[dev.typeparams] import stmt changes from dev.go2go

Import logic for typechecking statements involving generics from the
dev.go2go branch.  Notably, range type checking was simplified in
dev.go2go, resulting in the removal of the _InvalidChanRange error code.

Change-Id: I84c2665226c2b9b74e85f7fb6df257b0a292e5d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/282120
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>

src/go/types/errorcodes.go

@@ -1038,18 +1038,6 @@ const (
 	//  }
 	_InvalidPostDecl
 
-	// _InvalidChanRange occurs when a send-only channel used in a range
-	// expression.
-	//
-	// Example:
-	//  func sum(c chan<- int) {
-	//  	s := 0
-	//  	for i := range c {
-	//  		s += i
-	//  	}
-	//  }
-	_InvalidChanRange
-
 	// _InvalidIterVar occurs when two iteration variables are used while ranging
 	// over a channel.
 	//

src/go/types/stmt.go

@@ -49,6 +49,11 @@ func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body
 		check.error(atPos(body.Rbrace), _MissingReturn, "missing return")
 	}
 
+	// TODO(gri) Should we make it an error to declare generic functions
+	//           where the type parameters are not used?
+	// 12/19/2018: Probably not - it can make sense to have an API with
+	//           all functions uniformly sharing the same type parameters.
+
 	// spec: "Implementation restriction: A compiler may make it illegal to
 	// declare a variable inside a function body if the variable is never used."
 	check.usage(sig.scope)
@@ -147,9 +152,9 @@ func (check *Checker) multipleDefaults(list []ast.Stmt) {
 	}
 }
 
-func (check *Checker) openScope(s ast.Node, comment string) {
-	scope := NewScope(check.scope, s.Pos(), s.End(), comment)
-	check.recordScope(s, scope)
+func (check *Checker) openScope(node ast.Node, comment string) {
+	scope := NewScope(check.scope, node.Pos(), node.End(), comment)
+	check.recordScope(node, scope)
 	check.scope = scope
 }
 
@@ -273,6 +278,9 @@ L:
 		if T == Typ[Invalid] {
 			continue L
 		}
+		if T != nil {
+			check.ordinaryType(e, T)
+		}
 		// look for duplicate types
 		// (quadratic algorithm, but type switches tend to be reasonably small)
 		for t, other := range seen {
@@ -355,8 +363,8 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			return
 		}
 
-		tch, ok := ch.typ.Underlying().(*Chan)
-		if !ok {
+		tch := asChan(ch.typ)
+		if tch == nil {
 			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-chan type %s", ch.typ)
 			return
 		}
@@ -609,7 +617,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			return
 		}
 
-		// rhs must be of the form: expr.(type) and expr must be an interface
+		// rhs must be of the form: expr.(type) and expr must be an ordinary interface
 		expr, _ := rhs.(*ast.TypeAssertExpr)
 		if expr == nil || expr.Type != nil {
 			check.invalidAST(s, "incorrect form of type switch guard")
@@ -620,11 +628,12 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		if x.mode == invalid {
 			return
 		}
-		xtyp, _ := x.typ.Underlying().(*Interface)
+		xtyp, _ := under(x.typ).(*Interface)
 		if xtyp == nil {
 			check.errorf(&x, _InvalidTypeSwitch, "%s is not an interface", &x)
 			return
 		}
+		check.ordinaryType(&x, xtyp)
 
 		check.multipleDefaults(s.Body.List)
 
