SILGen: Fix invalid substitution map formation in coroutine witness thunk emission

If the conformance generic signature fixes all generic parameters,
F.getForwardingSubstitutionMap() is empty. Instead, map the
replacement types of the substitution map into the generic
environment earlier, before we strip off a fully-concrete generic
signature.

Author: slavapestov

lib/SILGen/SILGenPoly.cpp

@@ -7230,8 +7230,7 @@ void SILGenFunction::emitProtocolWitness(
     auto deallocCleanup = std::get<3>(tokenAndCleanups);
 
     YieldInfo witnessYieldInfo(SGM, witness, witnessFTy, witnessSubs);
-    YieldInfo reqtYieldInfo(SGM, requirement, thunkTy,
-                            reqtSubs.subst(getForwardingSubstitutionMap()));
+    YieldInfo reqtYieldInfo(SGM, requirement, thunkTy, reqtSubs);
 
     translateYields(*this, loc, witnessYields, witnessYieldInfo, reqtYieldInfo);
 

lib/SILGen/SILGenType.cpp

@@ -752,13 +752,6 @@ SILFunction *SILGenModule::emitProtocolWitness(
     reqtOrigTy->substGenericArgs(reqtSubMap)
       ->getReducedType(genericSig));
 
-  // Generic signatures where all parameters are concrete are lowered away
-  // at the SILFunctionType level.
-  if (genericSig && genericSig->areAllParamsConcrete()) {
-    genericSig = nullptr;
-    genericEnv = nullptr;
-  }
-
   // Rewrite the conformance in terms of the requirement environment's Self
   // type, which might have a different generic signature than the type
   // itself.
@@ -772,6 +765,16 @@ SILFunction *SILGenModule::emitProtocolWitness(
     auto self = requirement->getSelfInterfaceType()->getCanonicalType();
 
     conformance = reqtSubMap.lookupConformance(self, requirement);
+
+    if (genericEnv)
+      reqtSubMap = reqtSubMap.subst(genericEnv->getForwardingSubstitutionMap());
+  }
+
+  // Generic signatures where all parameters are concrete are lowered away
+  // at the SILFunctionType level.
+  if (genericSig && genericSig->areAllParamsConcrete()) {
+    genericSig = nullptr;
+    genericEnv = nullptr;
   }
 
   reqtSubstTy =