[01-02-2019] Opaque result types

include/swift/AST/ASTContext.h

@@ -84,6 +84,7 @@ namespace swift {
   class ModuleLoader;
   class NominalTypeDecl;
   class NormalProtocolConformance;
+  class OpaqueTypeDecl;
   class InheritedProtocolConformance;
   class SelfProtocolConformance;
   class SpecializedProtocolConformance;
@@ -942,6 +943,7 @@ class ASTContext final {
 
   friend TypeBase;
   friend ArchetypeType;
+  friend OpaqueTypeDecl;
 
   /// Provide context-level uniquing for SIL lowered type layouts and boxes.
   friend SILLayout;

include/swift/AST/Decl.h

@@ -157,6 +157,7 @@ enum class DescriptiveDeclKind : uint8_t {
   Module,
   MissingMember,
   Requirement,
+  OpaqueType,
 };
 
 /// Keeps track of stage of circularity checking for the given protocol.
@@ -2294,6 +2295,8 @@ class PoundDiagnosticDecl : public Decl {
     return D->getKind() == DeclKind::PoundDiagnostic;
   }
 };
+
+class OpaqueTypeDecl;
 
 /// ValueDecl - All named decls that are values in the language.  These can
 /// have a type, etc.
@@ -2650,6 +2653,15 @@ class ValueDecl : public Decl {
   /// True if this is a C function that was imported as a member of a type in
   /// Swift.
   bool isImportAsMember() const;
+
+  /// Get the decl for this value's opaque result type, if it has one.
+  OpaqueTypeDecl *getOpaqueResultTypeDecl() const;
+
+  /// Set the opaque return type decl for this decl.
+  ///
+  /// `this` must be of a decl type that supports opaque return types, and
+  /// must not have previously had an opaque result type set.
+  void setOpaqueResultTypeDecl(OpaqueTypeDecl *D);
 };
 
 /// This is a common base class for declarations which declare a type.
@@ -2700,7 +2712,7 @@ class TypeDecl : public ValueDecl {
   }
 };
 
-/// A type declaration that can have generic parameters attached to it.  Because
+/// A type declaration that can have generic parameters attached to it. Because
 /// it has these generic parameters, it is always a DeclContext.
 class GenericTypeDecl : public GenericContext, public TypeDecl {
 public:
@@ -2725,7 +2737,69 @@ class GenericTypeDecl : public GenericContext, public TypeDecl {
   }
 };
 
-
+/// OpaqueTypeDecl - This is a declaration of an opaque type. The opaque type
+/// is formally equivalent to its underlying type, but abstracts it away from
+/// clients of the opaque type, only exposing the type as something conforming
+/// to a given set of constraints.
+///
+/// Currently, opaque types do not normally have an explicit spelling in source
+/// code. One is formed implicitly when a declaration is written with an opaque
+/// result type, as in:
+///
+/// func foo() -> opaque SignedInteger { return 1 }
+///
+/// The declared type is a special kind of ArchetypeType representing the
+/// abstracted underlying type.
+class OpaqueTypeDecl : public GenericTypeDecl {
+  /// The original declaration that "names" the opaque type. Although a specific
+  /// opaque type cannot be explicitly named, oapque types can propagate
+  /// arbitrarily through expressions, so we need to know *which* opaque type is
+  /// propagated.
+  ValueDecl *NamingDecl;
+
+  /// The generic signature of the opaque interface to the type. This is the
+  /// outer generic signature with an added generic parameter representing the
+  /// underlying type.
+  GenericSignature *OpaqueInterfaceGenericSignature;
+
+  /// The generic parameter that represents the underlying type.
+  GenericTypeParamType *UnderlyingInterfaceType;
+
+  /// If known, the underlying type and conformances of the opaque type,
+  /// expressed as a SubstitutionMap for the opaque interface generic signature.
+  /// This maps types in the interface generic signature to the outer generic
+  /// signature of the original declaration.
+  Optional<SubstitutionMap> UnderlyingTypeSubstitutions;
+
+public:
+  OpaqueTypeDecl(ValueDecl *NamingDecl,
+                 GenericParamList *GenericParams,
+                 DeclContext *DC,
+                 GenericSignature *OpaqueInterfaceGenericSignature,
+                 GenericTypeParamType *UnderlyingInterfaceType);
+
+  ValueDecl *getNamingDecl() const { return NamingDecl; }
+
+  GenericSignature *getOpaqueInterfaceGenericSignature() const {
+    return OpaqueInterfaceGenericSignature;
+  }
+
+  GenericTypeParamType *getUnderlyingInterfaceType() const {
+    return UnderlyingInterfaceType;
+  }
+
+  Optional<SubstitutionMap> getUnderlyingTypeSubstitutions() const {
+    return UnderlyingTypeSubstitutions;
+  }
+
+  void setUnderlyingTypeSubstitutions(SubstitutionMap subs) {
+    assert(!UnderlyingTypeSubstitutions.hasValue() && "resetting underlying type?!");
+    UnderlyingTypeSubstitutions = subs;
+  }
+
+  // Opaque type decls are currently always implicit
+  SourceRange getSourceRange() const { return SourceRange(); }
+};
 
