SILGen: Fix crash when thrown error type is loadable and has a type parameter

Fixes swiftlang#74289.
Fixes rdar://problem/146677409.

Author: slavapestov

lib/SILGen/SILGenEpilog.cpp

@@ -83,10 +83,12 @@ void SILGenFunction::prepareEpilog(
 void SILGenFunction::prepareRethrowEpilog(
     DeclContext *dc, AbstractionPattern origErrorType, Type errorType,
     CleanupLocation cleanupLoc) {
+  ASSERT(!errorType->hasPrimaryArchetype());
 
   SILBasicBlock *rethrowBB = createBasicBlock(FunctionSection::Postmatter);
   if (!IndirectErrorResult) {
-    SILType loweredErrorType = getLoweredType(origErrorType, errorType);
+    auto errorTypeInContext = dc->mapTypeIntoContext(errorType);
+    SILType loweredErrorType = getLoweredType(origErrorType, errorTypeInContext);
     rethrowBB->createPhiArgument(loweredErrorType, OwnershipKind::Owned);
   }
 

test/SILGen/typed_throws_generic.swift

@@ -387,3 +387,10 @@ extension ReducedError where T == MyError {
     throw MyError.fail
   }
 }
+
+// https://github.com/swiftlang/swift/issues/74289
+struct LoadableGeneric<E>: Error {}
+
+func throwsLoadableGeneric<E>(_: E) throws(LoadableGeneric<E>) {
+  throw LoadableGeneric<E>()
+}