[AST] Make ValueDecls displayable via their DeclRefs.

Author: DougGregor

include/swift/AST/Decl.h

@@ -37,6 +37,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/Support/TrailingObjects.h"
+#include <type_traits>
 
 namespace swift {
   enum class AccessSemantics : unsigned char;
@@ -6765,6 +6766,9 @@ inline EnumElementDecl *EnumDecl::getUniqueElement(bool hasValue) const {
 std::pair<DefaultArgumentKind, Type>
 getDefaultArgumentInfo(ValueDecl *source, unsigned Index);
 
+/// Display ValueDecl subclasses.
+void simple_display(llvm::raw_ostream &out, const ValueDecl *&decl);
+
 } // end namespace swift
 
 #endif

include/swift/Basic/SimpleDisplay.h

@@ -25,7 +25,36 @@
 
 namespace swift {
   template<typename T>
-  void simple_display(llvm::raw_ostream &out, const T &value) {
+  struct HasTrivialDisplay {
+    static const bool value = false;
+  };
+
+#define HAS_TRIVIAL_DISPLAY(Type)     \
+  template<>                          \
+  struct HasTrivialDisplay<Type> {    \
+    static const bool value = true;   \
+  }
+
+  HAS_TRIVIAL_DISPLAY(unsigned char);
+  HAS_TRIVIAL_DISPLAY(signed char);
+  HAS_TRIVIAL_DISPLAY(char);
+  HAS_TRIVIAL_DISPLAY(short);
+  HAS_TRIVIAL_DISPLAY(unsigned short);
+  HAS_TRIVIAL_DISPLAY(int);
+  HAS_TRIVIAL_DISPLAY(unsigned int);
+  HAS_TRIVIAL_DISPLAY(long);
+  HAS_TRIVIAL_DISPLAY(unsigned long);
+  HAS_TRIVIAL_DISPLAY(long long);
+  HAS_TRIVIAL_DISPLAY(unsigned long long);
+  HAS_TRIVIAL_DISPLAY(float);
+  HAS_TRIVIAL_DISPLAY(double);
+  HAS_TRIVIAL_DISPLAY(bool);
+
+#undef HAS_TRIVIAL_DISPLAY
+
+  template<typename T>
+  typename std::enable_if<HasTrivialDisplay<T>::value>::type
+  simple_display(llvm::raw_ostream &out, const T &value) {
     out << value;
   }
 

lib/AST/Decl.cpp

@@ -5731,3 +5731,8 @@ NominalTypeDecl *TypeOrExtensionDecl::getBaseNominal() const {
   return getAsDeclContext()->getAsNominalTypeOrNominalTypeExtensionContext();
 }
 bool TypeOrExtensionDecl::isNull() const { return Decl.isNull(); }
+
+void swift::simple_display(llvm::raw_ostream &out, const ValueDecl *&decl) {
+  if (decl) decl->dumpRef(out);
+  else out << "(null)";
+}

test/decl/class/circular_inheritance.swift

@@ -42,7 +42,7 @@ class Outer3
 }
 
 // CHECK: ===CYCLE DETECTED===
-// CHECK-NEXT: `--{{.*}}SuperclassTypeRequest
+// CHECK-NEXT: `--{{.*}}SuperclassTypeRequest({{.*Left}}
 // CHECK-NEXT:      `--{{.*}}InheritedTypeRequest(circular_inheritance.(file).Left@
 // CHECK-NEXT:          `--{{.*}}SuperclassTypeRequest
 // CHECK-NEXT:              `--{{.*}}InheritedTypeRequest(circular_inheritance.(file).Right@