Index: clang/lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2083,6 +2083,11 @@
     if (Member->getDeclContext() != Pattern)
       continue;
 
+    // BlockDecls can appear in a default-member-initializer. They must be the
+    // child of a BlockExpr, so we only know how to instantiate them from there.
+    if (isa<BlockDecl>(Member))
+      continue;
+
     if (Member->isInvalidDecl()) {
       Instantiation->setInvalidDecl();
       continue;
Index: clang/test/SemaCXX/instantiate-blocks.cpp
===================================================================
--- clang/test/SemaCXX/instantiate-blocks.cpp
+++ clang/test/SemaCXX/instantiate-blocks.cpp
@@ -30,3 +30,12 @@
    noret((float)0.0, double(0.0)); // expected-note {{in instantiation of function template specialization 'noret<float, double>' requested here}}
 }
 
+namespace rdar41200624 {
+template <class T>
+struct S {
+  int (^p)() = ^{ return 0; };
+  T (^t)() = ^{ return T{}; };
+  T s = ^{ return T{}; }();
+};
+S<int> x;
+}