Eliminate non-generic dependent types in ASD::getValueInterfaceType()

lib/AST/Decl.cpp

@@ -5185,7 +5185,23 @@ Type SubscriptDecl::getElementInterfaceType() const {
   auto elementTy = getInterfaceType();
   if (elementTy->is<ErrorType>())
     return elementTy;
-  return elementTy->castTo<AnyFunctionType>()->getResult();
+  auto fnTy = elementTy->castTo<AnyFunctionType>();
+  auto resultTy = fnTy->getResult();
+
+  // Make sure we don't return a dependent type if the context is not
+  // actually generic.  This can only happen with subscripts because we're
+  // apparently somewhat eager to leave dependent types as dependent
+  // because of generic subscripts.
+  if (resultTy->hasTypeParameter()) {
+    auto genericFnTy = cast<GenericFunctionType>(fnTy);
+    auto sig = genericFnTy->getGenericSignature();
+    if (sig->areAllParamsConcrete()) {
+      // It's a bit unfortunate to canonicalize here.
+      return sig->getCanonicalTypeInContext(resultTy);
+    }
+  }
+
+  return resultTy;
 }
 
 void SubscriptDecl::computeType() {

test/SILGen/subscript_accessor.swift

@@ -18,3 +18,16 @@ func testXRead<T>(x: X<T>) -> T {
   return x[]!
 }
 // CHECK: $s18subscript_accessor1XVxSgycisTf4dn_n
+
+// Don't crash dealing with T? in a non-generic context.
+// rdar://44762116
+struct WillBeConcretelyConstrained<T> {}
+extension WillBeConcretelyConstrained where T == Int {
+  subscript(key: Int) -> T? {
+    get { return nil }
+    set {}
+  }
+}
+
+// CHECK-LABEL: sil hidden [transparent] @$s18subscript_accessor27WillBeConcretelyConstrainedVAASiRszlEySiSgSiciM
+// CHECK-SAME: $@yield_once @convention(method) (Int, @inout WillBeConcretelyConstrained<Int>) -> @yields @inout Optional<Int>