[CSDiagnostics] Improve requirement failure source detection

If affected declaration is a static or instance memeber (excluding
operators) and failed requirement is declared in other nominal type
or extension, diagnose such problems as `in reference` instead of
claiming that requirement belongs to the member itself.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -90,15 +90,6 @@ Type RequirementFailure::getOwnerType() const {
 }
 
 const GenericContext *RequirementFailure::getGenericContext() const {
-  // For conditional requirements, this is easy because all
-  // required information is contained in the conformance
-  // reference.
-  if (isConditional()) {
-    auto *conformance = getConformanceRef().getConcrete();
-    auto *DC = conformance->getDeclContext();
-    return DC->getAsDecl()->getAsGenericContext();
-  }
-
   if (auto *genericCtx = AffectedDecl->getAsGenericContext())
     return genericCtx;
   return AffectedDecl->getDeclContext()->getAsDecl()->getAsGenericContext();
@@ -199,6 +190,16 @@ const DeclContext *RequirementFailure::getRequirementDC() const {
   return AffectedDecl->getAsGenericContext();
 }
 
+bool RequirementFailure::isStaticOrInstanceMember(const ValueDecl *decl) {
+  if (decl->isInstanceMember())
+    return true;
+
+  if (auto *AFD = dyn_cast<AbstractFunctionDecl>(decl))
+    return AFD->isStatic() && !AFD->isOperator();
+
+  return decl->isStatic();
+}
+
 bool RequirementFailure::diagnoseAsError() {
   if (!canDiagnoseFailure())
     return false;
@@ -210,7 +211,8 @@ bool RequirementFailure::diagnoseAsError() {
   auto lhs = resolveType(getLHS());
   auto rhs = resolveType(getRHS());
 
-  if (reqDC != genericCtx) {
+  if (genericCtx != reqDC && (genericCtx->isChildContextOf(reqDC) ||
+                              isStaticOrInstanceMember(AffectedDecl))) {
     auto *NTD = reqDC->getSelfNominalTypeDecl();
     emitDiagnostic(anchor->getLoc(), getDiagnosticInRereference(),
                    AffectedDecl->getDescriptiveKind(),

lib/Sema/CSDiagnostics.h

@@ -269,6 +269,10 @@ class RequirementFailure : public FailureDiagnostic {
   ValueDecl *getDeclRef() const;
 
   void emitRequirementNote(const Decl *anchor, Type lhs, Type rhs) const;
+
+  /// Determine whether given declaration represents a static
+  /// or instance property/method, excluding operators.
+  static bool isStaticOrInstanceMember(const ValueDecl *decl);
 };
 
 /// Diagnostics for failed conformance checks originating from

test/Generics/conditional_conformances.swift

@@ -399,6 +399,6 @@ extension BinaryInteger {
     return self <= 1
             ? 1
             : (2...self).reduce(1, *)
-            // expected-error@-1 {{instance method 'reduce' requires that 'Self.Stride' conform to 'SignedInteger'}}
+            // expected-error@-1 {{referencing instance method 'reduce' on 'ClosedRange' requires that 'Self.Stride' conform to 'SignedInteger'}}
   }
 }

test/NameBinding/name_lookup_min_max_conditional_conformance.swift

@@ -53,6 +53,6 @@ extension Conditional {
         // expected-warning@-1{{use of 'min' as reference to global function in module 'Swift' will change in future versions of Swift to reference instance method in generic struct 'Conditional' which comes via a conditional conformance}}
         // expected-note@-2{{use 'Swift.' to continue to reference the global function}}
         _ = foo(5, 6)
-        // expected-error@-1{{instance method 'foo()' requires that 'T' conform to 'ContainsFoo'}}
+        // expected-error@-1{{referencing instance method 'foo()' on 'Conditional' requires that 'T' conform to 'ContainsFoo'}}
     }
 }