 /// TypeAliasDecl - This is a declaration of a typealias, for example:
 ///
@@ -5499,6 +5573,7 @@ class FuncDecl : public AbstractFunctionDecl {
   TypeLoc FnRetType;
 
   OperatorDecl *Operator = nullptr;
+  OpaqueTypeDecl *OpaqueReturn = nullptr;
 
 protected:
   FuncDecl(DeclKind Kind,
@@ -5652,6 +5727,14 @@ class FuncDecl : public AbstractFunctionDecl {
     Operator = o;
   }
 
+  OpaqueTypeDecl *getOpaqueResultTypeDecl() const {
+    return OpaqueReturn;
+  }
+  void setOpaqueResultTypeDecl(OpaqueTypeDecl *decl) {
+    assert(!OpaqueReturn && "already has opaque type decl");
+    OpaqueReturn = decl;
+  }
+
   /// Returns true if the function is forced to be statically dispatched.
   bool hasForcedStaticDispatch() const {
     return Bits.FuncDecl.ForcedStaticDispatch;

include/swift/AST/DeclNodes.def

@@ -152,6 +152,7 @@ ABSTRACT_DECL(Value, Decl)
         NOMINAL_TYPE_DECL(Class, NominalTypeDecl)
         NOMINAL_TYPE_DECL(Protocol, NominalTypeDecl)
         DECL_RANGE(NominalType, Enum, Protocol)
+      GENERIC_VALUE_DECL(OpaqueType, GenericTypeDecl)
       GENERIC_VALUE_DECL(TypeAlias, GenericTypeDecl)
       DECL_RANGE(GenericType, Enum, TypeAlias)
     ABSTRACT_DECL(AbstractTypeParam, TypeDecl)

include/swift/AST/DiagnosticsParse.def

@@ -784,6 +784,10 @@ ERROR(sil_box_expected_r_brace,none,
 ERROR(sil_box_expected_r_angle,none,
       "expected '>' to complete SIL box generic argument list", ())
 
+// Opaque types
+ERROR(opaque_mid_composition,none,
+      "'opaque' should appear at the beginning of a composition", ())
+
 //------------------------------------------------------------------------------
 // MARK: Layout constraint diagnostics
 //------------------------------------------------------------------------------

include/swift/AST/DiagnosticsSema.def

@@ -1503,6 +1503,11 @@ ERROR(noreturn_not_supported,none,
       "'@noreturn' has been removed; functions that never return should have a "
       "return type of 'Never' instead", ())
 
+// Opaque return types
+ERROR(opaque_type_invalid_constraint,none,
+      "an 'opaque' type must specify only 'Any', 'AnyObject', protocols, "
+      "and/or a base class", ())
+
 // Extensions
 ERROR(non_nominal_extension,none,
       "non-nominal type %0 cannot be extended", (Type))
@@ -3204,6 +3209,18 @@ ERROR(trailing_closure_requires_parens,none,
       "trailing closure requires parentheses for disambiguation in this"
       " context", ())
 
+ERROR(opaque_type_no_underlying_type_candidates,none,
+      "function declares an opaque return type, but has no return statements "
+      "in its body from which to infer an underlying type", ())
+ERROR(opaque_type_mismatched_underlying_type_candidates,none,
+      "function declares an opaque return type, but the return statements "
+      "in its body do not have matching underlying types", ())
+NOTE(opaque_type_underlying_type_candidate_here,none,
+     "return statement has underlying type %0", (Type))
+ERROR(opaque_type_self_referential_underlying_type,none,
+      "function opaque return type was inferred as %0, which defines the "
+      "opaque type in terms of itself", (Type))
+
 //------------------------------------------------------------------------------
 // MARK: Type Check Patterns
 //------------------------------------------------------------------------------
@@ -3514,6 +3531,10 @@ ERROR(unsupported_convention,none,
 ERROR(unreferenced_generic_parameter,none,
       "generic parameter '%0' is not used in function signature", (StringRef))
 
+// Opaque types
+ERROR(unsupported_opaque_type,none,
+      "'opaque' types are only implemented for the declared type of properties and subscripts and the return type of functions", ())
+
 // SIL
 ERROR(opened_non_protocol,none,
       "@opened cannot be applied to non-protocol type %0", (Type))

include/swift/AST/Expr.h

@@ -2871,6 +2871,17 @@ class UnevaluatedInstanceExpr : public ImplicitConversionExpr {
   }
 };
 
