Rename --enable-experimental-opaque-return-types and gate structural … (#38780)

* Rename --enable-experimental-opaque-return-types and gate structural opaque types with a flag

* Separate out structural opaque type result builder tests

Author: willtunnels

include/swift/AST/DiagnosticsSema.def

@@ -1861,6 +1861,9 @@ WARNING(spi_attribute_on_import_of_public_module,none,
         (DeclName, StringRef))
 
 // Opaque return types
+ERROR(structural_opaque_types_are_experimental,none,
+      "'opaque' types cannot be nested inside other types; "
+      "structural 'opaque' types are an experimental feature", ())
 ERROR(opaque_type_invalid_constraint,none,
       "an 'opaque' type must specify only 'Any', 'AnyObject', protocols, "
       "and/or a base class", ())

include/swift/Basic/LangOptions.h

@@ -298,10 +298,13 @@ namespace swift {
     /// Enable experimental concurrency model.
     bool EnableExperimentalConcurrency = false;
 
-    /// Enable experimental support for additional opaque return type features,
-    /// i.e. named opaque return types (with 'where' clause support), and opaque
-    /// types in nested position within the function return type.
-    bool EnableExperimentalOpaqueReturnTypes = false;
+    /// Enable experimental support for named opaque result types, e.g.
+    /// `func f() -> <T> T`.
+    bool EnableExperimentalNamedOpaqueTypes = false;
+
+    /// Enable experimental support for structural opaque result types, e.g.
+    /// `func f() -> (some P)?`.
+    bool EnableExperimentalStructuralOpaqueTypes = false;
 
     /// Enable experimental flow-sensitive concurrent captures.
     bool EnableExperimentalFlowSensitiveConcurrentCaptures = false;

include/swift/Option/FrontendOptions.td

@@ -431,9 +431,13 @@ def enable_experimental_static_assert :
   Flag<["-"], "enable-experimental-static-assert">,
   HelpText<"Enable experimental #assert">;
 
-def enable_experimental_opaque_return_types :
-  Flag<["-"], "enable-experimental-opaque-return-types">,
-  HelpText<"Enable experimental extensions to opaque return type support">;
+def enable_experimental_named_opaque_types :
+  Flag<["-"], "enable-experimental-named-opaque-types">,
+  HelpText<"Enable experimental support for named opaque result types">;
+
+def enable_experimental_structural_opaque_types :
+  Flag<["-"], "enable-experimental-structural-opaque-types">,
+  HelpText<"Enable experimental support for structural opaque result types">;
 
 def enable_deserialization_recovery :
   Flag<["-"], "enable-deserialization-recovery">,

lib/Frontend/CompilerInvocation.cpp

@@ -422,8 +422,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
 
-  Opts.EnableExperimentalOpaqueReturnTypes |=
-      Args.hasArg(OPT_enable_experimental_opaque_return_types);
+  Opts.EnableExperimentalNamedOpaqueTypes |=
+      Args.hasArg(OPT_enable_experimental_named_opaque_types);
+
+  Opts.EnableExperimentalStructuralOpaqueTypes |=
+      Args.hasArg(OPT_enable_experimental_structural_opaque_types);
 
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);

lib/Parse/ParseType.cpp

@@ -550,7 +550,7 @@ ParserResult<TypeRepr> Parser::parseType(
 
 ParserResult<TypeRepr> Parser::parseTypeWithOpaqueParams(Diag<> MessageID) {
   GenericParamList *genericParams = nullptr;
-  if (Context.LangOpts.EnableExperimentalOpaqueReturnTypes) {
+  if (Context.LangOpts.EnableExperimentalNamedOpaqueTypes) {
     auto result = maybeParseGenericParams();
     genericParams = result.getPtrOrNull();
     if (result.hasCodeCompletion())

lib/Sema/TypeCheckGeneric.cpp

@@ -116,6 +116,14 @@ OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
   auto *dc = originatingDecl->getInnermostDeclContext();
   auto &ctx = dc->getASTContext();
 
+  // Support for structural opaque result types is hidden behind a compiler flag
+  // until the proposal gets approved.
+  if (!ctx.LangOpts.EnableExperimentalStructuralOpaqueTypes &&
+      !isa<OpaqueReturnTypeRepr>(repr)) {
+    ctx.Diags.diagnose(repr->getLoc(), diag::structural_opaque_types_are_experimental);
+    return nullptr;
+  }
+
   // Protocol requirements can't have opaque return types.
   //
   // TODO: Maybe one day we could treat this as sugar for an associated type.
@@ -136,8 +144,8 @@ OpaqueResultTypeRequest::evaluate(Evaluator &evaluator,
       out << "associatedtype " << placeholder << ": ";
       // FIXME [OPAQUE SUPPORT]: to produce the right associate type for the
       // replacement in general, we would need to recurse into the type repr and
-      // replace every `OpaqueReturnType` with its 'base'. Things get trickier
-      // when we allow named opaque return types.
+      // replace every `OpaqueReturnType` with its 'constraint'. Things get
+      // trickier when we allow named opaque return types.
       if (isa<OpaqueReturnTypeRepr>(repr)) {
         cast<OpaqueReturnTypeRepr>(repr)->getConstraint()->print(out);
       } else {

test/Constraints/result_builder_infer.swift

@@ -66,13 +66,6 @@ struct TupleMe: Tupled {
   }
 }
 
-struct TupleMeStructural: Tupled {
-  var tuple: (some Any, Int) {
-    "hello"
-    0
-  }
-}
-
 // Witness is separated from the context declaring conformance, so don't infer
 // the result builder.
 struct DoNotTupleMe {

