Sema: Fix another crash with an invalid extension

Perhaps we should refactor everything so that extensions
that are not at the top-level are still bound and type
checked normally. However that requires a bit of work, so
keep playing wack-a-mole for now to handle these invalid
states when they come up.

However while we're at it, make the code a little better
by removing a bogus diagnostic path that was not used.

Fixes <rdar://problem/45290211>, <https://bugs.swift.org/browse/SR-9009>.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -3225,19 +3225,6 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     TC.checkDeclAttributesEarly(ED);
 
-    if (auto extendedTy = ED->getExtendedType()) {
-      if (!extendedTy->is<NominalType>() &&
-          !extendedTy->is<BoundGenericType>() &&
-          !extendedTy->hasError()) {
-        // FIXME: Redundant diagnostic test here?
-        TC.diagnose(ED->getStartLoc(), diag::non_nominal_extension,
-                    extendedTy);
-        // FIXME: It would be nice to point out where we found the named type
-        // declaration, if any.
-        ED->setInvalid();
-      }
-    }
-
     checkInheritanceClause(ED);
 
     if (auto nominal = ED->getExtendedNominal()) {
@@ -3273,8 +3260,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     // nominal type.
     // FIXME: This is a hack to make sure that the type checker precomputes
     // enough information for later passes that might query conformances.
-    if (auto nominal = ED->getSelfNominalTypeDecl())
-      (void)nominal->getAllConformances();
+    if (auto nominal = ED->getExtendedNominal())
+      (void) nominal->getAllConformances();
   }
 
   void visitTopLevelCodeDecl(TopLevelCodeDecl *TLCD) {

lib/Sema/TypeCheckType.cpp

@@ -149,6 +149,9 @@ Type TypeResolution::resolveDependentMemberType(
   }
 
   assert(stage == TypeResolutionStage::Interface);
+  if (!getGenericSignature())
+    return ErrorType::get(baseTy);
+
   auto builder = getGenericSignatureBuilder();
   auto baseEquivClass =
     builder->resolveEquivalenceClass(

test/decl/ext/generic.swift

@@ -167,3 +167,11 @@ public typealias Array2 = Array
 extension Array2 where QQQ : VVV {}
 // expected-error@-1 {{use of undeclared type 'QQQ'}}
 // expected-error@-2 {{use of undeclared type 'VVV'}}
+
+// https://bugs.swift.org/browse/SR-9009
+func foo() {
+  extension Array where Element : P1 {
+  // expected-error@-1 {{declaration is only valid at file scope}}
+    func foo() -> Element.AssocType {}
+  }
+}