[4.2] [SILGen] Don't assert on subscripts with no index arguments.

include/swift/AST/Types.h

@@ -1617,12 +1617,6 @@ class NameAliasType final
     return getNumSubstitutions();
   }
 
-  /// Retrieve the set of substitutions to be applied to the declaration to
-  /// produce the underlying type.
-  SubstitutionList getSubstitutionList() const {
-    return {getTrailingObjects<Substitution>(), getNumSubstitutions()};
-  }
-
   /// Retrieve the generic signature used for substitutions.
   GenericSignature *getGenericSignature() const {
     return getNumSubstitutions() > 0
@@ -1648,6 +1642,12 @@ class NameAliasType final
                                              : Type();
   }
 
+  /// Retrieve the set of substitutions to be applied to the declaration to
+  /// produce the underlying type.
+  SubstitutionList getSubstitutionList() const {
+    return {getTrailingObjects<Substitution>(), getNumSubstitutions()};
+  }
+
   /// Retrieve the substitution map applied to the declaration's underlying
   /// to produce the described type.
   SubstitutionMap getSubstitutionMap() const;

lib/SILGen/SILGenApply.cpp

@@ -5073,7 +5073,7 @@ void SILGenFunction::emitSetAccessor(SILLocation loc, SILDeclRef set,
   if (!subscripts.isNull()) {
     // If we have a value and index list, create a new rvalue to represent the
     // both of them together.
-    auto inputTupleType = cast<TupleType>(accessType.getInput());
+    auto inputTupleType = dyn_cast<TupleType>(accessType.getInput());
 
     SmallVector<ArgumentSource, 4> eltSources;
 
@@ -5086,18 +5086,25 @@ void SILGenFunction::emitSetAccessor(SILLocation loc, SILDeclRef set,
     // TODO: we should really take an array of RValues.
     if (accessType->getNumParams() != 2) {
       auto subscriptsTupleType = cast<TupleType>(subscripts.getType());
-      assert(inputTupleType->getNumElements()
+      assert(accessType.getParams().size()
               == 1 + subscriptsTupleType->getNumElements());
       SmallVector<RValue, 8> eltRVs;
       std::move(subscripts).extractElements(eltRVs);
       for (auto &elt : eltRVs)
         eltSources.emplace_back(loc, std::move(elt));
     } else {
+      assert(inputTupleType && "Must have an input tuple here");
       subscripts.rewriteType(inputTupleType.getElementType(1));
       eltSources.emplace_back(loc, std::move(subscripts));
     }
 
-    setValue = ArgumentSource(loc, inputTupleType, eltSources);
+    if (eltSources.size() == 1) {
+      setValue = std::move(eltSources.front());
+    } else {
+      assert(inputTupleType);
+      setValue = ArgumentSource(loc, inputTupleType, eltSources);
+    }
+
   } else {
     setValue.rewriteType(accessType.getInput());
   }

lib/Serialization/Serialization.cpp

@@ -3631,10 +3631,7 @@ void Serializer::writeType(Type ty) {
                              addTypeRef(alias->getParent()),
                              addTypeRef(alias->getSinglyDesugaredType()));
     // Write the set of substitutions.
-    SmallVector<Substitution, 4> flatSubs;
-    if (auto genericSig = typeAlias->getGenericSignature())
-      genericSig->getSubstitutions(alias->getSubstitutionMap(), flatSubs);
-    writeSubstitutions(flatSubs, DeclTypeAbbrCodes);
+    writeSubstitutions(alias->getSubstitutionList(), DeclTypeAbbrCodes);
     break;
   }
 

test/SILGen/subscript_accessor.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -enable-sil-ownership -O -emit-sil -primary-file %s | %FileCheck %s
+
+// CHECK-LABEL: sil private [transparent] @$S18subscript_accessor1XVxSgycimytfU_
+// CHECK: [[SETTER:%.*]] = function_ref @$S18subscript_accessor1XVxSgycis
+// CHECK-NEXT: apply [[SETTER]]<T>
+struct X<T> {
+  subscript () -> T? {
+    get {
+      return nil
+    }
+    set { }
+  }
+}
+
+  // CHECK: sil{{.*}}S18subscript_accessor9testXRead1xxAA1XVyxG_tlF
+@_specialize(where T == (Int, Int))
+func testXRead<T>(x: X<T>) -> T {
+  return x[]!
+}
+// CHECK: $S18subscript_accessor1XVxSgycisTf4dn_n