Provide proper error message when trying access an unknown unit field.

spicy/toolchain/src/ast/operators/unit.cc

@@ -31,7 +31,7 @@ QualifiedType* itemType(hilti::Builder* builder, const Expressions& operands) {
     else if ( auto bitrange = unit->findRangeInAnonymousBitField(id).second )
         return bitrange->itemType();
     else
-        return builder->qualifiedType(builder->typeAuto(), hilti::Constness::Const);
+        return builder->qualifiedType(builder->typeUnknown(), hilti::Constness::Const);
 }
 
 QualifiedType* contextResult(hilti::Builder* builder, const Expressions& operands, hilti::Constness constness) {

spicy/toolchain/src/ast/types/unit.cc

@@ -97,7 +97,7 @@ static std::pair<unit::item::Field*, hilti::type::bitfield::BitRange*> findRange
             if ( ! field->isAnonymous() )
                 continue;
 
-            auto t = field->itemType()->type()->tryAs<hilti::type::Bitfield>();
+            auto t = field->originalType()->type()->tryAs<hilti::type::Bitfield>();
             if ( ! t )
                 continue;
 

tests/Baseline/spicy.types.unit.unknown-field/output

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+[error] <...>/unknown-field.spicy:13:14-13:22: unit does not have field 'bar_'
+[error] spicyc: aborting after errors

tests/spicy/types/unit/unknown-field.spicy

@@ -0,0 +1,15 @@
+# @TEST-EXEC-FAIL: spicyc -p %INPUT >output 2>&1
+# @TEST-EXEC: btest-diff output
+#
+# @TEST-DOC: Check that we get a proper error message when trying to access an unknown unit field; regression test for #1790
+
+module Test;
+
+type Msg = unit {
+  var x: uint16;
+  bar: uint16;
+
+  on %done {
+    self.x = self.bar_;
+  }
+};