go/types: generalize instanceHash to accept any type, rename to typeHash

This is a port of CL 345791 to go/types.

Change-Id: I673c22ad8b668f07aae4117555b1c0efb273fb78
Reviewed-on: https://go-review.googlesource.com/c/go/+/346556
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/instantiate.go

@@ -119,7 +119,7 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, posList
 func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) Type {
 	switch t := typ.(type) {
 	case *Named:
-		h := instantiatedHash(t, targs)
+		h := typeHash(t, targs)
 		if check != nil {
 			// typ may already have been instantiated with identical type arguments. In
 			// that case, re-use the existing instance.

src/go/types/named.go

@@ -258,7 +258,7 @@ func (n *Named) expand(typMap map[string]*Named) *Named {
 					// type-checking pass. In that case we won't have a pre-existing
 					// typMap, but don't want to create a duplicate of the current instance
 					// in the process of expansion.
-					h := instantiatedHash(n.orig, n.targs.list())
+					h := typeHash(n.orig, n.targs.list())
 					typMap = map[string]*Named{h: n}
 				}
 			}

src/go/types/stmt.go

@@ -302,7 +302,7 @@ L:
 				// talk about "case" rather than "type" because of nil case
 				Ts := "nil"
 				if T != nil {
-					Ts = T.String()
+					Ts = TypeString(T, check.qualifier)
 				}
 				check.errorf(e, _DuplicateCase, "duplicate case %s in type switch", Ts)
 				check.error(other, _DuplicateCase, "\tprevious case") // secondary error, \t indented
@@ -317,6 +317,47 @@ L:
 	return
 }
 
+// TODO(gri) Once we are certain that typeHash is correct in all situations, use this version of caseTypes instead.
+//           (Currently it may be possible that different types have identical names and import paths due to ImporterFrom.)
+//
+// func (check *Checker) caseTypes(x *operand, xtyp *Interface, types []ast.Expr, seen map[string]ast.Expr) (T Type) {
+// 	var dummy operand
+// L:
+// 	for _, e := range types {
+// 		// The spec allows the value nil instead of a type.
+// 		var hash string
+// 		if check.isNil(e) {
+// 			check.expr(&dummy, e) // run e through expr so we get the usual Info recordings
+// 			T = nil
+// 			hash = "<nil>" // avoid collision with a type named nil
+// 		} else {
+// 			T = check.varType(e)
+// 			if T == Typ[Invalid] {
+// 				continue L
+// 			}
+// 			hash = typeHash(T, nil)
+// 		}
+// 		// look for duplicate types
+// 		if other := seen[hash]; other != nil {
+// 			// talk about "case" rather than "type" because of nil case
+// 			Ts := "nil"
+// 			if T != nil {
+// 				Ts = TypeString(T, check.qualifier)
+// 			}
+// 			var err error_
+// 			err.errorf(e, "duplicate case %s in type switch", Ts)
+// 			err.errorf(other, "previous case")
+// 			check.report(&err)
+// 			continue L
+// 		}
+// 		seen[hash] = e
+// 		if T != nil {
+// 			check.typeAssertion(e.Pos(), x, xtyp, T)
+// 		}
+// 	}
+// 	return
+// }
+
 // stmt typechecks statement s.
 func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 	// statements must end with the same top scope as they started with

src/go/types/subst.go

@@ -217,7 +217,7 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 		// before creating a new named type, check if we have this one already
-		h := instantiatedHash(t, newTArgs)
+		h := typeHash(t, newTArgs)
 		dump(">>> new type hash: %s", h)
 		if named, found := subst.typMap[h]; found {
 			dump(">>> found %s", named)
@@ -256,17 +256,29 @@ func (subst *subster) typ(typ Type) Type {
 	return typ
 }
 
-var instanceHashing = 0
+var typeHashing = 0
 
-func instantiatedHash(typ *Named, targs []Type) string {
+// typeHash returns a string representation of typ, which can be used as an exact
+// type hash: types that are identical produce identical string representations.
+// If typ is a *Named type and targs is not empty, typ is printed as if it were
+// instantiated with targs.
+func typeHash(typ Type, targs []Type) string {
+	assert(typ != nil)
 	var buf bytes.Buffer
 
-	assert(instanceHashing == 0)
-	instanceHashing++
+	assert(typeHashing == 0)
+	typeHashing++
 	w := newTypeWriter(&buf, nil)
-	w.typeName(typ.obj)
+	if named, _ := typ.(*Named); named != nil && len(targs) > 0 {
+		// Don't use WriteType because we need to use the provided targs
+		// and not any targs that might already be with the *Named type.
+		w.typeName(named.obj)
 		w.typeList(targs)
-	instanceHashing--
+	} else {
+		assert(targs == nil)
+		w.typ(typ)
+	}
+	typeHashing--
 
 	if debug {
 		// there should be no instance markers in type hashes

src/go/types/typestring.go

@@ -208,7 +208,7 @@ func (w *typeWriter) typ(typ Type) {
 		// types. Write them to aid debugging, but don't write
 		// them when we need an instance hash: whether a type
 		// is fully expanded or not doesn't matter for identity.
-		if instanceHashing == 0 && t.instPos != nil {
+		if typeHashing == 0 && t.instPos != nil {
 			w.byte(instanceMarker)
 		}
 		w.typeName(t.obj)
@@ -292,7 +292,7 @@ func (w *typeWriter) tParamList(list []*TypeParam) {
 
 func (w *typeWriter) typeName(obj *TypeName) {
 	if obj == nil {
-		assert(instanceHashing == 0) // we need an object for instance hashing
+		assert(typeHashing == 0) // we need an object for type hashing
 		w.string("<Named w/o object>")
 		return
 	}
@@ -301,10 +301,10 @@ func (w *typeWriter) typeName(obj *TypeName) {
 	}
 	w.string(obj.name)
 
-	if instanceHashing != 0 {
+	if typeHashing != 0 {
 		// For local defined types, use the (original!) TypeName's scope
 		// numbers to disambiguate.
-		typ := obj.typ.(*Named)
+		if typ, _ := obj.typ.(*Named); typ != nil {
 			// TODO(gri) Figure out why typ.orig != typ.orig.orig sometimes
 			//           and whether the loop can iterate more than twice.
 			//           (It seems somehow connected to instance types.)
@@ -313,6 +313,7 @@ func (w *typeWriter) typeName(obj *TypeName) {
 			}
 			w.writeScopeNumbers(typ.obj.parent)
 		}
+	}
 }
 
 // writeScopeNumbers writes the number sequence for this scope to buf
@@ -333,7 +334,8 @@ func (w *typeWriter) tuple(tup *Tuple, variadic bool) {
 			if i > 0 {
 				w.string(", ")
 			}
-			if v.name != "" {
+			// parameter names are ignored for type identity and thus type hashes
+			if typeHashing == 0 && v.name != "" {
 				w.string(v.name)
 				w.byte(' ')
 			}
@@ -381,8 +383,8 @@ func (w *typeWriter) signature(sig *Signature) {
 	}
 
 	w.byte(' ')
-	if n == 1 && sig.results.vars[0].name == "" {
-		// single unnamed result
+	if n == 1 && (typeHashing != 0 || sig.results.vars[0].name == "") {
+		// single unnamed result (if typeHashing, name must be ignored)
 		w.typ(sig.results.vars[0].typ)
 		return
 	}