diff --git a/packages/core/src/runtime/quasivalue.ts b/packages/core/src/runtime/quasivalue.ts
index 93842ee..ab5ceaf 100644
--- a/packages/core/src/runtime/quasivalue.ts
+++ b/packages/core/src/runtime/quasivalue.ts
@@ -211,23 +211,37 @@ export function ctor(info: QuasiValueConstructorInfo, ... items: QuasiValue[]):
             }
         }
 
-        switch (def._variant) {
-            case 'or': throw new Error("Cannot use union definition as pattern");
-            case 'and': throw new Error("Cannot use intersection definition as pattern");
-            case 'Pattern': {
-                const p = def.value;
-                switch (p._variant) {
-                    case 'SimplePattern': {
-                        if (ctorArgs.length === 0) {
-                            throw new Error(`Missing argument to ${defNameStr}`);
-                        }
-                        return ctorArgs[0];
+        function qTopPattern(p: Meta.Pattern): QuasiValue {
+            switch (p._variant) {
+                case 'SimplePattern': {
+                    if (ctorArgs.length === 0) {
+                        throw new Error(`Missing argument to ${defNameStr}`);
                     }
-                    case 'CompoundPattern':
-                        return qCompound(p.value);
+                    return ctorArgs[0];
                 }
+                case 'CompoundPattern':
+                    return qCompound(p.value);
             }
         }
+
+        switch (def._variant) {
+            case 'or': {
+                const variant = definfo.variant?.description;
+                if (variant === void 0) {
+                    throw new Error("Cannot use union definition as pattern");
+                }
+                for (const p of [def.pattern0, def.pattern1, ...def.patternN]) {
+                    if (p.variantLabel === variant) {
+                        return qTopPattern(p.pattern);
+                    }
+                }
+                throw new Error(`Unknown variant ${variant} in definition ${defNameStr}`);
+            }
+            case 'and':
+                throw new Error("Cannot use intersection definition as pattern");
+            case 'Pattern':
+                return qTopPattern(def.value);
+        }
     } else if ('quasiValue' in info) {
         return info.quasiValue(... items);
     } else {