SIL: Change rule regarding shared_external linkage

The existence of a shared_external function in itself is not
an error; it just means we deserialized a witness table or
vtable but did not need to deserialize a thunk.

However, a direct reference to such a function is an error,
because we should have deserialized the body in that case.

This fixes a crasher, but the SIL crashers are kind of silly
because the SIL parser does not try at all not to crash on
invalid input.

Author: slavapestov

lib/SIL/SILVerifier.cpp

@@ -1131,8 +1131,25 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
                                     "result of function_ref");
     require(!fnType->getExtInfo().hasContext(),
             "function_ref should have a context-free function result");
+
+    // Note: in SingleFunction mode, we relax some of these checks because
+    // we may not have linked everything yet.
+
+    SILFunction *RefF = FRI->getReferencedFunction();
+
+    // A direct reference to a shared_external declaration is an error; we
+    // should have deserialized a body.
+    if (RefF->isExternalDeclaration()) {
+      require(SingleFunction ||
+              !hasSharedVisibility(RefF->getLinkage()) ||
+              RefF->hasForeignBody(),
+              "external declarations of SILFunctions with shared visibility is "
+              "not allowed");
+    }
+
+    // A direct reference to a non-public or shared but not fragile function
+    // from a fragile function is an error.
     if (F.isSerialized()) {
-      SILFunction *RefF = FRI->getReferencedFunction();
       require((SingleFunction && RefF->isExternalDeclaration()) ||
               RefF->hasValidLinkageForFragileRef(),
               "function_ref inside fragile function cannot "
@@ -3912,9 +3929,6 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
 
       assert(F->isAvailableExternally() &&
              "external declaration of internal SILFunction not allowed");
-      assert(!hasSharedVisibility(F->getLinkage()) &&
-             "external declarations of SILFunctions with shared visibility is not "
-             "allowed");
       // If F is an external declaration, there is nothing further to do,
       // return.
       return;

validation-test/SIL/crashers/005-swift-silfunction-verify.sil

@@ -0,0 +1,2 @@
+// RUN: %target-sil-opt %s
+sil shared_external@a:$()->()