[Diagnostics] Unsatisfied requirement in reference diagnostics expect type context

Resolves: rdar://problem/50666427

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -336,8 +336,9 @@ bool RequirementFailure::diagnoseAsError() {
     return true;
   }
 
-  if (genericCtx != reqDC && (genericCtx->isChildContextOf(reqDC) ||
-                              isStaticOrInstanceMember(AffectedDecl))) {
+  if (reqDC->isTypeContext() && genericCtx != reqDC &&
+      (genericCtx->isChildContextOf(reqDC) ||
+       isStaticOrInstanceMember(AffectedDecl))) {
     auto *NTD = reqDC->getSelfNominalTypeDecl();
     emitDiagnostic(anchor->getLoc(), getDiagnosticInRereference(),
                    AffectedDecl->getDescriptiveKind(),

validation-test/Sema/type_checker_crashers_fixed/rdar49712364.swift

@@ -0,0 +1,15 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+protocol A {}
+
+class C<T> where T: A {}
+
+extension C {
+  func foo() {
+    extension C where T: Undefined {
+      class Inner: Encodable {
+        var foo: Int
+      }
+    }
+  }
+}