Sema: Fix crash with recursive subscript validation via associated type inference

Looks like subscript validation wasn't checking or setting
ValueDecl::isBeingValidated(). This caused a stack overflow
with associated type inference. The way to trigger this
during normal use is to invoke the fixit for adding
missing protocol requirements -- they're written in terms
of the protocol's associated type, which the user might
not have defined yet.

Fixes <rdar://problem/26680599>.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -4093,9 +4093,11 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       return;
     }
 
-    if (SD->hasInterfaceType())
+    if (SD->hasInterfaceType() || SD->isBeingValidated())
       return;
 
+    SD->setIsBeingValidated();
+
     auto dc = SD->getDeclContext();
     assert(dc->isTypeContext() &&
            "Decl parsing must prevent subscripts outside of types!");
@@ -4139,6 +4141,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         TC.configureInterfaceType(SD, SD->getGenericSignature());
     }
 
+    SD->setIsBeingValidated(false);
+
     TC.checkDeclAttributesEarly(SD);
     TC.computeAccessibility(SD);
 

test/decl/protocol/req/associated_type_inference.swift

@@ -129,7 +129,11 @@ struct XProp0b : PropertyP0 { // expected-error{{type 'XProp0b' does not conform
 // Inference from subscripts
 protocol SubscriptP0 {
   associatedtype Index
-  associatedtype Element : PSimple // expected-note{{unable to infer associated type 'Element' for protocol 'SubscriptP0'}}
+  // expected-note@-1 2 {{protocol requires nested type 'Index'; do you want to add it?}}
+
+  associatedtype Element : PSimple
+  // expected-note@-1 {{unable to infer associated type 'Element' for protocol 'SubscriptP0'}}
+  // expected-note@-2 2 {{protocol requires nested type 'Element'; do you want to add it?}}
 
   subscript (i: Index) -> Element { get }
 }
@@ -138,14 +142,27 @@ struct XSubP0a : SubscriptP0 {
   subscript (i: Int) -> Int { get { return i } }
 }
 
-struct XSubP0b : SubscriptP0 { // expected-error{{type 'XSubP0b' does not conform to protocol 'SubscriptP0'}}
+struct XSubP0b : SubscriptP0 {
+// expected-error@-1{{type 'XSubP0b' does not conform to protocol 'SubscriptP0'}}
   subscript (i: Int) -> Float { get { return Float(i) } } // expected-note{{inferred type 'Float' (by matching requirement 'subscript') is invalid: does not conform to 'PSimple'}}
 }
 
+struct XSubP0c : SubscriptP0 {
+// expected-error@-1 {{type 'XSubP0c' does not conform to protocol 'SubscriptP0'}}
+  subscript (i: Index) -> Element { get { } }
+}
+
+struct XSubP0d : SubscriptP0 {
+// expected-error@-1 {{type 'XSubP0d' does not conform to protocol 'SubscriptP0'}}
+  subscript (i: XSubP0d.Index) -> XSubP0d.Element { get { } }
+}
+
 // Inference from properties and subscripts
 protocol CollectionLikeP0 {
   associatedtype Index
+  // expected-note@-1 {{protocol requires nested type 'Index'; do you want to add it?}}
   associatedtype Element
+  // expected-note@-1 {{protocol requires nested type 'Element'; do you want to add it?}}
 
   var startIndex: Index { get }
   var endIndex: Index { get }
@@ -164,6 +181,13 @@ struct XCollectionLikeP0a<T> : CollectionLikeP0 {
   subscript (r: Range<Int>) -> SomeSlice<T> { get { return SomeSlice() } }
 }
 
+struct XCollectionLikeP0b : CollectionLikeP0 {
+// expected-error@-1 {{type 'XCollectionLikeP0b' does not conform to protocol 'CollectionLikeP0'}}
+  var startIndex: XCollectionLikeP0b.Index
+  // expected-error@-1 {{'startIndex' used within its own type}}
+  var startElement: XCollectionLikeP0b.Element
+}
+
 // rdar://problem/21304164
 public protocol Thenable {
     associatedtype T // expected-note{{protocol requires nested type 'T'}}