test/Constraints/result_builder_opaque_result_structural.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-structural-opaque-types -disable-availability-checking
+
+@resultBuilder
+struct TupleBuilder {
+  static func buildBlock<T1, T2>(_ t1: T1, _ t2: T2) -> (T1, T2) {
+    return (t1, t2)
+  }
+}
+
+protocol Tupled {
+  associatedtype TupleType
+  
+  @TupleBuilder var tuple: TupleType { get }
+}
+
+struct TupleMeStructural: Tupled {
+  var tuple: (some Any, Int) {
+    "hello"
+    0
+  }
+}

test/IDE/print_ast_named_opaque_return.swift

@@ -5,7 +5,7 @@ func f1() -> <T> Int {
 func f2() -> <T: SignedInteger, U: SignedInteger> Int {
 }
 
-// RUN: %target-swift-ide-test -print-ast-typechecked -enable-experimental-opaque-return-types -source-filename %s | %FileCheck %s -check-prefix=CHECK1
+// RUN: %target-swift-ide-test -print-ast-typechecked -enable-experimental-named-opaque-types -source-filename %s | %FileCheck %s -check-prefix=CHECK1
 // CHECK1: {{^}}func f0() {{{$}}
 // CHECK1: {{^}}}{{$}}
 // CHECK1: {{^}}func f1() -> <T> Int {{{$}}

test/type/opaque.swift

@@ -75,81 +75,11 @@ struct Test {
 
 let zingle = {() -> some P in 1 } // expected-error{{'some' types are only implemented}}
 
-// Structural positions
 
-struct S1<T> {
-  var x: T
-}
-struct S2<T, U> {
-  var x: T
-  var y: U
-}
-struct R1<T: P> {
-  var x: T
-}
-struct R2<T: P, U: Q> {
-  var x: T
-  var y: U
-}
-
-// TODO: cases that we should support, but don't yet:
-//
-// WARNING: using '!' is not allowed here; treating this as '?' instead
-// func asUnwrappedOptionalBase() -> (some P)! { return 1 }
-//
-// ERROR: generic parameter 'τ_0_0' could not be inferred
-// func asHOFRetRet() -> () -> some P { return { 1 } }
-//
-// ERROR: function declares an opaque return type, but has no return statements in its body from which to infer an underlying type
-// func asHOFRetArg() -> (some P) -> () { return { (x: String) -> () in } }
-//
-// ERROR: 'some' types are only implemented for the declared type of properties and subscripts and the return type of functions
-// let x = { () -> some P in return ConcreteP() }
-
-func twoOpaqueTypes() -> (some P, some P) { return (1, 2) } // expected-error{{only one 'opaque' type is supported}}
-func asTupleElemBad() -> (P, some Q) { return (1, C()) } // expected-note{{opaque return type declared here}} expected-error{{requires that 'C' conform to 'Q'}}
-
-func asTupleElem() -> (P, some Q) { return (1, 2) }
-func asArrayElem() -> [some P] { return [1] }
-func asOptionalBase() -> (some P)? { return 1 }
-
-let asTupleElemLet: (P, some Q) = (1, 2)
-let asArrayElemLet: [some P] = [1]
-let asOptionalBaseLet: (some P)? = 1
-
-func asUnconstrainedGeneric1() -> S1<some P> { return S1(x: 1) }
-func asUnconstrainedGeneric2() -> S2<P, some Q> { return S2(x: 1, y: 2) }
-func asConstrainedGeneric1() -> R1<some P> { return R1(x: 1) }
-func asConstrainedGeneric2() -> R2<Int, some Q> { return R2(x: 1, y: 2) }
-func asNestedGenericDirect() -> S1<S1<some P>> { return S1(x: S1(x: 1)) }
-func asNestedGenericIndirect() -> S1<S1<(Int, some P)>> { return S1(x: S1(x: (1, 2))) }
-
-let asUnconstrainedGeneric2Let: S2<P, some Q> = S2(x: 1, y: 2)
-let asNestedGenericIndirectLet: S1<S1<(Int, some P)>> = S1(x: S1(x: (1, 2)))
-
-// Tests an interesting SILGen case. For the underlying opaque type, we have to
-// use the generic calling convention for closures.
-func funcToAnyOpaqueCoercion() -> S1<some Any> {
-  let f: () -> () = {}
-  return S1(x: f)
-}
-
-// TODO: We should give better error messages here. The opaque types have
-// underlying types 'Int' and 'String', but the return statments have underlying
-// types '(Int, Int)' and '(String, Int)'.
-func structuralMismatchedReturnTypes(_ x: Bool, _ y: Int, _ z: String) -> (some P, Int) { // expected-error{{do not have matching underlying types}}
-  if x {
-    return (y, 1) // expected-note{{return statement has underlying type 'Int'}}
-  } else {
-    return (z, 1) // expected-note{{return statement has underlying type 'String'}}
-  }
-}
-
-func structuralMemberLookupBad() {
-  var tup: (some P, Int) = (1, 1)
-  tup.0.paul();
-  tup.0.d(); // expected-error{{value of type 'some P' has no member 'd'}}
-}
+// Support for structural opaque result types is hidden behind a compiler flag
+// until the proposal gets approved.
+func twoOpaqueTypes() -> (some P, some P) { return (1, 2) } // expected-error{{'opaque' types cannot be nested inside other types}}
+func asArrayElem() -> (some P)! { return [1] } // expected-error{{'opaque' types cannot be nested inside other types}}
 
 // Invalid positions