diff --git a/lib/SILAnalysis/CallGraphAnalysis.cpp b/lib/SILAnalysis/CallGraphAnalysis.cpp
index 2e8c6cc6114..7f4bc06de8b 100644
--- a/lib/SILAnalysis/CallGraphAnalysis.cpp
+++ b/lib/SILAnalysis/CallGraphAnalysis.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/SILAnalysis/CallGraphAnalysis.h"
+#include "swift/Basic/Fallthrough.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
@@ -64,6 +65,9 @@ bool CallGraph::tryGetCalleeSet(SILValue Callee,
                                 bool &Complete) {
 
   switch (Callee->getKind()) {
+  case ValueKind::ThinToThickFunctionInst:
+    Callee = cast<ThinToThickFunctionInst>(Callee)->getOperand();
+    SWIFT_FALLTHROUGH;
   case ValueKind::FunctionRefInst: {
     auto *CalleeFn = cast<FunctionRefInst>(Callee)->getReferencedFunction();
     auto *CalleeNode = getCallGraphNode(CalleeFn);
@@ -82,14 +86,6 @@ bool CallGraph::tryGetCalleeSet(SILValue Callee,
     //       graph.
     return false;
 
-  case ValueKind::ThinToThickFunctionInst: {
-    auto ThinCallee = cast<ThinToThickFunctionInst>(Callee)->getOperand();
-    // TODO: We want to see through these to the underlying function.
-    assert(cast<FunctionRefInst>(ThinCallee) && "Expected function reference!");
-    (void)ThinCallee;
-    return false;
-  }
-
   case ValueKind::SILArgument:
     // First-pass call-graph construction will not do anything with
     // these, but a second pass can potentially statically determine
diff --git a/test/DebugInfo/closure-multivalue.swift b/test/DebugInfo/closure-multivalue.swift
index 235dd5a453e..562324ae63d 100644
--- a/test/DebugInfo/closure-multivalue.swift
+++ b/test/DebugInfo/closure-multivalue.swift
@@ -1,5 +1,5 @@
 // RUN: %target-swift-frontend -Xllvm -debug-values-propagate-liveness -O %s -disable-llvm-optzns -emit-ir -g -o - | FileCheck %s
-// CHECK: define {{.*}}4main4sort
+// CHECK: define hidden i1 {{.*}}4main4sort
 // CHECK: call void @llvm.dbg.value({{.*}}, metadata ![[A:.*]], metadata ![[P1:.*]])
 // CHECK: call void @llvm.dbg.value({{.*}}, metadata ![[A]], metadata ![[P2:.*]])
 // CHECK: call void @llvm.dbg.value({{.*}}, metadata ![[A]], metadata ![[P3:.*]])
diff --git a/test/SILPasses/functionsigopts.sil b/test/SILPasses/functionsigopts.sil
index fae3f9b0823..d08847b88c1 100644
--- a/test/SILPasses/functionsigopts.sil
+++ b/test/SILPasses/functionsigopts.sil
@@ -107,9 +107,10 @@ bb0(%0 : $Builtin.NativeObject, %1 : $Builtin.NativeObject):
   return %5 : $()
 }
 
-// This test case verifies how dumb our implementation is currently.
+// This test case verifies we can specialize functions called through thin_to_thick_function.
 // CHECK-LABEL: sil [fragile] @dead_arg_obfuscated_callsite : $@thin (Builtin.NativeObject, Builtin.NativeObject) -> () {
-// CHECK-NOT: function_ref @_TTSf4d_n__dead_arg_with_callsites : $@thin (Builtin.NativeObject) -> ()
+// CHECK: [[OPT_FUN:%[0-9]+]] = function_ref @_TTSf4d_n__dead_arg_with_callsites : $@thin (Builtin.NativeObject) -> ()
+// CHECK-NEXT: apply [[OPT_FUN]](
 sil [fragile] @dead_arg_obfuscated_callsite : $@thin (Builtin.NativeObject, Builtin.NativeObject) -> () {
 bb0(%0 : $Builtin.NativeObject, %1 : $Builtin.NativeObject):
   %2 = function_ref @dead_arg_with_callsites : $@thin (Builtin.NativeObject, Builtin.NativeObject) -> ()