Sema: Fix a recusive validation crasher

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -3723,6 +3723,13 @@ void TypeChecker::validateDecl(ValueDecl *D) {
       return;
   }
 
+  // Validating the parent may have triggered validation of this declaration,
+  // so just return if that was the case.
+  if (D->hasValidationStarted()) {
+    assert(D->hasValidSignature());
+    return;
+  }
+
   if (Context.Stats)
     Context.Stats->getFrontendCounters().NumDeclsValidated++;
 

test/decl/nested/protocol.swift

@@ -79,3 +79,13 @@ protocol SelfDotTest {
   class Class {}
   // expected-error@-1{{type 'Class' cannot be nested in protocol 'SelfDotTest'}}
 }
+
+struct Outer {
+  typealias E = NestedValidation.T
+  protocol NestedValidation { // expected-error {{protocol 'NestedValidation' cannot be nested inside another declaration}}
+    typealias T = A.B
+    class A { // expected-error {{type 'A' cannot be nested in protocol 'NestedValidation'}}
+      typealias B = Int
+    }
+  }
+}

validation-test/compiler_crashers/28797-declbits-beingvalidated-ibv.swift renamed to validation-test/compiler_crashers_fixed/28797-declbits-beingvalidated-ibv.swift

@@ -5,7 +5,7 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+// RUN: not %target-swift-frontend %s -emit-ir
 {typealias e:P.a
 protocol P{typealias a=a.b:class a