[ConstraintSystem] Adjust the way pack parameters are opened

Although variadic type is declared without `repeat` each of
its generic arguments is supposed to be a PackType which is
modeled in the interface type as `Pack{repeat ...}`. When
reference to such a type is opened by the constraint solver
we need to drop the Pack{repeat ...} structure because the
type variable would represent a pack type so `S<each T>`
is opened as `S<$T0>` instead of `S<Pack{repeat $T0}>`.

Resolves: #66095

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -837,7 +837,21 @@ Type ConstraintSystem::openUnboundGenericType(GenericTypeDecl *decl,
     result = DC->mapTypeIntoContext(result);
   }
 
-  return result.transform([&](Type type) {
+  return result.transform([&](Type type) -> Type {
+    // Although generic parameters are declared with just `each`
+    // their interface types introduce a pack expansion which
+    // means that the solver has to extact generic argument type
+    // variable from Pack{repeat ...} and drop that structure to
+    // make sure that generic argument gets inferred to a pack type.
+    if (auto *packTy = type->getAs<PackType>()) {
+      assert(packTy->getNumElements() == 1);
+      auto *expansion = packTy->getElementType(0)->castTo<PackExpansionType>();
+      auto *typeVar = expansion->getPatternType()->castTo<TypeVariableType>();
+      assert(typeVar->getImpl().getGenericParameter() &&
+             typeVar->getImpl().canBindToPack());
+      return typeVar;
+    }
+
     if (auto *expansion = dyn_cast<PackExpansionType>(type.getPointer()))
       return openPackExpansionType(expansion, replacements, locator);
     return type;
@@ -990,6 +1004,15 @@ Type ConstraintSystem::openType(Type type, OpenedTypeMap &replacements,
         }
       }
 
+      // While opening variadic generic types that appear in other types
+      // we need to extract generic parameter from Pack{repeat ...} structure
+      // that gets introduced by the interface type, see
+      // \c openUnboundGenericType for more details.
+      if (auto *packTy = type->getAs<PackType>()) {
+        if (auto expansion = packTy->unwrapSingletonPackExpansion())
+          type = expansion->getPatternType();
+      }
+
       if (auto *expansion = type->getAs<PackExpansionType>()) {
         return openPackExpansionType(expansion, replacements, locator);
       }

test/Constraints/variadic_generic_types.swift

@@ -29,3 +29,30 @@ struct MissingMemberError<each T> {
     // expected-error@-1 {{value of type 'MissingMemberError<repeat each T>' has no member 'doesNotExist'}}
   }
 }
+
+// https://github.com/apple/swift/issues/66095
+do {
+  struct Test<each S> {
+    init(_ s: repeat each S) {}
+  }
+
+  func test1<each T>(_ v: repeat each T) -> Test<repeat each T> {
+    return Test(repeat each v) // Ok
+  }
+
+  func test2<each T>(_ v: repeat each T) -> Test<repeat each T> {
+    return Test<repeat each T>(repeat each v) // Ok
+  }
+
+  func test3<each T>(_ v: repeat each T) -> Test<String, repeat each T, Int> {
+    return Test("a", repeat each v, 42) // Ok
+  }
+
+  func test4<each T>(_ v: repeat each T) -> Test<repeat each T, String, Int> {
+    return Test<repeat each T, String, Int>(repeat each v, "a", 42) // Ok
+  }
+
+  func test5<each T>(_ v: repeat each T) -> Test<String, Int, repeat each T> {
+    return Test<String, Int, repeat each T>("a", 42, repeat each v) // Ok
+  }
+}