@@ -761,43 +770,22 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		// determine key/value types
 		var key, val Type
 		if x.mode != invalid {
-			switch typ := x.typ.Underlying().(type) {
-			case *Basic:
-				if isString(typ) {
-					key = Typ[Int]
-					val = universeRune // use 'rune' name
-				}
-			case *Array:
-				key = Typ[Int]
-				val = typ.elem
-			case *Slice:
-				key = Typ[Int]
-				val = typ.elem
-			case *Pointer:
-				if typ, _ := typ.base.Underlying().(*Array); typ != nil {
-					key = Typ[Int]
-					val = typ.elem
-				}
-			case *Map:
-				key = typ.key
-				val = typ.elem
-			case *Chan:
-				key = typ.elem
-				val = Typ[Invalid]
-				if typ.dir == SendOnly {
-					check.errorf(&x, _InvalidChanRange, "cannot range over send-only channel %s", &x)
-					// ok to continue
-				}
-				if s.Value != nil {
-					check.errorf(atPos(s.Value.Pos()), _InvalidIterVar, "iteration over %s permits only one iteration variable", &x)
-					// ok to continue
-				}
+			typ := optype(x.typ)
+			if _, ok := typ.(*Chan); ok && s.Value != nil {
+				// TODO(gri) this also needs to happen for channels in generic variables
+				check.softErrorf(atPos(s.Value.Pos()), _InvalidIterVar, "range over %s permits only one iteration variable", &x)
+				// ok to continue
+			}
+			var msg string
+			key, val, msg = rangeKeyVal(typ, isVarName(s.Key), isVarName(s.Value))
+			if key == nil || msg != "" {
+				if msg != "" {
+					// TODO(rFindley) should this be parenthesized, to be consistent with other qualifiers?
+					msg = ": " + msg
+				}
+				check.softErrorf(&x, _InvalidRangeExpr, "cannot range over %s%s", &x, msg)
+				// ok to continue
 			}
-		}
-
-		if key == nil {
-			check.errorf(&x, _InvalidRangeExpr, "cannot range over %s", &x)
-			// ok to continue
 		}
 
 		// check assignment to/declaration of iteration variables
@@ -879,3 +867,72 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		check.invalidAST(s, "invalid statement")
 	}
 }
+
+// isVarName reports whether x is a non-nil, non-blank (_) expression.
+func isVarName(x ast.Expr) bool {
+	if x == nil {
+		return false
+	}
+	ident, _ := unparen(x).(*ast.Ident)
+	return ident == nil || ident.Name != "_"
+}
+
+// rangeKeyVal returns the key and value type produced by a range clause
+// over an expression of type typ, and possibly an error message. If the
+// range clause is not permitted the returned key is nil or msg is not
+// empty (in that case we still may have a non-nil key type which can be
+// used to reduce the chance for follow-on errors).
+// The wantKey, wantVal, and hasVal flags indicate which of the iteration
+// variables are used or present; this matters if we range over a generic
+// type where not all keys or values are of the same type.
+func rangeKeyVal(typ Type, wantKey, wantVal bool) (Type, Type, string) {
+	switch typ := typ.(type) {
+	case *Basic:
+		if isString(typ) {
+			return Typ[Int], universeRune, "" // use 'rune' name
+		}
+	case *Array:
+		return Typ[Int], typ.elem, ""
+	case *Slice:
+		return Typ[Int], typ.elem, ""
+	case *Pointer:
+		if typ := asArray(typ.base); typ != nil {
+			return Typ[Int], typ.elem, ""
+		}
+	case *Map:
+		return typ.key, typ.elem, ""
+	case *Chan:
+		var msg string
+		if typ.dir == SendOnly {
+			msg = "send-only channel"
+		}
+		return typ.elem, Typ[Invalid], msg
+	case *Sum:
+		first := true
+		var key, val Type
+		var msg string
+		typ.is(func(t Type) bool {
+			k, v, m := rangeKeyVal(under(t), wantKey, wantVal)
+			if k == nil || m != "" {
+				key, val, msg = k, v, m
+				return false
+			}
+			if first {
+				key, val, msg = k, v, m
+				first = false
+				return true
+			}
+			if wantKey && !Identical(key, k) {
+				key, val, msg = nil, nil, "all possible values must have the same key type"
+				return false
+			}
+			if wantVal && !Identical(val, v) {
+				key, val, msg = nil, nil, "all possible values must have the same element type"
+				return false
+			}
+			return true
+		})
+		return key, val, msg
+	}
+	return nil, nil, ""
+}

src/go/types/testdata/stmt0.src

@@ -886,7 +886,7 @@ func rangeloops1() {
 		ee = e
 		_ = ee
 	}
-	for _ = range sc /* ERROR "cannot range over send-only channel" */ {}
+	for _ = range sc /* ERROR "cannot range over" */ {}
 	for _ = range rc {}
 
 	// constant strings