IRGen: Don't attempt to emit debug info for type-erased generic params.

Fixes a crash when emitting debug info for ObjC generic extensions. Part of rdar://problem/26867815.

Author: jckarter

lib/IRGen/GenMeta.cpp

@@ -267,8 +267,8 @@ static bool isTypeErasedGenericClassType(CanType type) {
 }
 
 // Get the type that exists at runtime to represent a compile-time type.
-static CanType
-getRuntimeReifiedType(IRGenModule &IGM, CanType type) {
+CanType
+irgen::getRuntimeReifiedType(IRGenModule &IGM, CanType type) {
   return CanType(type.transform([&](Type t) -> Type {
     if (isTypeErasedGenericClassType(CanType(t))) {
       return t->getAnyNominal()->getDeclaredType()->getCanonicalType();

lib/IRGen/GenMeta.h

@@ -85,6 +85,10 @@ namespace irgen {
                                                    CanType type,
                                                    SymbolReferenceKind refKind);
 
+  /// Get the type as it exists in Swift's runtime type system, removing any
+  /// erased generic parameters.
+  CanType getRuntimeReifiedType(IRGenModule &IGM, CanType type);
+
   /// Emit a reference to a compile-time constant piece of heap metadata, or
   /// return a null pointer if the type's heap metadata cannot be represented
   /// by a constant.

lib/IRGen/IRGenSIL.cpp

@@ -747,8 +747,10 @@ class IRGenSILFunction :
     // Force all archetypes referenced by the type to be bound by this point.
     // TODO: just make sure that we have a path to them that the debug info
     //       can follow.
-    if (!IGM.IRGen.Opts.Optimize && Ty.getType()->hasArchetype())
-      Ty.getType()->getCanonicalType().visit([&](Type t) {
+    auto runtimeTy = getRuntimeReifiedType(IGM,
+                                           Ty.getType()->getCanonicalType());
+    if (!IGM.IRGen.Opts.Optimize && runtimeTy->hasArchetype())
+      runtimeTy.visit([&](Type t) {
         if (auto archetype = dyn_cast<ArchetypeType>(CanType(t)))
           emitTypeMetadataRef(archetype);
        });

test/IRGen/objc_generic_class_debug_info.swift

@@ -0,0 +1,13 @@
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) %s -emit-ir -g -verify
+
+// REQUIRES: objc_interop
+
+import Swift
+import Foundation
+import objc_generics
+
+extension GenericClass {
+  func method() {}
+  class func classMethod() {}
+}