Infer existential element/key/value types for collection literals as a fallback

include/swift/AST/DiagnosticsSema.def

@@ -2097,7 +2097,11 @@ ERROR(discard_expr_outside_of_assignment,none,
       ())
 ERROR(inout_expr_outside_of_call,none,
       "'&' can only appear immediately in a call argument list", ())
-
+ERROR(collection_literal_heterogenous,none,
+      "heterogenous collection literal could only be inferred to %0; add"
+      " explicit type annotation if this is intentional", (Type))
+ERROR(collection_literal_empty,none,
+      "empty collection literal requires an explicit type", ())
 
 ERROR(unresolved_member_no_inference,none,
       "reference to member %0 cannot be resolved without a contextual type",

include/swift/AST/Expr.h

@@ -2140,6 +2140,10 @@ class CollectionExpr : public Expr {
 
   Expr *SemanticExpr = nullptr;
 
+  /// True if the type of this collection expr was inferred by the collection
+  /// fallback type, like [Any].
+  bool IsTypeDefaulted = false;
+
 protected:
   CollectionExpr(ExprKind Kind, SourceLoc LBracketLoc,
                  MutableArrayRef<Expr*> Elements,
@@ -2157,6 +2161,9 @@ class CollectionExpr : public Expr {
   void setElement(unsigned i, Expr *E) { Elements[i] = E; }
   unsigned getNumElements() const { return Elements.size(); }
 
+  bool isTypeDefaulted() const { return IsTypeDefaulted; }
+  void setIsTypeDefaulted(bool value = true) { IsTypeDefaulted = value; }
+
   SourceLoc getLBracketLoc() const { return LBracketLoc; }
   SourceLoc getRBracketLoc() const { return RBracketLoc; }
   SourceRange getSourceRange() const {

include/swift/AST/Type.h

@@ -183,7 +183,30 @@ class Type {
 
   /// Get the canonical type, or return null if the type is null.
   CanType getCanonicalTypeOrNull() const; // in Types.h
-
+
+  /// Computes the meet between two types.
+  ///
+  /// The meet of two types is the most specific type that is a supertype of
+  /// both \c type1 and \c type2. For example, given a simple class hierarchy as
+  /// follows:
+  ///
+  /// \code
+  /// class A { }
+  /// class B : A { }
+  /// class C : A { }
+  /// class D { }
+  /// \endcode
+  ///
+  /// The meet of B and C is A, the meet of A and B is A. However, there is no
+  /// meet of D and A (or D and B, or D and C) because there is no common
+  /// superclass. One would have to jump to an existential (e.g., \c AnyObject)
+  /// to find a common type.
+  /// 
+  /// \returns the meet of the two types, if there is a concrete type that can
+  /// express the meet, or a null type if the only meet would be a more-general
+  /// existential type (e.g., \c Any).
+  static Type meet(Type type1, Type type2);
+
 private:
   // Direct comparison is disabled for types, because they may not be canonical.
   void operator==(Type T) const = delete;
@@ -432,6 +455,7 @@ class CanGenericSignature {
     return Signature;
   }
 };
+
 } // end namespace swift
 
 namespace llvm {

lib/AST/CMakeLists.txt

@@ -38,6 +38,7 @@ add_swift_library(swiftAST STATIC
   SourceEntityWalker.cpp
   Substitution.cpp
   Type.cpp
+  TypeJoinMeet.cpp
   TypeRefinementContext.cpp
   TypeRepr.cpp
   TypeWalker.cpp

lib/AST/TypeJoinMeet.cpp

@@ -0,0 +1,70 @@
+//===--- TypeJoinMeet.cpp - Swift Type "Join" and "Meet"  -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file implements the "meet" operation for types (and, eventually,
+//  "join").
+//
+//===----------------------------------------------------------------------===//
+#include "swift/AST/ASTContext.h"
+#include "swift/AST/Type.h"
+#include "swift/AST/Types.h"
+#include "llvm/ADT/SmallPtrSet.h"
+using namespace swift;
+
+Type Type::meet(Type type1, Type type2) {
+  assert(!type1->hasTypeVariable() && !type2->hasTypeVariable() &&
+         "Cannot compute meet of types involving type variables");
+
+  // FIXME: This algorithm is woefully incomplete, and is only currently used
+  // for optimizing away extra exploratory work in the constraint solver. It
+  // should eventually encompass all of the subtyping rules of the language.
+
+  // If the types are equivalent, the meet is obvious.
+  if (type1->isEqual(type2))
+    return type1;
+
+  // If both are class types or opaque types that potentially have superclasses,
+  // find the common superclass.
+  if (type1->mayHaveSuperclass() && type2->mayHaveSuperclass()) {
+    ASTContext &ctx = type1->getASTContext();
+    LazyResolver *resolver = ctx.getLazyResolver();
+
+    /// Walk the superclasses of type1 looking for type2. Record them for our
+    /// second step.
+    llvm::SmallPtrSet<CanType, 8> superclassesOfType1;
+    CanType canType2 = type2->getCanonicalType();
+    for (Type super1 = type1; super1; super1 = super1->getSuperclass(resolver)){
+      CanType canSuper1 = super1->getCanonicalType();
+
+      // If we have found the second type, we're done.
+      if (canSuper1 == canType2) return super1;
+
+      superclassesOfType1.insert(canSuper1);
+    }
+
+    // Look through the superclasses of type2 to determine if any were also
+    // superclasses of type1.
+    for (Type super2 = type2; super2; super2 = super2->getSuperclass(resolver)){
+      CanType canSuper2 = super2->getCanonicalType();
+
+      // If we found the first type, we're done.
+      if (superclassesOfType1.count(canSuper2)) return super2;
+    }
+
+    // There is no common superclass; we're done.
+    return nullptr;
+  }
+
+  // The meet can only be an existential.
+  return nullptr;
+}
+

