[ES|QL] To_DatePeriod and To_TimeDuration return better error messages on union_type fields (#114934)

* better error messages with union_type fields

docs/changelog/114934.yaml

@@ -0,0 +1,6 @@
+pr: 114934
+summary: "[ES|QL] To_DatePeriod and To_TimeDuration return better error messages on\
+  \ `union_type` fields"
+area: ES|QL
+type: bug
+issues: []

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java

@@ -51,6 +51,7 @@ import org.elasticsearch.xpack.esql.expression.function.UnresolvedFunction;
 import org.elasticsearch.xpack.esql.expression.function.UnsupportedAttribute;
 import org.elasticsearch.xpack.esql.expression.function.scalar.EsqlScalarFunction;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.AbstractConvertFunction;
+import org.elasticsearch.xpack.esql.expression.function.scalar.convert.FoldablesConvertFunction;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToDouble;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToInteger;
 import org.elasticsearch.xpack.esql.expression.function.scalar.convert.ToLong;
@@ -1226,6 +1227,16 @@ public class Analyzer extends ParameterizedRuleExecutor<LogicalPlan, AnalyzerCon
             if (convert.field() instanceof FieldAttribute fa && fa.field() instanceof InvalidMappedField imf) {
                 HashMap<TypeResolutionKey, Expression> typeResolutions = new HashMap<>();
                 Set<DataType> supportedTypes = convert.supportedTypes();
+                if (convert instanceof FoldablesConvertFunction fcf) {
+                    // FoldablesConvertFunction does not accept fields as inputs, they only accept constants
+                    String unresolvedMessage = "argument of ["
+                        + fcf.sourceText()
+                        + "] must be a constant, received ["
+                        + Expressions.name(fa)
+                        + "]";
+                    Expression ua = new UnresolvedAttribute(fa.source(), fa.name(), unresolvedMessage);
+                    return fcf.replaceChildren(Collections.singletonList(ua));
+                }
                 imf.types().forEach(type -> {
                     if (supportedTypes.contains(type.widenSmallNumeric())) {
                         TypeResolutionKey key = new TypeResolutionKey(fa.name(), type);

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/convert/FoldablesConvertFunction.java

@@ -59,7 +59,8 @@ public abstract class FoldablesConvertFunction extends AbstractConvertFunction i
 
     @Override
     protected final Map<DataType, BuildFactory> factories() {
-        // TODO if a union type field is provided as an input, the correct error message is not shown, #112668 is a follow up
+        // This is used by ResolveUnionTypes, which is expected to be applied to ES fields only
+        // FoldablesConvertFunction takes only constants as inputs, so this is empty
         return Map.of();
     }
 

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/analysis/VerifierTests.java

@@ -244,6 +244,34 @@ public class VerifierTests extends ESTestCase {
                 + " [ip] in [test1, test2, test3] and [2] other indices, [keyword] in [test6]",
             error("from test* | where multi_typed is not null", analyzer)
         );
+
+        for (String functionName : List.of("to_timeduration", "to_dateperiod")) {
+            String lineNumber = functionName.equalsIgnoreCase("to_timeduration") ? "47" : "45";
+            String errorType = functionName.equalsIgnoreCase("to_timeduration") ? "time_duration" : "date_period";
+            assertEquals(
+                "1:" + lineNumber + ": Cannot use field [unsupported] with unsupported type [flattened]",
+                error("from test* | eval x = now() + " + functionName + "(unsupported)", analyzer)
+            );
+            assertEquals(
+                "1:" + lineNumber + ": argument of [" + functionName + "(multi_typed)] must be a constant, received [multi_typed]",
+                error("from test* | eval x = now() + " + functionName + "(multi_typed)", analyzer)
+            );
+            assertThat(
+                error("from test* | eval x = unsupported, y = now() + " + functionName + "(x)", analyzer),
+                containsString("1:23: Cannot use field [unsupported] with unsupported type [flattened]")
+            );
+            assertThat(
+                error("from test* | eval x = multi_typed, y = now() + " + functionName + "(x)", analyzer),
+                containsString(
+                    "1:48: argument of ["
+                        + functionName
+                        + "(x)] must be ["
+                        + errorType
+                        + " or string], "
+                        + "found value [x] type [unsupported]"
+                )
+            );
+        }
     }
 
     public void testRoundFunctionInvalidInputs() {