Fix an edge case in ExistentialSpecializer

ExistentialSpecializer could be building the function type from a
witness_method. We cannot attach a witnessProtocolConformance to the
newly specialized thin function.

Author: meg-gupta

lib/SILOptimizer/FunctionSignatureTransforms/ExistentialTransform.cpp

@@ -376,15 +376,14 @@ ExistentialTransform::createExistentialSpecializedFunctionType() {
   /// Finally the ExtInfo.
   auto ExtInfo = FTy->getExtInfo();
   ExtInfo = ExtInfo.withRepresentation(SILFunctionTypeRepresentation::Thin);
-  auto witnessMethodConformance = FTy->getWitnessMethodConformanceOrInvalid();
 
   /// Return the new signature.
   return SILFunctionType::get(
       NewGenericSig, ExtInfo, FTy->getCoroutineKind(),
       FTy->getCalleeConvention(), InterfaceParams, FTy->getYields(),
       FTy->getResults(), InterfaceErrorResult,
       SubstitutionMap(), SubstitutionMap(),
-      Ctx, witnessMethodConformance);
+      Ctx);
 }
 
 /// Create the Thunk Body with always_inline attribute.

test/SILOptimizer/existential_spl_witness_method.swift

@@ -0,0 +1,45 @@
+// RUN: %target-swift-frontend -O -Xllvm -sil-disable-pass=GenericSpecializer -Xllvm -sil-disable-pass=EarlyInliner -Xllvm -sil-disable-pass=PerfInliner -Xllvm -sil-disable-pass=LateInliner -emit-sil -sil-verify-all %s | %FileCheck %s
+
+// Test for ExistentialSpecializer when an existential type is passed to a witness_method func representation
+protocol P {
+@inline(never)
+  func myfuncP(_ q:Q) -> Int
+}
+
+protocol Q {
+@inline(never)
+  func myfuncQ() -> Int
+}
+
+class C : P {
+  var id = 10
+@inline(never)
+  func myfuncP(_ q:Q) -> Int {
+    return id
+  }
+}
+
+class D : Q {
+  var id = 20
+@inline(never)
+  func myfuncQ() -> Int {
+    return id
+  }
+}
+
+// CHECK-LABEL: @$s30existential_spl_witness_method1CCAA1PA2aDP7myfuncPySiAA1Q_pFTW : $@convention(witness_method: P) (@in_guaranteed Q, @in_guaranteed C) -> Int {
+// CHECK: [[FR1:%.*]] = function_ref @$s30existential_spl_witness_method1CCAA1PA2aDP7myfuncPySiAA1Q_pFTWTf4en_n : $@convention(thin) <τ_0_0 where τ_0_0 : Q> (@in_guaranteed τ_0_0, @in_guaranteed C) -> Int
+// CHECK: apply [[FR1]]
+// CHECK-LABEL: } // end sil function '$s30existential_spl_witness_method1CCAA1PA2aDP7myfuncPySiAA1Q_pFTW'
+
+// CHECK-LABEL : @$s30existential_spl_witness_method3bazyyAA1P_p_AA1Q_ptFTf4ee_n : $@convention(thin) <τ_0_0, τ_0_1 where τ_0_0 : P, τ_0_1 : Q> (@in_guaranteed τ_0_0, @in_guaranteed τ_0_1) -> () {
+// CHECK: [[FR2:%.*]] = function_ref @$s30existential_spl_witness_method1CCAA1PA2aDP7myfuncPySiAA1Q_pFTW : $@convention(witness_method: P) (@in_guaranteed Q, @in_guaranteed C) -> Int
+// CHECK: apply [[FR2]]
+// CHECK-LABEL: } // end sil function '$s30existential_spl_witness_method3bazyyAA1P_p_AA1Q_ptFTf4ee_n'
+@inline(never)
+func baz(_ p : P, _ q : Q) {
+  p.myfuncP(q)
+}
+
+baz(C(), D());
+