IRGen: Fix assertion failure with typed throws

The mapTypeIntoContext() call in visitFullApplySite() was not
necessary.

The type in question is already fully substituted and should
no longer contain type parameters.

However, it can contain archetypes, which is what caused
mapTypeIntoContext() to assert.

Indeed, this case where the return type is generic but still
loadable wasn't covered by our test suite.

- Fixes #80020.
- Fixes rdar://147051717.

Author: slavapestov

lib/IRGen/IRGenSIL.cpp

@@ -4040,8 +4040,9 @@ void IRGenSILFunction::visitFullApplySite(FullApplySite site) {
       Builder.emitBlock(typedErrorLoadBB);
 
       auto &errorTI = cast<LoadableTypeInfo>(IGM.getTypeInfo(errorType));
-      auto silResultTy = CurSILFn->mapTypeIntoContext(
-          substConv.getSILResultType(IGM.getMaximalTypeExpansionContext()));
+      auto silResultTy =
+          substConv.getSILResultType(IGM.getMaximalTypeExpansionContext());
+      ASSERT(!silResultTy.hasTypeParameter());
       auto &resultTI = cast<LoadableTypeInfo>(IGM.getTypeInfo(silResultTy));
 
       auto &resultSchema = resultTI.nativeReturnValueSchema(IGM);

test/IRGen/typed_throws_generic.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-irgen
+
+// https://github.com/swiftlang/swift/issues/80020
+//
+// We used to assert if you had a loadable return type that contained
+// a generic parameter.
+
+public enum MyError: Error {
+  case error
+}
+
+public struct G<T> {}  // Note: G<T> is loadable
+
+public func f<T>(t: T) throws(MyError) -> G<T> {
+  return G<T>()
+}
+
+public func g<U>(u: U?) throws(MyError) -> G<U?> {
+  return try f(t: u)
+}