[Diagnostics] Extract requirement declaration context retrieval into its own method

Change logic to traverse the chain of declaration contexts
starting for "affected" declaration's parent and check if the
requirement is satisfied by one of them, if so return it as
requirement's declaration context.

Extract this logic into a method on `RequirementFailure` to make
it accessible for other types of requirement failure diagnostics.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -14,10 +14,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "ConstraintSystem.h"
 #include "CSDiagnostics.h"
+#include "ConstraintSystem.h"
 #include "MiscDiagnostics.h"
 #include "swift/AST/Expr.h"
+#include "swift/AST/GenericSignature.h"
 #include "swift/AST/Types.h"
 #include "llvm/ADT/ArrayRef.h"
 
@@ -67,7 +68,7 @@ Type RequirementFailure::getOwnerType() const {
   return getType(getAnchor())->getInOutObjectType()->getMetatypeInstanceType();
 }
 
-const Requirement &RequirementFailure::getRequirement() {
+const Requirement &RequirementFailure::getRequirement() const {
   auto *genericCtx = AffectedDecl->getAsGenericContext();
   return genericCtx->getGenericRequirements()[getRequirementIndex()];
 }
@@ -100,6 +101,27 @@ ValueDecl *RequirementFailure::getDeclRef() const {
   return ownerType->getAnyGeneric();
 }
 
+const DeclContext *RequirementFailure::getRequirementDC() const {
+  const auto &req = getRequirement();
+  auto *DC = AffectedDecl->getDeclContext();
+
+  do {
+    auto *D = DC->getInnermostDeclarationDeclContext();
+    if (!D)
+      break;
+
+    if (auto *GC = D->getAsGenericContext()) {
+      auto *sig = GC->getGenericSignature();
+      if (sig && sig->isRequirementSatisfied(req))
+        return DC;
+    }
+
+    DC = DC->getParent();
+  } while (DC);
+
+  return AffectedDecl->getAsGenericContext();
+}
+
 bool MissingConformanceFailure::diagnose() {
   auto *anchor = getAnchor();
   auto ownerType = getOwnerType();
@@ -175,21 +197,10 @@ bool MissingConformanceFailure::diagnose() {
   } else {
     const auto &req = getRequirement();
     auto *genericCtx = AffectedDecl->getAsGenericContext();
+    const auto *reqDC = getRequirementDC();
 
-    std::function<const DeclContext *(Type)> getAffectedCtx =
-        [&](Type type) -> const DeclContext * {
-      if (auto *DMT = type->getAs<DependentMemberType>())
-        return getAffectedCtx(DMT->getBase());
-
-      if (auto *GPT = type->getAs<GenericTypeParamType>())
-        return GPT->getDecl()->getDeclContext();
-
-      return genericCtx;
-    };
-
-    const auto *affected = getAffectedCtx(req.getFirstType());
-    if (affected != genericCtx) {
-      auto *NTD = affected->getAsNominalTypeOrNominalTypeExtensionContext();
+    if (reqDC != genericCtx) {
+      auto *NTD = reqDC->getAsNominalTypeOrNominalTypeExtensionContext();
       emitDiagnostic(anchor->getLoc(), diag::type_does_not_conform_in_decl_ref,
                      AffectedDecl->getDescriptiveKind(),
                      AffectedDecl->getFullName(), NTD->getDeclaredType(), type,
@@ -200,7 +211,7 @@ bool MissingConformanceFailure::diagnose() {
                      AffectedDecl->getFullName(), type, protocolType);
     }
 
-    emitDiagnostic(affected->getAsDeclOrDeclExtensionContext(),
+    emitDiagnostic(reqDC->getAsDeclOrDeclExtensionContext(),
                    diag::where_type_does_not_conform_type, req.getFirstType(),
                    type);
   }

lib/Sema/CSDiagnostics.h

@@ -134,7 +134,12 @@ class RequirementFailure : public FailureDiagnostic {
   Type getOwnerType() const;
 
   /// Generic requirement associated with the failure.
-  const Requirement &getRequirement();
+  const Requirement &getRequirement() const;
+
+protected:
+  /// Retrieve declaration contextual where current
+  /// requirement has been introduced.
+  const DeclContext *getRequirementDC() const;
 
 private:
   /// Retrieve declaration associated with failing generic requirement.