AST: Fix ProtocolConformanceRef::subst() with self-conforming existentials

Author: slavapestov

lib/AST/ProtocolConformance.cpp

@@ -103,15 +103,23 @@ ProtocolConformanceRef::subst(Type origType,
   auto substType = origType.subst(subs, conformances,
                                   SubstFlags::UseErrorType);
 
-  if (substType->isOpenedExistential())
-    return *this;
-
   // If we have a concrete conformance, we need to substitute the
   // conformance to apply to the new type.
   if (isConcrete())
     return ProtocolConformanceRef(
       getConcrete()->subst(substType, subs, conformances));
 
+  // Opened existentials trivially conform and do not need to go through
+  // substitution map lookup.
+  if (substType->isOpenedExistential())
+    return *this;
+
+  // If the substituted type is an existential, we have a self-conforming
+  // existential being substituted in place of itself. There's no
+  // conformance information in this case, so just return.
+  if (substType->isObjCExistentialType())
+    return *this;
+
   auto *proto = getRequirement();
 
   // If the original type was an archetype, check the conformance map.

test/SILOptimizer/specialize_self_conforming.swift

@@ -0,0 +1,24 @@
+// RUN: %target-swift-frontend -emit-sil -O -primary-file %s | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+@objc protocol P {}
+
+@_semantics("optimize.sil.never")
+func takesP<T : P>(_: T) {}
+
+@inline(__always)
+func callsTakesP<T : P>(_ t: T) {
+  takesP(t)
+}
+
+// CHECK-LABEL: sil hidden @_T026specialize_self_conforming16callsTakesPWithPyAA1P_pF : $@convention(thin) (@owned P) -> () {
+// CHECK: [[FN:%.*]] = function_ref @_T026specialize_self_conforming6takesPyxAA1PRzlF : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@owned τ_0_0) -> ()
+// CHECK: apply [[FN]]<P>(%0) : $@convention(thin) <τ_0_0 where τ_0_0 : P> (@owned τ_0_0) -> ()
+// CHECK: return
+
+func callsTakesPWithP(_ p: P) {
+  callsTakesP(p)
+}