[Typed throws] Properly reabstract closure types with originally-indirect errors

Thank you, John McCall for debugging this with me!

Author: DougGregor

lib/SIL/IR/SILFunctionType.cpp

@@ -2306,10 +2306,13 @@ static CanSILFunctionType getSILFunctionType(
     auto &errorTLConv = TC.getTypeLowering(origErrorType, errorType,
                                            TypeExpansionContext::minimal());
 
+    bool isFormallyIndirectError =
+        origErrorType.isTypeParameter() || errorTLConv.isAddressOnly();
+
     errorResult = SILResultInfo(errorTLConv.getLoweredType().getASTType(),
-                                errorTLConv.isAddressOnly()
-                                ? ResultConvention::Indirect
-                                : ResultConvention::Owned);
+                                isFormallyIndirectError
+                                  ? ResultConvention::Indirect
+                                  : ResultConvention::Owned);
   }
 
   // Lower the result type.

test/SILGen/typed_throws_generic.swift

@@ -249,8 +249,8 @@ extension Collection {
   }
 }
 
-// CHECK-LABEL: sil private [ossa] @$s20typed_throws_generic9forcedMapySayq_GSayxGr0_lFq_xXEfU_ : $@convention(thin) <T, U> (@in_guaranteed T) -> (@out U, @error Never)
+// CHECK-LABEL: sil private [ossa] @$s20typed_throws_generic9forcedMapySayq_GSayxGr0_lFq_xXEfU_ : $@convention(thin) <T, U> (@in_guaranteed T) -> (@out U, @error_indirect Never)
 func forcedMap<T, U>(_ source: [T]) -> [U] {
-  // CHECK: bb0(%0 : $*U, %1 : $*T)
+  // CHECK: bb0(%0 : $*U, %1 : $*Never, %2 : $*T)
   return source.typedMap { $0 as! U }
 }