [this, &wkt]() -> BaseObjectNNPtr {
+        size_t indexEnd;
+        WKTNodeNNPtr root = WKTNode::createFrom(wkt, 0, 0, indexEnd);
+        const std::string &name(root->GP()->value());
+        if (ci_equal(name, WKTConstants::DATUM) ||
+            ci_equal(name, WKTConstants::GEODETICDATUM) ||
+            ci_equal(name, WKTConstants::TRF)) {
+
+            auto primeMeridian = PrimeMeridian::GREENWICH;
+            if (indexEnd < wkt.size()) {
+                indexEnd = skipSpace(wkt, indexEnd);
+                if (indexEnd < wkt.size() && wkt[indexEnd] == ',') {
+                    ++indexEnd;
+                    indexEnd = skipSpace(wkt, indexEnd);
+                    if (indexEnd < wkt.size() &&
+                        ci_starts_with(wkt.c_str() + indexEnd,
+                                       WKTConstants::PRIMEM.c_str())) {
+                        primeMeridian = d->buildPrimeMeridian(
+                            WKTNode::createFrom(wkt, indexEnd + 1, 0, indexEnd),
+                            UnitOfMeasure::DEGREE);
+                    }
+                }
+            }
+            return d->buildGeodeticReferenceFrame(root, primeMeridian,
+                                                  null_node);
+        }
+        return d->build(root);
+    };
+
+    auto obj = build();
 
     const auto dialect = guessDialect(wkt);
     if (dialect == WKTGuessedDialect::WKT1_GDAL ||