Sema: Allow T == NonCopyableOrEscapable same-type constraints without a redundant T: ~Copyable.

lib/AST/CMakeLists.txt

@@ -95,6 +95,7 @@ add_swift_host_library(swiftAST STATIC
   RawComment.cpp
   Requirement.cpp
   RequirementEnvironment.cpp
+  RequirementMachine/ApplyInverses.cpp
   RequirementMachine/ConcreteContraction.cpp
   RequirementMachine/ConcreteTypeWitness.cpp
   RequirementMachine/Diagnostics.cpp

lib/AST/RequirementMachine/ApplyInverses.cpp

@@ -0,0 +1,268 @@
+//===--- ApplyInverses.cpp - Resolve `~Protocol` anti-constraints ---------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// The `applyInverses` function takes the syntactic representation of a generic
+// signature and applies implicit default constraints on generic parameters for
+// core type capabilities like `Copyable` and `Escapable`. In doing so, it
+// looks for explicit constraint suppression "requirements" like `T: ~Copyable`
+// or same-type constraints that would contradict the implicit requirements and
+// filters out unwanted default requirements.
+//
+//===----------------------------------------------------------------------===//
+
+#include "RequirementLowering.h"
+#include "swift/AST/ASTContext.h"
+#include "swift/AST/ConformanceLookup.h"
+#include "swift/AST/Decl.h"
+#include "swift/AST/DiagnosticsSema.h"
+#include "swift/AST/Requirement.h"
+#include "swift/AST/RequirementSignature.h"
+#include "swift/AST/TypeCheckRequests.h"
+#include "swift/AST/TypeMatcher.h"
+#include "swift/AST/TypeRepr.h"
+#include "swift/Basic/Assertions.h"
+#include "swift/Basic/Defer.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/SetVector.h"
+#include "Diagnostics.h"
+#include "RewriteContext.h"
+#include "NameLookup.h"
+
+using namespace swift;
+using namespace rewriting;
+
+void swift::rewriting::applyInverses(
+    ASTContext &ctx,
+    ArrayRef<Type> gps,
+    ArrayRef<InverseRequirement> inverseList,
+    ArrayRef<StructuralRequirement> explicitRequirements,
+    SmallVectorImpl<StructuralRequirement> &result,
+    SmallVectorImpl<RequirementError> &errors) {
+
+  // Are there even any inverses or same-type requirements to validate?
+  if (inverseList.empty() && explicitRequirements.empty()) {
+    return;
+  }
+
+  const bool allowInverseOnAssocType =
+      ctx.LangOpts.hasFeature(Feature::SuppressedAssociatedTypes);
+
+  llvm::DenseMap<CanType, CanType> representativeGPs;
+
+  // Start with an identity mapping.
+  for (auto gp : gps) {
+    auto canGP = gp->getCanonicalType();
+    representativeGPs.insert({canGP, canGP});
+  }
+  bool hadSameTypeConstraintInScope = false;
+
+  // Return the in-scope generic parameter that represents the equivalence class
+  // for `gp`, or return null if the parameter is constrained out of scope.
+  auto representativeGPFor = [&](CanType gp) -> CanType {
+    while (true) {
+      auto found = representativeGPs.find(gp);
+      if (found == representativeGPs.end()) {
+        return CanType();
+      }
+      if (found->second == CanType()) {
+        return CanType();
+      }
+
+      if (found->second == gp) {
+        return gp;
+      }
+
+      gp = found->second;
+    }
+  };
+
+  // Look for same-type constraints that equate multiple generic parameters
+  // within the scope so we can treat the equivalence class as a unit.
+  for (auto &explicitReqt : explicitRequirements) {
+    if (explicitReqt.req.getKind() != RequirementKind::SameType) {
+      continue;
+    }
+
+    // If one end of the same-type requirement is in scope, and the other is
+    // a concrete type or out-of-scope generic parameter, then the other
+    // parameter is also effectively out of scope.
+    auto firstTy = explicitReqt.req.getFirstType()->getCanonicalType();
+    auto secondTy = explicitReqt.req.getSecondType()->getCanonicalType();
+    if (!representativeGPs.count(firstTy)
+        && !representativeGPs.count(secondTy)) {
+      // Same type constraint doesn't involve any in-scope generic parameters.
+      continue;
+    }
+
+    CanType typeInScope;
+    CanType typeOutOfScope;
+
+    if (representativeGPs.count(firstTy)
+        && !representativeGPs.count(secondTy)){
+      // First type is constrained out of scope.
+      typeInScope = firstTy;
+      typeOutOfScope = secondTy;
+    } else if (!representativeGPs.count(firstTy)
+               && representativeGPs.count(secondTy)) {
+      // Second type is constrained out of scope.
+      typeInScope = secondTy;
+      typeOutOfScope = firstTy;
+    } else {
+      // Otherwise, both ends of the same-type constraint are in scope.
+      // Fold the lexicographically-greater parameter with the lesser.
+      auto firstGP = cast<GenericTypeParamType>(firstTy);
+      auto secondGP = cast<GenericTypeParamType>(secondTy);
+
+      if (firstGP == secondGP) {
+        // `T == T` has no effect.
+        continue;
+      }
+
+      if (firstGP->getDepth() > secondGP->getDepth()
+          || (firstGP->getDepth() == secondGP->getDepth()
+              && firstGP->getIndex() > secondGP->getIndex())) {
