SILVerifier: allow that a shared_external function references a shared function.

A serialized specialized function can reference another specialized function.
In case the other specialization is already generated by the optimizer before the de-serialization point,
we can end up that a shared_external function references a shared function. This is okay.

Author: eeckstein

lib/SIL/Verifier/SILVerifier.cpp

@@ -2074,7 +2074,16 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
     // from a fragile function is an error.
     if (F.isSerialized()) {
       require((SingleFunction && RefF->isExternalDeclaration()) ||
-              RefF->hasValidLinkageForFragileRef(),
+              RefF->hasValidLinkageForFragileRef() ||
+
+              // A serialized specialized function can reference another
+              // specialized function. In case the other specialization is already
+              // generated by the optimizer before the de-serialization point,
+              // we can end up that a shared_external function references a
+              // shared function. This is okay.
+              (F.getLinkage() == SILLinkage::SharedExternal &&
+               RefF->getLinkage() == SILLinkage::Shared),
+
               "function_ref inside fragile function cannot "
               "reference a private or hidden symbol");
     }