Prespecialization: reduce the strength of an assertion.

If we find a specialization within the current module, it's okay if it
has a body. This can come up if we compile something with optimizations
and then compile the resulting SIL (or SIB) without optimizations.

Author: jrose-apple

lib/SILOptimizer/IPO/UsePrespecialized.cpp

@@ -135,15 +135,14 @@ bool UsePrespecialized::replaceByPrespecialized(SILFunction &F) {
             << PrevF << "\n");
       if (!PrevF)
         continue;
+      assert(PrevF->isExternalDeclaration() &&
+             "Prespecialized function should be an external declaration");
       NewF = PrevF;
     }
 
     if (!NewF)
       continue;
 
-    assert(NewF->isExternalDeclaration() &&
-           "Prespecialized function should be an external declaration");
-
     // An existing specialization was found.
     DEBUG(llvm::dbgs() << "Found a specialization of " << ReferencedF->getName()
                        << " : " << NewF->getName() << "\n");

test/SILOptimizer/prespecialization_with_definition.sil

@@ -0,0 +1,38 @@
+// RUN: %target-swift-frontend -emit-ir %s -module-name main | FileCheck %s
+
+sil_stage canonical
+
+struct Box<T> {
+  @inline(never)
+  static func foo() {}
+}
+
+// Reference a function in a way that will be specialized.
+sil @test : $@convention(thin) () -> () {
+bb0:
+  // function_ref static Box.foo() -> ()
+  %0 = function_ref @_TZFV4main3Box3foofT_T_ : $@convention(thin) <τ_0_0> (@thin Box<τ_0_0>.Type) -> () // user: %2
+  %1 = metatype $@thin Box<()>.Type               // user: %2
+  %2 = apply %0<()>(%1) : $@convention(thin) <τ_0_0> (@thin Box<τ_0_0>.Type) -> ()
+  %3 = tuple ()                                   // user: %4
+  return %3 : $()                                 // id: %4
+}
+
+// This is the unspecialized form.
+sil hidden @_TZFV4main3Box3foofT_T_ : $@convention(thin) <T> (@thin Box<T>.Type) -> () {
+bb0(%0 : $@thin Box<T>.Type):
+  %1 = tuple ()
+  return %1 : $()
+}
+
+// generic specialization <()> of static main.Box.foo () -> ()
+// This specialization is one that would be matched by the prespecialization
+// pass.
+sil shared @_TTSg5T____TZFV4main3Box3foofT_T_ : $@convention(thin) (@thin Box<()>.Type) -> () {
+bb0(%0 : $@thin Box<()>.Type):
+  %1 = tuple ()
+  return %1 : $()
+}
+
+// CHECK-DAG: define linkonce_odr hidden void @_TTSg5T____TZFV4main3Box3foofT_T_() {{.*}}{
+// CHECK-DAG: call void @_TTSg5T____TZFV4main3Box3foofT_T_()