[Conformance checking] Diagnose the actual problem with near-misses.

Use the diagnostics machinery of the protocol conformance checker to
say why each near-miss actually missed, e.g., a type conflict. This
gives better information regarding how to fix the actual problem. Yet
more QoI for rdar://problem/25159872.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -1204,7 +1204,7 @@ NOTE(ambiguous_witnesses_type,none,
 NOTE(protocol_witness_exact_match,none,
      "candidate exactly matches%0", (StringRef))
 NOTE(protocol_witness_renamed,none,
-     "candidate matches (with renaming)%0", (StringRef))
+     "rename to %0 to satisfy this requirement%1", (DeclName, StringRef))
 NOTE(protocol_witness_kind_conflict,none,
      "candidate is not %select{an initializer|a function|a variable|"
      "a subscript}0", (RequirementKind))
@@ -1272,9 +1272,6 @@ NOTE(protocol_conformance_implied_here,none,
 WARNING(optional_req_near_match,none,
         "%0 %1 nearly matches optional requirement %2 of protocol %3",
         (DescriptiveDeclKind, DeclName, DeclName, DeclName))
-NOTE(optional_req_near_match_rename,none,
-     "rename to %0 to satisfy this requirement",
-     (DeclName))
 NOTE(optional_req_nonobjc_near_match_add_objc,none,
      "add '@objc' to provide an Objective-C entrypoint", ())
 NOTE(optional_req_nonobjc_to_objc,none,

lib/Sema/TypeCheckProtocol.cpp

@@ -1708,9 +1708,18 @@ diagnoseMatch(TypeChecker &tc, Module *module,
                 withAssocTypes);
     break;
 
-  case MatchKind::RenamedMatch:
-    tc.diagnose(match.Witness, diag::protocol_witness_renamed, withAssocTypes);
+  case MatchKind::RenamedMatch: {
+    auto diag = tc.diagnose(match.Witness, diag::protocol_witness_renamed,
+                            req->getFullName(), withAssocTypes);
+
+    // Fix the name.
+    fixDeclarationName(diag, match.Witness, req->getFullName());
+
+    // Also fix the Objective-C name, if needed.
+    if (req->isObjC())
+      fixDeclarationObjCName(diag, match.Witness, req->getObjCRuntimeName());
     break;
+  }
 
   case MatchKind::KindConflict:
     tc.diagnose(match.Witness, diag::protocol_witness_kind_conflict,
@@ -4344,7 +4353,9 @@ static bool shouldWarnAboutPotentialWitness(ValueDecl *req,
 }
 
 /// Diagnose a potential witness.
-static void diagnosePotentialWitness(TypeChecker &tc, ValueDecl *req,
+static void diagnosePotentialWitness(TypeChecker &tc,
+                                     NormalProtocolConformance *conformance,
+                                     ValueDecl *req,
                                      ValueDecl *witness,
                                      Accessibility accessibility) {
   auto proto = cast<ProtocolDecl>(req->getDeclContext());
@@ -4356,20 +4367,18 @@ static void diagnosePotentialWitness(TypeChecker &tc, ValueDecl *req,
               req->getFullName(),
               proto->getFullName());
 
-  if (req->getFullName() != witness->getFullName()) {
-    // Note to fix the names.
-    auto diag = tc.diagnose(witness, diag::optional_req_near_match_rename,
-                            req->getFullName());
-    fixDeclarationName(diag, witness, req->getFullName());
-
-    // Also fix the Objective-C name, if needed.
-    if (req->isObjC())
-      fixDeclarationObjCName(diag, witness, req->getObjCRuntimeName());
-  } else if (req->isObjC() && !witness->isObjC()) {
-    // Note to add @objc.
+  // Describe why the witness didn't satisfy the requirement.
+  auto match = matchWitness(tc, conformance->getProtocol(), conformance,
+                            conformance->getDeclContext(), req, witness);
+  if (match.Kind == MatchKind::ExactMatch &&
+      req->isObjC() && !witness->isObjC()) {
+    // Special case: note to add @objc.
     auto diag = tc.diagnose(witness,
                             diag::optional_req_nonobjc_near_match_add_objc);
-    fixDeclarationObjCName(diag, witness, req->getObjCRuntimeName());    
+    fixDeclarationObjCName(diag, witness, req->getObjCRuntimeName());
+  } else {
+    diagnoseMatch(tc, conformance->getDeclContext()->getParentModule(),
+                  conformance, req, match);
   }
 
   // If moving the declaration can help, suggest that.
@@ -4559,7 +4568,18 @@ void TypeChecker::checkConformancesInContext(DeclContext *dc,
       // If we have something to complain about, do so.
       if (!bestOptionalReqs.empty()) {
         auto req = bestOptionalReqs[0];
-        diagnosePotentialWitness(*this, req, value, defaultAccessibility);
+        bool diagnosed = false;
+        for (auto conformance : conformances) {
+          if (conformance->getProtocol() == req->getDeclContext()) {
+            diagnosePotentialWitness(*this,
+                                     conformance->getRootNormalConformance(),
+                                     req, value, defaultAccessibility);
+            diagnosed = true;
+            break;
+          }
+        }
+        assert(diagnosed && "Failed to find conformance to diagnose?");
+        (void)diagnosed;
 
         // Remove this optional requirement from the list. We don't want to
         // complain about it twice.

test/decl/protocol/conforms/near_miss_objc.swift

@@ -104,3 +104,18 @@ class C6a : P6 {
   // expected-note@-3{{move 'methodWithInt(_:forSomeClass:dividingDouble:)' to an extension to silence this warning}}
   // expected-note@-4{{make 'methodWithInt(_:forSomeClass:dividingDouble:)' private to silence this warning}}{{3-3=private }}
 }
+
+// Use the first note to always describe why it didn't match.
+@objc protocol P7 {
+  optional func method(foo: Float)
+  // expected-note@-1{{requirement 'method(foo:)' declared here}}
+}
+
+class C7a : P7 {
+  @objc func method(foo: Double) { }
+  // expected-warning@-1{{instance method 'method(foo:)' nearly matches optional requirement 'method(foo:)' of protocol 'P7'}}
+  // expected-note@-2{{candidate has non-matching type '(foo: Double) -> ()'}}
+  // expected-note@-3{{move 'method(foo:)' to an extension to silence this warning}}
+  // expected-note@-4{{make 'method(foo:)' private to silence this warning}}
+}
+