[Sema] Diagnose deprecated default implementations in the witness checker.

If a protocol provides a deprecated default implementation for a requirement
that is not deprecated, the compiler should emit a warning so the programmer
can provide an explicit implementation of the requirement. This is helpful
for staging in new protocol requirements that should be implemented in
conforming types.

Author: hborla

include/swift/AST/Attr.h

@@ -2705,6 +2705,10 @@ class DeclAttributes {
     return getUnavailable(ctx) != nullptr;
   }
 
+  bool isDeprecated(const ASTContext &ctx) const {
+    return getDeprecated(ctx) != nullptr;
+  }
+
   /// Determine whether there is a swiftVersionSpecific attribute that's
   /// unavailable relative to the provided language version.
   bool

include/swift/AST/DiagnosticsSema.def

@@ -3137,6 +3137,11 @@ ERROR(witness_unavailable,none,
         "unavailable %kind0 was used to satisfy a requirement of protocol %1%select{|: %2}2",
         (const ValueDecl *, Identifier, StringRef))
 
+WARNING(witness_deprecated,none,
+        "deprecated default implementation is used to satisfy %kind0 required by "
+        "protocol %1%select{|: %2}2",
+        (const ValueDecl *, Identifier, StringRef))
+
 ERROR(redundant_conformance,none,
       "redundant conformance of %0 to protocol %1", (Type, Identifier))
 ERROR(redundant_conformance_conditional,none,

include/swift/AST/RequirementMatch.h

@@ -239,6 +239,10 @@ enum class CheckKind : unsigned {
 
   /// The witness itself is inaccessible.
   WitnessUnavailable,
+
+  /// The witness is a deprecated default implementation provided by the
+  /// protocol.
+  DefaultWitnessDeprecated,
 };
 /// Describes the suitability of the chosen witness for
 /// the requirement.
@@ -464,4 +468,4 @@ struct RequirementMatch {
 
 }
 
-#endif // SWIFT_AST_REQUIREMENTMATCH_H
+#endif // SWIFT_AST_REQUIREMENTMATCH_H

lib/Sema/TypeCheckProtocol.cpp

@@ -1740,6 +1740,8 @@ checkWitnessAvailability(ValueDecl *requirement,
 
 RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
                                               const RequirementMatch &match) {
+  auto &ctx = getASTContext();
+
   if (!match.OptionalAdjustments.empty())
     return CheckKind::OptionalityConflict;
 
@@ -1769,7 +1771,7 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
     return RequirementCheck(CheckKind::Availability, requiredAvailability);
   }
 
-  if (requirement->getAttrs().isUnavailable(getASTContext()) &&
+  if (requirement->getAttrs().isUnavailable(ctx) &&
       match.Witness->getDeclContext() == DC) {
     return RequirementCheck(CheckKind::Unavailable);
   }
@@ -1792,11 +1794,11 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
     }
   }
 
-  if (match.Witness->getAttrs().isUnavailable(getASTContext()) &&
-      !requirement->getAttrs().isUnavailable(getASTContext())) {
+  if (match.Witness->getAttrs().isUnavailable(ctx) &&
+      !requirement->getAttrs().isUnavailable(ctx)) {
     auto nominalOrExtensionIsUnavailable = [&]() {
       if (auto extension = dyn_cast<ExtensionDecl>(DC)) {
-        if (extension->getAttrs().isUnavailable(getASTContext()))
+        if (extension->getAttrs().isUnavailable(ctx))
           return true;
       }
 
@@ -1813,6 +1815,20 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
       return CheckKind::WitnessUnavailable;
   }
 
+  // Warn about deprecated default implementations if the requirement is
+  // not deprecated, and the conformance is not deprecated.
+  bool isDefaultWitness = false;
+  if (auto *nominal = match.Witness->getDeclContext()->getSelfNominalTypeDecl())
+    isDefaultWitness = isa<ProtocolDecl>(nominal);
+  if (isDefaultWitness &&
+      match.Witness->getAttrs().isDeprecated(ctx) &&
+      !requirement->getAttrs().isDeprecated(ctx)) {
+    auto conformanceContext = ExportContext::forConformance(DC, Proto);
+    if (!conformanceContext.isDeprecated()) {
+      return RequirementCheck(CheckKind::DefaultWitnessDeprecated);
+    }
+  }
+
   return CheckKind::Success;
 }
 
@@ -4374,6 +4390,25 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
                          requirement->getName());
         });
       break;
+
+      case CheckKind::DefaultWitnessDeprecated:
+        getASTContext().addDelayedConformanceDiag(
+          Conformance, /*isError=*/false,
+          [witness, requirement](NormalProtocolConformance *conformance) {
+            auto &ctx = witness->getASTContext();
+            auto &diags = ctx.Diags;
+            SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);
+            auto *attr = witness->getAttrs().getDeprecated(ctx);
+            EncodedDiagnosticMessage EncodedMessage(attr->Message);
+            diags.diagnose(diagLoc, diag::witness_deprecated,
+                           witness, conformance->getProtocol()->getName(),
+                           EncodedMessage.Message);
+            emitDeclaredHereIfNeeded(diags, diagLoc, witness);
+            diags.diagnose(requirement, diag::kind_declname_declared_here,
+                           DescriptiveDeclKind::Requirement,
+                           requirement->getName());
+          });
+        break;
     }
 
     if (auto *classDecl = DC->getSelfClassDecl()) {

test/decl/protocol/req/deprecated.swift

@@ -0,0 +1,60 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol DeprecatedRequirement {
+  @available(*, deprecated)
+  func f()
+}
+
+extension DeprecatedRequirement {
+  @available(*, deprecated)
+  func f() {}
+}
+
+// No warning if both the requirement and the default implementation are deprecated
+struct S1: DeprecatedRequirement {}
+
+protocol DeprecatedDefault {
+  func f() // expected-note {{requirement 'f()' declared here}}
+}
+
+extension DeprecatedDefault {
+  @available(*, deprecated)
+  func f() {}  // expected-note {{'f()' declared here}}
+}
+
+// expected-warning@+1 {{deprecated default implementation is used to satisfy instance method 'f()' required by protocol 'DeprecatedDefault'}}
+struct S2: DeprecatedDefault {}
+
+// No warning if the conformance itself is deprecated
+@available(*, deprecated)
+struct S3: DeprecatedDefault {
+}
+
+struct S4: DeprecatedDefault {
+  func f() {}
+}
+
+struct S5 {}
+
+// No warning if the conformance itself is deprecated
+@available(*, deprecated)
+extension S5: DeprecatedDefault {}
+
+@available(*, deprecated)
+enum UnavailableEnum {
+  struct Nested: DeprecatedDefault {}
+}
+
+// Include message string from @available attribute if provided
+protocol DeprecatedDefaultWithMessage {
+  func f() // expected-note {{requirement 'f()' declared here}}
+}
+
+extension DeprecatedDefaultWithMessage {
+  @available(*, deprecated, message: "write it yourself")
+  func f() {} // expected-note {{'f()' declared here}}
+}
+
+
+// expected-warning@+1 {{deprecated default implementation is used to satisfy instance method 'f()' required by protocol 'DeprecatedDefaultWithMessage': write it yourself}}
+struct S6: DeprecatedDefaultWithMessage {}