Sema: Diagnose variadic types with more than one type parameter pack

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -5411,6 +5411,9 @@ ERROR(vararg_not_allowed,none,
 ERROR(experimental_type_with_parameter_pack,none,
       "generic types with parameter packs are experimental",
       ())
+ERROR(more_than_one_parameter_pack_in_type,none,
+      "generic type cannot have more than one pack", ())
+
 ERROR(expansion_not_allowed,none,
       "pack expansion %0 can only appear in a function parameter list, "
       "tuple element, or generic argument list", (Type))
@@ -5432,7 +5435,7 @@ ERROR(pack_reference_must_be_in_expansion,none,
       "pack reference %0 requires expansion using keyword 'repeat'",
       (TypeRepr*))
 ERROR(pack_type_requires_keyword_each,none,
-      "pack type %0 must be referenced with 'each'",
+      "type pack %0 must be referenced with 'each'",
       (TypeRepr*))
 ERROR(tuple_duplicate_label,none,
       "cannot create a tuple with a duplicate element label", ())

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -471,14 +471,24 @@ static void checkGenericParams(GenericContext *ownerCtx) {
   if (!genericParams)
     return;
 
+  auto *decl = ownerCtx->getAsDecl();
+  bool isGenericType = isa<GenericTypeDecl>(decl);
+  bool hasPack = false;
+
   for (auto gp : *genericParams) {
     // Diagnose generic types with a parameter packs if VariadicGenerics
     // is not enabled.
-    auto *decl = ownerCtx->getAsDecl();
     auto &ctx = decl->getASTContext();
-    if (gp->isParameterPack() && isa<GenericTypeDecl>(decl) &&
-        !ctx.LangOpts.hasFeature(Feature::VariadicGenerics)) {
-      decl->diagnose(diag::experimental_type_with_parameter_pack);
+    if (gp->isParameterPack() && isGenericType) {
+      if (!ctx.LangOpts.hasFeature(Feature::VariadicGenerics)) {
+        decl->diagnose(diag::experimental_type_with_parameter_pack);
+      }
+
+      if (hasPack) {
+        gp->diagnose(diag::more_than_one_parameter_pack_in_type);
+      }
+
+      hasPack = true;
     }
 
     TypeChecker::checkDeclAttributes(gp);

test/Constraints/pack-expansion-expressions.swift

@@ -103,28 +103,28 @@ func returnEachPackReference<each T>(_ t: repeat each T) -> each T { // expected
   fatalError()
 }
 
-// expected-error@+1 {{pack type 'T' must be referenced with 'each'}}{{63-63=each }}
+// expected-error@+1 {{type pack 'T' must be referenced with 'each'}}{{63-63=each }}
 func returnRepeatTuple<each T>(_ t: repeat each T) -> (repeat T) {
   fatalError()
 }
 
 // expected-error@+2 {{pack reference 'T' requires expansion using keyword 'repeat'}}
-// expected-error@+1 {{pack type 'T' must be referenced with 'each'}}{{55-55=each }}
+// expected-error@+1 {{type pack 'T' must be referenced with 'each'}}{{55-55=each }}
 func parameterAsPackTypeWithoutExpansion<each T>(_ t: T) {
 }
 
 // expected-error@+2 {{pack reference 'T' requires expansion using keyword 'repeat'}}
-// expected-error@+1 {{pack type 'T' must be referenced with 'each'}}{{57-57=each }}
+// expected-error@+1 {{type pack 'T' must be referenced with 'each'}}{{57-57=each }}
 func returnPackReference<each T>(_ t: repeat each T) -> T {
   fatalError()
 }
 
 func packTypeParameterOutsidePackExpansionType<each T>(_ t: T,
   // expected-error@-1 {{pack reference 'T' requires expansion using keyword 'repeat'}}
-  // expected-error@-2 {{pack type 'T' must be referenced with 'each'}}{{61-61=each }}
+  // expected-error@-2 {{type pack 'T' must be referenced with 'each'}}{{61-61=each }}
                                                        _ a: Array<T>) {
   // expected-error@-1 {{pack reference 'T' requires expansion using keyword 'repeat'}}
-  // expected-error@-2 {{pack type 'T' must be referenced with 'each'}}{{67-67=each }}
+  // expected-error@-2 {{type pack 'T' must be referenced with 'each'}}{{67-67=each }}
 }
 
 func expansionOfNonPackType<T>(_ t: repeat each T) {}

test/Generics/variadic_generic_types.swift

@@ -2,6 +2,9 @@
 
 // REQUIRES: asserts
 
+struct MultiplePack<each T, each U> {} // expected-error {{generic type cannot have more than one pack}}
+typealias MultiplePackAlias<each T, each U> = (repeat each T, repeat each U) // expected-error {{generic type cannot have more than one pack}}
+
 func bindAll() {
   struct Bind<each U> {}