lib/Sema/CSApply.cpp

@@ -2711,6 +2711,14 @@ namespace {
 
       expr->setSemanticExpr(result);
       expr->setType(arrayTy);
+
+      // If the array element type was defaulted, note that in the expression.
+      auto elementTypeVariable = cs.ArrayElementTypeVariables.find(expr);
+      if (elementTypeVariable != cs.ArrayElementTypeVariables.end()) {
+        if (solution.DefaultedTypeVariables.count(elementTypeVariable->second))
+          expr->setIsTypeDefaulted();
+      }
+
       return expr;
     }
 
@@ -2780,6 +2788,18 @@ namespace {
 
       expr->setSemanticExpr(result);
       expr->setType(dictionaryTy);
+
+      // If the dictionary key or value type was defaulted, note that in the
+      // expression.
+      auto elementTypeVariable = cs.DictionaryElementTypeVariables.find(expr);
+      if (elementTypeVariable != cs.DictionaryElementTypeVariables.end()) {
+        if (solution.DefaultedTypeVariables.count(
+              elementTypeVariable->second.first) ||
+            solution.DefaultedTypeVariables.count(
+              elementTypeVariable->second.second))
+          expr->setIsTypeDefaulted();
+      }
+
       return expr;
     }
 

lib/Sema/CSGen.cpp

@@ -1728,12 +1728,18 @@ namespace {
                            LocatorPathElt::getTupleElement(index++)));
       }
 
+      // The array element type defaults to 'Any'.
+      if (auto elementTypeVar = arrayElementTy->getAs<TypeVariableType>()) {
+        CS.addConstraint(ConstraintKind::Defaultable, arrayElementTy,
+                         tc.Context.TheAnyType, locator);
+        CS.ArrayElementTypeVariables[expr] = elementTypeVar;
+      }
+
       return arrayTy;
     }
 
     static bool isMergeableValueKind(Expr *expr) {
-      return isa<CollectionExpr>(expr) ||
-             isa<StringLiteralExpr>(expr) || isa<IntegerLiteralExpr>(expr) ||
+      return isa<StringLiteralExpr>(expr) || isa<IntegerLiteralExpr>(expr) ||
              isa<FloatLiteralExpr>(expr);
     }
 
@@ -1811,16 +1817,22 @@ namespace {
               auto keyTyvar2 = tty2->getElementTypes()[0]->
                                 getAs<TypeVariableType>();
 
-              mergedKey = mergeRepresentativeEquivalenceClasses(CS, 
+              auto keyExpr1 = cast<TupleExpr>(element1)->getElements()[0];
+              auto keyExpr2 = cast<TupleExpr>(element2)->getElements()[0];
+
+              if (keyExpr1->getKind() == keyExpr2->getKind() &&
+                  isMergeableValueKind(keyExpr1)) {
+                mergedKey = mergeRepresentativeEquivalenceClasses(CS,
                             keyTyvar1, keyTyvar2);
+              }
 
               auto valueTyvar1 = tty1->getElementTypes()[1]->
                                   getAs<TypeVariableType>();
               auto valueTyvar2 = tty2->getElementTypes()[1]->
                                   getAs<TypeVariableType>();
 
-              auto elemExpr1 = dyn_cast<TupleExpr>(element1)->getElements()[1];
-              auto elemExpr2 = dyn_cast<TupleExpr>(element2)->getElements()[1];
+              auto elemExpr1 = cast<TupleExpr>(element1)->getElements()[1];
+              auto elemExpr2 = cast<TupleExpr>(element2)->getElements()[1];
 
               if (elemExpr1->getKind() == elemExpr2->getKind() &&
                 isMergeableValueKind(elemExpr1)) {
@@ -1849,6 +1861,26 @@ namespace {
                              LocatorPathElt::getTupleElement(index++)));
       }
 
+      // The dictionary key type defaults to 'AnyHashable'.
+      auto keyTypeVar = dictionaryKeyTy->getAs<TypeVariableType>();
+      if (keyTypeVar && tc.Context.getAnyHashableDecl()) {
+        auto anyHashable = tc.Context.getAnyHashableDecl();
+        tc.validateDecl(anyHashable);
+        CS.addConstraint(ConstraintKind::Defaultable, dictionaryKeyTy,
+                         anyHashable->getDeclaredInterfaceType(), locator);
+      }
+
+      // The dictionary value type defaults to 'Any'.
+      auto valueTypeVar = dictionaryValueTy->getAs<TypeVariableType>();
+      if (valueTypeVar) {
+        CS.addConstraint(ConstraintKind::Defaultable, dictionaryValueTy,
+                         tc.Context.TheAnyType, locator);
+      }
+
+      // Record key/value type variables.
+      if (keyTypeVar || valueTypeVar)
+        CS.DictionaryElementTypeVariables[expr] = { keyTypeVar, valueTypeVar };
+
       return dictionaryTy;
     }