+/// Use an opaque type to abstract a value of the underlying concrete type.
+class UnderlyingToOpaqueExpr : public ImplicitConversionExpr {
+public:
+  UnderlyingToOpaqueExpr(Expr *subExpr, Type ty)
+    : ImplicitConversionExpr(ExprKind::UnderlyingToOpaque, subExpr, ty) {}
+
+  static bool classof(const Expr *E) {
+    return E->getKind() == ExprKind::UnderlyingToOpaque;
+  }
+};
+
 /// TupleShuffleExpr - This represents a permutation of a tuple value to a new
 /// tuple type.
 ///

include/swift/AST/ExprNodes.def

@@ -168,7 +168,8 @@ ABSTRACT_EXPR(ImplicitConversion, Expr)
   EXPR(PointerToPointer, ImplicitConversionExpr)
   EXPR(ForeignObjectConversion, ImplicitConversionExpr)
   EXPR(UnevaluatedInstance, ImplicitConversionExpr)
-  EXPR_RANGE(ImplicitConversion, Load, UnevaluatedInstance)
+  EXPR(UnderlyingToOpaque, ImplicitConversionExpr)
+  EXPR_RANGE(ImplicitConversion, Load, UnderlyingToOpaque)
 ABSTRACT_EXPR(ExplicitCast, Expr)
   ABSTRACT_EXPR(CheckedCast, ExplicitCastExpr)
     EXPR(ForcedCheckedCast, CheckedCastExpr)

include/swift/AST/GenericSignature.h

@@ -307,6 +307,10 @@ class alignas(1 << TypeAlignInBits) GenericSignature final
   ///   <t_0_0, t_0_1, t_1_0>
   /// then this will return 0 for t_0_0, 1 for t_0_1, and 2 for t_1_0.
   unsigned getGenericParamOrdinal(GenericTypeParamType *param);
+
+  /// Get a substitution map that maps all of the generic signature's
+  /// generic parameters to themselves.
+  SubstitutionMap getIdentitySubstitutionMap() const;
 
   static void Profile(llvm::FoldingSetNodeID &ID,
                       TypeArrayView<GenericTypeParamType> genericParams,

include/swift/AST/ProtocolConformance.h

@@ -323,7 +323,8 @@ class alignas(1 << DeclAlignInBits) ProtocolConformance {
   /// Substitute the conforming type and produce a ProtocolConformance that
   /// applies to the substituted type.
   ProtocolConformance *subst(TypeSubstitutionFn subs,
-                             LookupConformanceFn conformances) const;
+                             LookupConformanceFn conformances,
+                             SubstOptions options = None) const;
 
   void dump() const;
   void dump(llvm::raw_ostream &out, unsigned indent = 0) const;

include/swift/AST/ProtocolConformanceRef.h

@@ -106,8 +106,13 @@ class ProtocolConformanceRef {
   /// Apply a substitution to the conforming type.
   ProtocolConformanceRef subst(Type origType,
                                TypeSubstitutionFn subs,
-                               LookupConformanceFn conformances) const;
+                               LookupConformanceFn conformances,
+                               SubstOptions options = None) const;
 
+  /// Replace opaque types in the conforming type with their underlying types,
+  /// and resolve opaque conformances to their underlying conformances.
+  ProtocolConformanceRef substOpaqueTypesWithUnderlyingTypes(Type origType) const;
+
   /// Given a dependent type (expressed in terms of this conformance's
   /// protocol), follow it from the conforming type.
   Type getAssociatedType(Type origType, Type dependentType,

include/swift/AST/SubstitutionMap.h

@@ -169,7 +169,12 @@ class SubstitutionMap {
   /// Apply a substitution to all replacement types in the map. Does not
   /// change keys.
   SubstitutionMap subst(TypeSubstitutionFn subs,
-                        LookupConformanceFn conformances) const;
+                        LookupConformanceFn conformances,
+                        SubstOptions options = None) const;
+
+  /// Replace opaque types in the replacement types in the map with their
+  /// underlying types. Does not change keys.
+  SubstitutionMap substOpaqueTypesWithUnderlyingTypes() const;
 