+        std::swap(firstGP, secondGP);
+      }
+
+      hadSameTypeConstraintInScope = true;
+      representativeGPs.insert_or_assign(secondGP, representativeGPFor(firstGP));
+      continue;
+    }
+
+    // If the out-of-scope type is another type parameter or associated type,
+    // then ignore this same-type constraint and allow defaulting to continue.
+    //
+    // It would probably have been more principled to suppress any defaulting
+    // in this case, but this behavior shipped in Swift 6.0 and 6.1, so we
+    // need to maintain source compatibility.
+    if (typeOutOfScope->isTypeParameter()) {
+      continue;
+    }
+
+    // If the out-of-scope type contains errors, then similarly, ignore the
+    // same type constraint. Any additional diagnostics arising from the type
+    // parameter being left ~Copyable or ~Escapable might be misleading if the
+    // corrected code is attempting to refer to a Copyable or Escapable type.
+    if (typeOutOfScope->hasError()) {
+      continue;
+    }
+
+    representativeGPs.insert_or_assign(representativeGPFor(typeInScope),
+                                       CanType());
+    hadSameTypeConstraintInScope = true;
+  }
+
+  // Summarize the inverses and diagnose ones that are incorrect.
+  llvm::DenseMap<CanType, InvertibleProtocolSet> inverses;
+  for (auto inverse : inverseList) {
+    auto canSubject = inverse.subject->getCanonicalType();
+
+    // Inverses on associated types are experimental.
+    if (!allowInverseOnAssocType && canSubject->is<DependentMemberType>()) {
+      // Special exception: allow if we're building the stdlib.
+      if (!ctx.MainModule->isStdlibModule()) {
+        errors.push_back(RequirementError::forInvalidInverseSubject(inverse));
+        continue;
+      }
+    }
+
+    // Noncopyable checking support for parameter packs is not implemented yet.
+    if (canSubject->isParameterPack()) {
+      errors.push_back(RequirementError::forInvalidInverseSubject(inverse));
+      continue;
+    }
+
+    // Value generics never have inverse requirements (or the positive thereof).
+    if (canSubject->isValueParameter()) {
+      continue;
+    }
+
+    // If the inverse is on a subject that wasn't permitted by our caller, then
+    // remove and diagnose as an error. This can happen when an inner context
+    // has a constraint on some outer generic parameter, e.g.,
+    //
+    //     protocol P {
+    //       func f() where Self: ~Copyable
+    //     }
+    //
+    if (representativeGPs.find(canSubject) == representativeGPs.end()) {
+      errors.push_back(
+          RequirementError::forInvalidInverseOuterSubject(inverse));
+      continue;
+    }
+
+    auto representativeSubject = representativeGPFor(canSubject);
+
+    // If the subject is in scope, but same-type constrained to a type out of
+    // scope, then allow inverses to be stated even though they are redundant.
+    // This is because older versions of Swift not only accepted but required
+    // `extension Foo where T == NonCopyableType, T: ~Copyable {}` to be
+    // written, so we need to continue to accept that formulation for source
+    // compatibility.
+    if (!representativeSubject) {
+      continue;
+    }
+
+    auto state = inverses.getOrInsertDefault(representativeSubject);
+
+    // Check if this inverse has already been seen.
+    auto inverseKind = inverse.getKind();
+    if (state.contains(inverseKind))
+      continue;
+
+    state.insert(inverseKind);
+    inverses[representativeSubject] = state;
+  }
+
+  // Fast-path: if there are no valid inverses or same-type constraints, then
+  // there are no requirements to be removed.
+  if (inverses.empty() && !hadSameTypeConstraintInScope) {
+    return;
+  }
+
+  // Scan the structural requirements and cancel out any inferred requirements
+  // based on the inverses we saw.
+  result.erase(llvm::remove_if(result, [&](StructuralRequirement structReq) {
+    auto req = structReq.req;
+
+    if (req.getKind() != RequirementKind::Conformance)
+      return false;
+
+    // Only consider requirements involving an invertible protocol.
+    auto proto = req.getProtocolDecl()->getInvertibleProtocolKind();
+    if (!proto) {
+      return false;
+    }
+
+    // See if this subject is in-scope.
+    auto subject = req.getFirstType()->getCanonicalType();
+    auto representative = representativeGPs.find(subject);
+    if (representative == representativeGPs.end()) {
+      return false;
+    }
+
+    // If this type is same-type constrained into another equivalence class,
+    // then it doesn't need its own defaulted requirements.
+    if (representative->second != subject) {
+      return true;
+    }
+
+    // We now have found the inferred constraint 'Subject : Proto'.
+    // So, remove it if we have recorded a 'Subject : ~Proto'.
+    auto foundInverses = inverses.find(subject);
+    if (foundInverses == inverses.end()) {
+      return false;
+    }
+    auto recordedInverses = foundInverses->getSecond();
+    return recordedInverses.contains(*proto);
+  }), result.end());
+}