[Diagnostics] Implement diagnostic-as-note for requirement failures

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -1539,6 +1539,10 @@ NOTE(where_requirement_failure_both_subst,none,
      "where %0 = %1, %2 = %3", (Type, Type, Type, Type))
 NOTE(requirement_implied_by_conditional_conformance,none,
      "requirement from conditional conformance of %0 to %1", (Type, Type))
+NOTE(candidate_types_conformance_requirement,none,
+     "candidate requires that %0 conform to %1 "
+     "(requirement specified as %2 == %3%4)",
+     (Type, Type, Type, Type, StringRef))
 NOTE(candidate_types_equal_requirement,none,
      "candidate requires that the types %0 and %1 be equivalent "
      "(requirement specified as %2 == %3%4)",

lib/Sema/CSDiagnostics.cpp

@@ -117,13 +117,20 @@ const DeclContext *RequirementFailure::getRequirementDC() const {
 }
 
 bool RequirementFailure::diagnose(bool asNote) {
-  if (!canDiagnoseFailure() || asNote)
+  if (!canDiagnoseFailure())
     return false;
 
   auto *anchor = getAnchor();
   const auto *reqDC = getRequirementDC();
   auto *genericCtx = AffectedDecl->getAsGenericContext();
 
+  if (asNote) {
+    const auto &req = getRequirement();
+    emitDiagnostic(reqDC->getAsDecl(), getDiagnosticAsNote(), getLHS(),
+                   getRHS(), req.getFirstType(), req.getSecondType(), "");
+    return true;
+  }
+
   if (reqDC != genericCtx) {
     auto *NTD = reqDC->getSelfNominalTypeDecl();
     emitDiagnostic(anchor->getLoc(), getDiagnosticInRereference(),
@@ -160,9 +167,12 @@ void RequirementFailure::emitRequirementNote(const Decl *anchor) const {
 }
 
 bool MissingConformanceFailure::diagnose(bool asNote) {
-  if (!canDiagnoseFailure() || asNote)
+  if (!canDiagnoseFailure())
     return false;
 
+  if (asNote)
+    return RequirementFailure::diagnose(asNote);
+
   auto *anchor = getAnchor();
   auto ownerType = getOwnerType();
   auto nonConformingType = getLHS();

lib/Sema/CSDiagnostics.h

@@ -133,6 +133,7 @@ class RequirementFailure : public FailureDiagnostic {
   using PathEltKind = ConstraintLocator::PathElementKind;
   using DiagOnDecl = Diag<DescriptiveDeclKind, DeclName, Type, Type>;
   using DiagInReference = Diag<DescriptiveDeclKind, DeclName, Type, Type, Type>;
+  using DiagAsNote = Diag<Type, Type, Type, Type, StringRef>;
 
   const ValueDecl *AffectedDecl;
   /// If possible, find application expression associated
@@ -182,6 +183,7 @@ class RequirementFailure : public FailureDiagnostic {
 
   virtual DiagOnDecl getDiagnosticOnDecl() const = 0;
   virtual DiagInReference getDiagnosticInRereference() const = 0;
+  virtual DiagAsNote getDiagnosticAsNote() const = 0;
 
   /// Determine whether it would be possible to diagnose
   /// current requirement failure.
@@ -242,6 +244,10 @@ class MissingConformanceFailure final : public RequirementFailure {
   DiagInReference getDiagnosticInRereference() const override {
     return diag::type_does_not_conform_in_decl_ref;
   }
+
+  DiagAsNote getDiagnosticAsNote() const override {
+    return diag::candidate_types_conformance_requirement;
+  }
 };
 
 /// Diagnose failures related to same-type generic requirements, e.g.
@@ -278,6 +284,10 @@ class SameTypeRequirementFailure final : public RequirementFailure {
   DiagInReference getDiagnosticInRereference() const override {
     return diag::types_not_equal_in_decl_ref;
   }
+
+  DiagAsNote getDiagnosticAsNote() const override {
+    return diag::candidate_types_equal_requirement;
+  }
 };
 
 /// Diagnose failures related to superclass generic requirements, e.g.
@@ -312,6 +322,10 @@ class SuperclassRequirementFailure final : public RequirementFailure {
   DiagInReference getDiagnosticInRereference() const override {
     return diag::types_not_inherited_in_decl_ref;
   }
+
+  DiagAsNote getDiagnosticAsNote() const override {
+    return diag::candidate_types_inheritance_requirement;
+  }
 };
 
 /// Diagnose errors associated with missing, extraneous