cmd/compile/internal/types2: generalize instanceHash to accept any type, rename to typeHash

Rename instanceHashing accordingly.

Eventually, this will make it possible to use typeHash to detect
multiple identical types in type switch cases and other places.

Also fix some bugs: When creating a type hash, the name of function
parameters must be ignored because they don't matter for type
identity. And when printing a type name, don't assume its type
is a *Named type; it could be a *Basic type as well.

Finally, use a correctly qualified type string when reporting
a duplicate type error in a type switch case rather than the
(debugging) type string.

Change-Id: Ida3873f6259b51847843b0e2d7e3aa2fcdc3a0c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/345791
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/instantiate.go

@@ -125,7 +125,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 func (check *Checker) instance(pos syntax.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/cmd/compile/internal/types2/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/cmd/compile/internal/types2/stmt.go

@@ -295,7 +295,7 @@ L:
 				// talk about "case" rather than "type" because of nil case
 				Ts := "nil"
 				if T != nil {
-					Ts = T.String()
+					Ts = TypeString(T, check.qualifier)
 				}
 				var err error_
 				err.errorf(e, "duplicate case %s in type switch", Ts)
@@ -312,6 +312,47 @@ 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 []syntax.Expr, seen map[string]syntax.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 syntax.Stmt) {
 	// statements must end with the same top scope as they started with

src/cmd/compile/internal/types2/subst.go

@@ -214,7 +214,7 @@ 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)
@@ -253,17 +253,29 @@ 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/cmd/compile/internal/types2/typestring.go

@@ -207,7 +207,7 @@ 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)
@@ -291,7 +291,7 @@ 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
 	}
@@ -300,10 +300,10 @@ 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.)
@@ -312,6 +312,7 @@ func (w *typeWriter) typeName(obj *TypeName) {
 			}
 			w.writeScopeNumbers(typ.obj.parent)
 		}
+	}
 }
 
 // writeScopeNumbers writes the number sequence for this scope to buf
@@ -332,7 +333,8 @@ 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(' ')
 			}
@@ -380,8 +382,8 @@ 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
 	}