go/types: slightly relax notion of structural type

This is a port of CL 363075 from types2 to go/types, adjusted for the
different error reporting API, and to adjust positions of error messages
in tests.

Change-Id: Ic6bfedf1152eff94bad20725b56e6ba804b2e3e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/363991
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/expr.go

@@ -174,29 +174,26 @@ func (check *Checker) unary(x *operand, e *ast.UnaryExpr) {
 		return
 
 	case token.ARROW:
-		var elem Type
-		if !underIs(x.typ, func(u Type) bool {
-			ch, _ := u.(*Chan)
-			if ch == nil {
-				check.invalidOp(x, _InvalidReceive, "cannot receive from non-channel %s", x)
-				return false
-			}
-			if ch.dir == SendOnly {
-				check.invalidOp(x, _InvalidReceive, "cannot receive from send-only channel %s", x)
-				return false
-			}
-			if elem != nil && !Identical(ch.elem, elem) {
-				check.invalidOp(x, _InvalidReceive, "channels of %s must have the same element type", x)
-				return false
-			}
-			elem = ch.elem
-			return true
-		}) {
+		u := structuralType(x.typ)
+		if u == nil {
+			check.invalidOp(x, _InvalidReceive, "cannot receive from %s: no structural type", x)
 			x.mode = invalid
 			return
 		}
+		ch, _ := u.(*Chan)
+		if ch == nil {
+			check.invalidOp(x, _InvalidReceive, "cannot receive from non-channel %s", x)
+			x.mode = invalid
+			return
+		}
+		if ch.dir == SendOnly {
+			check.invalidOp(x, _InvalidReceive, "cannot receive from send-only channel %s", x)
+			x.mode = invalid
+			return
+		}
+
 		x.mode = commaok
-		x.typ = elem
+		x.typ = ch.elem
 		check.hasCallOrRecv = true
 		return
 	}

src/go/types/stmt.go

@@ -417,27 +417,21 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		if ch.mode == invalid || val.mode == invalid {
 			return
 		}
-		var elem Type
-		if !underIs(ch.typ, func(u Type) bool {
-			uch, _ := u.(*Chan)
-			if uch == nil {
-				check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-channel %s", &ch)
-				return false
-			}
-			if uch.dir == RecvOnly {
-				check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to receive-only channel %s", &ch)
-				return false
-			}
-			if elem != nil && !Identical(uch.elem, elem) {
-				check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "channels of %s must have the same element type", &ch)
-				return false
-			}
-			elem = uch.elem
-			return true
-		}) {
+		u := structuralType(ch.typ)
+		if u == nil {
+			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to %s: no structural type", &ch)
 			return
 		}
-		check.assignment(&val, elem, "send")
+		uch, _ := u.(*Chan)
+		if uch == nil {
+			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-channel %s", &ch)
+			return
+		}
+		if uch.dir == RecvOnly {
+			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to receive-only channel %s", &ch)
+			return
+		}
+		check.assignment(&val, uch.elem, "send")
 
 	case *ast.IncDecStmt:
 		var op token.Token

src/go/types/testdata/check/typeparams.go2

@@ -210,7 +210,7 @@ func _[
         for _, _ /* ERROR permits only one iteration variable */ = range c1 {}
 
         var c2 C2
-        for range c2 /* ERROR cannot range over c2.*no structural type */ {}
+        for range c2 {}
 
         var c3 C3
         for range c3 /* ERROR receive from send-only channel */ {}

src/go/types/testdata/fixedbugs/issue43671.go2

@@ -12,11 +12,11 @@ type C4 interface{ chan int | chan<- int }
 type C5[T any] interface{ ~chan T | <-chan T }
 
 func _[T any](ch T) {
-	<-ch // ERROR cannot receive from non-channel
+	<-ch // ERROR cannot receive from ch .* no structural type
 }
 
 func _[T C0](ch T) {
-	<-ch // ERROR cannot receive from non-channel
+	<-ch // ERROR cannot receive from non-channel ch
 }
 
