Sema: Ensure that generic parameters appearing in pack expansion patterns all have the same shape

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -5144,6 +5144,10 @@ ERROR(tuple_duplicate_label,none,
 ERROR(multiple_ellipsis_in_tuple,none,
       "only a single element can be variadic", ())
 
+ERROR(expansion_not_same_shape,none,
+      "variadic expansion %0 requires that %1 and %2 have the same shape",
+      (Type, Type, Type))
+
 ERROR(enum_element_ellipsis,none,
       "variadic enum cases are not supported", ())
 ERROR(unlabeled_parameter_following_variadic_parameter,none,

lib/Sema/TypeCheckType.cpp

@@ -4238,7 +4238,18 @@ NeverNullType TypeResolver::resolvePackExpansionType(PackExpansionTypeRepr *repr
   if (resolution.getStage() == TypeResolutionStage::Interface) {
     auto genericSig = resolution.getGenericSignature();
     auto shapeType = genericSig->getReducedShape(pair.second);
-    return PackExpansionType::get(pair.first, shapeType);
+
+    auto result = PackExpansionType::get(pair.first, shapeType);
+
+    SmallVector<Type, 2> rootParameterPacks;
+    pair.first->getTypeParameterPacks(rootParameterPacks);
+    for (auto type : rootParameterPacks) {
+      if (!genericSig->haveSameShape(type, shapeType)) {
+        ctx.Diags.diagnose(repr->getLoc(), diag::expansion_not_same_shape,
+                           result, shapeType, type);
+      }
+    }
+    return result;
   }
 
   return PackExpansionType::get(pair.first, pair.second);

test/type/pack_expansion.swift

@@ -50,3 +50,17 @@ enum E<T...> {
 
 func withWhereClause<T...>(_ x: T...) where T...: P {}
 // expected-error@-1 {{variadic expansion 'T' cannot appear outside of a function parameter list, function result, tuple element or generic argument list}}
+
+struct Outer<T...> {
+  struct Bad<U...> {
+    typealias Value = ((T, U)...) // expected-error {{variadic expansion '(T, U)...' requires that 'T' and 'U' have the same shape}}
+  }
+
+  struct Good<U...> where ((T, U)...): Any {
+    typealias Value = ((T, U)...)
+  }
+
+  struct AlsoGood<U...> {
+    typealias Value = ((T, E<U... >)...)
+  }
+}