Fix printing of types in reporter output

cmp/compare.go

@@ -40,6 +40,8 @@ import (
 	"github.com/google/go-cmp/cmp/internal/value"
 )
 
+// TODO(≥go1.18): Use any instead of interface{}.
+
 // Equal reports whether x and y are equal by recursively applying the
 // following rules in the given order to x and y and all of their sub-values:
 //

cmp/compare_test.go

@@ -104,6 +104,7 @@ func mustFormatGolden(path string, in []struct{ Name, Data string }) {
 
 var now = time.Date(2009, time.November, 10, 23, 00, 00, 00, time.UTC)
 
+// TODO(≥go1.18): Define a generic function that boxes a value on the heap.
 func newInt(n int) *int { return &n }
 
 type Stringer string
@@ -885,6 +886,7 @@ func reporterTests() []test {
 			FloatsB []MyFloat
 			FloatsC MyFloats
 		}
+		PointerString *string
 	)
 
 	return []test{{
@@ -1351,6 +1353,38 @@ using the AllowUnexported option.`, "\n"),
 	"bar": true,
 }`,
 		reason: "short multiline JSON should prefer triple-quoted string diff as it is more readable",
+	}, {
+		label: label + "/PointerToStringOrAny",
+		x: func() *string {
+			var v string = "hello"
+			return &v
+		}(),
+		y: func() *interface{} {
+			var v interface{} = "hello"
+			return &v
+		}(),
+		reason: "mismatched types between any and *any should print differently",
+	}, {
+		label: label + "/NamedPointer",
+		x: func() *string {
+			v := "hello"
+			return &v
+		}(),
+		y: func() PointerString {
+			v := "hello"
+			return &v
+		}(),
+		reason: "mismatched pointer types should print differently",
+	}, {
+		label:  label + "/MapStringAny",
+		x:      map[string]interface{}{"key": int(0)},
+		y:      map[string]interface{}{"key": uint(0)},
+		reason: "mismatched underlying value within interface",
+	}, {
+		label:  label + "/StructFieldAny",
+		x:      struct{ X interface{} }{int(0)},
+		y:      struct{ X interface{} }{uint(0)},
+		reason: "mismatched underlying value within interface",
 	}}
 }
 

cmp/report_reflect.go

@@ -282,7 +282,12 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 		}
 		defer ptrs.Pop()
 
-		skipType = true // Let the underlying value print the type instead
+		// Skip the name only if this is an unnamed pointer type.
+		// Otherwise taking the address of a value does not reproduce
+		// the named pointer type.
+		if v.Type().Name() == "" {
+			skipType = true // Let the underlying value print the type instead
+		}
 		out = opts.FormatValue(v.Elem(), t.Kind(), ptrs)
 		out = wrapTrunkReference(ptrRef, opts.PrintAddresses, out)
 		out = &textWrap{Prefix: "&", Value: out}
@@ -293,7 +298,6 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 		}
 		// Interfaces accept different concrete types,
 		// so configure the underlying value to explicitly print the type.
-		skipType = true // Print the concrete type instead
 		return opts.WithTypeMode(emitType).FormatValue(v.Elem(), t.Kind(), ptrs)
 	default:
 		panic(fmt.Sprintf("%v kind not handled", v.Kind()))

cmp/testdata/diffs

@@ -1134,6 +1134,30 @@
   	"""
   )
 >>> TestDiff/Reporter/ShortJSON
+<<< TestDiff/Reporter/PointerToStringOrAny
+  any(
+- 	&string("hello"),
++ 	&any(string("hello")),
+  )
+>>> TestDiff/Reporter/PointerToStringOrAny
+<<< TestDiff/Reporter/NamedPointer
+  any(
+- 	&string("hello"),
++ 	cmp_test.PointerString(&string("hello")),
+  )
+>>> TestDiff/Reporter/NamedPointer
+<<< TestDiff/Reporter/MapStringAny
+  map[string]any{
+- 	"key": int(0),
++ 	"key": uint(0),
+  }
+>>> TestDiff/Reporter/MapStringAny
+<<< TestDiff/Reporter/StructFieldAny
+  struct{ X any }{
+- 	X: int(0),
++ 	X: uint(0),
+  }
+>>> TestDiff/Reporter/StructFieldAny
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{