 func _[T C1](ch T) {
@@ -28,7 +28,7 @@ func _[T C2](ch T) {
 }
 
 func _[T C3](ch T) {
-	<-ch // ERROR channels of ch .* must have the same element type
+	<-ch // ERROR cannot receive from ch .* no structural type
 }
 
 func _[T C4](ch T) {

src/go/types/testdata/fixedbugs/issue45920.go2

@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[T any, C chan T | <-chan T](ch C) {}
+
+func _(ch chan int)   { f1(ch) }
+func _(ch <-chan int) { f1(ch) }
+func _(ch chan<- int) { f1 /* ERROR chan<- int does not satisfy chan T\|<-chan T */ (ch) }
+
+func f2[T any, C chan T | chan<- T](ch C) {}
+
+func _(ch chan int)   { f2(ch) }
+func _(ch <-chan int) { f2 /* ERROR <-chan int does not satisfy chan T\|chan<- T */ (ch)}
+func _(ch chan<- int) { f2(ch) }

src/go/types/testdata/fixedbugs/issue47115.go2

@@ -12,7 +12,7 @@ type C4 interface{ chan int | chan<- int }
 type C5[T any] interface{ ~chan T | chan<- T }
 
 func _[T any](ch T) {
-	ch <- /* ERROR cannot send to non-channel */ 0
+	ch <- /* ERROR cannot send to ch .* no structural type */ 0
 }
 
 func _[T C0](ch T) {
@@ -28,7 +28,7 @@ func _[T C2](ch T) {
 }
 
 func _[T C3](ch T) {
-	ch <- /* ERROR channels of ch .* must have the same element type */ 0
+	ch <- /* ERROR cannot send to ch .* no structural type */ 0
 }
 
 func _[T C4](ch T) {

src/go/types/type.go

@@ -27,17 +27,51 @@ func under(t Type) Type {
 	return t
 }
 
+// If x and y are identical, match returns x.
+// If x and y are identical channels but for their direction
+// and one of them is unrestricted, match returns the channel
+// with the restricted direction.
+// In all other cases, match returns nil.
+func match(x, y Type) Type {
+	// Common case: we don't have channels.
+	if Identical(x, y) {
+		return x
+	}
+
+	// We may have channels that differ in direction only.
+	if x, _ := x.(*Chan); x != nil {
+		if y, _ := y.(*Chan); y != nil && Identical(x.elem, y.elem) {
+			// We have channels that differ in direction only.
+			// If there's an unrestricted channel, select the restricted one.
+			switch {
+			case x.dir == SendRecv:
+				return y
+			case y.dir == SendRecv:
+				return x
+			}
+		}
+	}
+
+	// types are different
+	return nil
+}
+
 // If typ is a type parameter, structuralType returns the single underlying
-// type of all types in the corresponding type constraint if it exists,
-// or nil otherwise. If typ is not a type parameter, structuralType returns
-// the underlying type.
+// type of all types in the corresponding type constraint if it exists, or
+// nil otherwise. If the type set contains only unrestricted and restricted
+// channel types (with identical element types), the single underlying type
+// is the restricted channel type if the restrictions are always the same.
+// If typ is not a type parameter, structuralType returns the underlying type.
 func structuralType(typ Type) Type {
 	var su Type
 	if underIs(typ, func(u Type) bool {
-		if su != nil && !Identical(su, u) {
-			return false
+		if su != nil {
+			u = match(su, u)
+			if u == nil {
+				return false
+			}
 		}
-		// su == nil || Identical(su, u)
+		// su == nil || match(su, u) != nil
 		su = u
 		return true
 	}) {
@@ -55,10 +89,13 @@ func structuralString(typ Type) Type {
 		if isString(u) {
 			u = NewSlice(universeByte)
 		}
-		if su != nil && !Identical(su, u) {
-			return false
+		if su != nil {
+			u = match(su, u)
+			if u == nil {
+				return false
+			}
 		}
-		// su == nil || Identical(su, u)
+		// su == nil || match(su, u) != nil
 		su = u
 		return true
 	}) {