[OpaqueValues] Map back-deploy thunk results.

When a throwing function which returns a generic (this is the simplest
example) is back deployed, it gets a back deploy thunk.  This back
deploy thunk calls one of two variations of the function, depending on
availability, both which have the same signature and in particular both
return a generic.  The result type of that function signature is an
unbound generic.

Previously, that result type was used as is.  Consequently the success
blocks for the try_apply instructions in these thunks had arguments of
unbound generic type.

Here, that type is mapped into the context of the function being
emitted.  The thunks now have the appropriate bound generic type.

Author: nate-chandler

lib/SILGen/SILGenBackDeploy.cpp

@@ -133,14 +133,16 @@ static void emitBackDeployForwardApplyAndReturnOrThrow(
 
     // Emit error block.
     SGF.B.emitBlock(errorBB);
-    SILValue error = errorBB->createPhiArgument(fnConv.getSILErrorType(TEC),
-                                                OwnershipKind::Owned);
+    SILValue error = errorBB->createPhiArgument(
+        SGF.F.mapTypeIntoContext(fnConv.getSILErrorType(TEC)),
+        OwnershipKind::Owned);
     SGF.B.createBranch(loc, SGF.ThrowDest.getBlock(), {error});
 
     // Emit normal block.
     SGF.B.emitBlock(normalBB);
-    SILValue result = normalBB->createPhiArgument(fnConv.getSILResultType(TEC),
-                                                  OwnershipKind::Owned);
+    SILValue result = normalBB->createPhiArgument(
+        SGF.F.mapTypeIntoContext(fnConv.getSILResultType(TEC)),
+        OwnershipKind::Owned);
     SmallVector<SILValue, 4> directResults;
     extractAllElements(result, loc, SGF.B, directResults);
 

test/SILGen/opaque_values_silgen.swift

@@ -636,3 +636,12 @@ func takeKeyPathString<T>(_ kp: KeyPath<T, String>) {
 func giveKeyPathString() {
   takeKeyPathString(\StructWithAReadableStringProperty.s)
 }
+
+// CHECK-LABEL: sil {{.*}}@$s20opaque_values_silgen29backDeployingReturningGenericyxxKlFTwb : {{.*}} <T> {{.*}} {
+// Ensure that there aren't any "normal" (in the sense of try_apply) blocks that
+// take unbound generic parameters (τ_0_0).
+// CHECK-NOT: {{bb[0-9]+}}({{%[^,]+}} : @owned $τ_0_0):
+// CHECK-LABEL: } // end sil function '$s20opaque_values_silgen29backDeployingReturningGenericyxxKlFTwb'
+@available(SwiftStdlib 5.1, *)
+@_backDeploy(before: SwiftStdlib 5.8)
+public func backDeployingReturningGeneric<T>(_ t: T) throws -> T { t }