   /// Create a substitution map for a protocol conformance.
   static SubstitutionMap

include/swift/AST/Type.h

@@ -53,7 +53,8 @@ class TypeBase;
 class Type;
 class TypeWalker;
 struct ExistentialLayout;
-
+enum class ResilienceExpansion : unsigned;
+
 /// Type substitution mapping from substitutable types to their
 /// replacements.
 typedef llvm::DenseMap<SubstitutableType *, Type> TypeSubstitutionMap;
@@ -147,6 +148,8 @@ enum class SubstFlags {
   AllowLoweredTypes = 0x02,
   /// Map member types to their desugared witness type.
   DesugarMemberTypes = 0x04,
+  /// Substitute types involving opaque type archetypes.
+  SubstituteOpaqueArchetypes = 0x08,
 };
 
 /// Options for performing substitutions into a type.
@@ -312,6 +315,10 @@ class Type {
 
   /// Replace references to substitutable types with error types.
   Type substDependentTypesWithErrorTypes() const;
+
+  /// Replace opaque types with their underlying types when visible at the given
+  /// resilience expansion.
+  Type substOpaqueTypesWithUnderlyingTypes() const;
 
   bool isPrivateStdlibType(bool treatNonBuiltinProtocolsAsPublic = true) const;
 

include/swift/AST/TypeNodes.def

@@ -133,6 +133,7 @@ TYPE(DynamicSelf, Type)
 ABSTRACT_TYPE(Substitutable, Type)
   ABSTRACT_TYPE(Archetype, SubstitutableType)
     ALWAYS_CANONICAL_TYPE(PrimaryArchetype, ArchetypeType)
+    ALWAYS_CANONICAL_TYPE(OpaqueTypeArchetype, ArchetypeType)
     ALWAYS_CANONICAL_TYPE(OpenedArchetype, ArchetypeType)
     ALWAYS_CANONICAL_TYPE(NestedArchetype, ArchetypeType)
     TYPE_RANGE(Archetype, PrimaryArchetype, NestedArchetype)

include/swift/AST/TypeRepr.h

@@ -1010,6 +1010,45 @@ class SILBoxTypeReprField {
   TypeRepr *getFieldType() const { return FieldTypeAndMutable.getPointer(); }
   bool isMutable() const { return FieldTypeAndMutable.getInt(); }
 };
+
+/// TypeRepr for opaque return types.
+///
+/// This can occur in the return position of a function declaration, or the
+/// top-level type of a property, to specify that the concrete return type
+/// should be abstracted from callers, given a set of generic constraints that
+/// the concrete return type satisfies:
+///
+/// func foo() -> opaque Collection { return [1,2,3] }
+/// var bar: opaque SignedInteger = 1
+///
+/// It is currently illegal for this to appear in any other position.
+class OpaqueReturnTypeRepr : public TypeRepr {
+  /// The type repr for the immediate constraints on the opaque type.
+  /// In valid code this must resolve to a class, protocol, or composition type.
+  TypeRepr *Constraint;
+  SourceLoc OpaqueLoc;
+
+public:
+  OpaqueReturnTypeRepr(SourceLoc opaqueLoc, TypeRepr *constraint)
+    : TypeRepr(TypeReprKind::OpaqueReturn), Constraint(constraint),
+      OpaqueLoc(opaqueLoc)
+  {}
+
+  TypeRepr *getConstraint() const { return Constraint; }
+  SourceLoc getOpaqueLoc() const { return OpaqueLoc; }
+
+  static bool classof(const TypeRepr *T) {
+    return T->getKind() == TypeReprKind::OpaqueReturn;
+  }
+  static bool classof(const OpaqueReturnTypeRepr *T) { return true; }
+
+private:
+  SourceLoc getStartLocImpl() const { return OpaqueLoc; }
+  SourceLoc getEndLocImpl() const { return Constraint->getEndLoc(); }
+  SourceLoc getLocImpl() const { return OpaqueLoc; }
+  void printImpl(ASTPrinter &Printer, const PrintOptions &Opts) const;
+  friend class TypeRepr;
+};
 
 /// SIL-only TypeRepr for box types.
 ///
@@ -1108,6 +1147,7 @@ inline bool TypeRepr::isSimple() const {
   case TypeReprKind::Function:
   case TypeReprKind::InOut:
   case TypeReprKind::Composition:
+  case TypeReprKind::OpaqueReturn:
     return false;
   case TypeReprKind::SimpleIdent:
   case TypeReprKind::GenericIdent:

include/swift/AST/TypeReprNodes.def

@@ -53,6 +53,7 @@ TYPEREPR(Tuple, TypeRepr)
 TYPEREPR(Composition, TypeRepr)
 TYPEREPR(Metatype, TypeRepr)
 TYPEREPR(Protocol, TypeRepr)
+TYPEREPR(OpaqueReturn, TypeRepr)
 ABSTRACT_TYPEREPR(Specifier, TypeRepr)
   TYPEREPR(InOut, SpecifierTypeRepr)
   TYPEREPR(Shared, SpecifierTypeRepr)