go/types: refer to structural rather than single underlying type in errors

This is a port of CL 362997 from types2 to go/types. Some error
positions were adjusted in tests.

Change-Id: I6a932aee1a8d9bcbf4cd8c16a95bbb41b5c7e13f
Reviewed-on: https://go-review.googlesource.com/c/go/+/363990
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/builtins.go

@@ -479,7 +479,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		case *Map, *Chan:
 			min = 1
 		case nil:
-			check.errorf(arg0, _InvalidMake, "cannot make %s; type set has no single underlying type", arg0)
+			check.errorf(arg0, _InvalidMake, "cannot make %s: no structural type", arg0)
 			return
 		default:
 			check.invalidArg(arg0, _InvalidMake, "cannot make %s; type must be slice, map, or channel", arg0)

src/go/types/index.go

@@ -213,7 +213,7 @@ func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 	length := int64(-1) // valid if >= 0
 	switch u := structuralType(x.typ).(type) {
 	case nil:
-		check.errorf(x, _NonSliceableOperand, "cannot slice %s: type set has no single underlying type", x)
+		check.invalidOp(x, _NonSliceableOperand, "cannot slice %s: %s has no structural type", x, x.typ)
 		x.mode = invalid
 		return
 

src/go/types/stmt.go

@@ -832,12 +832,12 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		// determine key/value types
 		var key, val Type
 		if x.mode != invalid {
-			// Ranging over a type parameter is permitted if it has a single underlying type.
+			// Ranging over a type parameter is permitted if it has a structural type.
 			var cause string
 			u := structuralType(x.typ)
 			switch t := u.(type) {
 			case nil:
-				cause = "type set has no single underlying type"
+				cause = check.sprintf("%s has no structural type", x.typ)
 			case *Chan:
 				if s.Value != nil {
 					check.softErrorf(s.Value, _InvalidIterVar, "range over %s permits only one iteration variable", &x)

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

@@ -148,7 +148,7 @@ func _[
 	_ = make /* ERROR expects 2 or 3 arguments */ (S1)
 	_ = make(S1, 10, 20)
 	_ = make /* ERROR expects 2 or 3 arguments */ (S1, 10, 20, 30)
-	_ = make(S2 /* ERROR cannot make .* no single underlying type */ , 10)
+	_ = make(S2 /* ERROR cannot make S2: no structural type */ , 10)
 
 	type M0 map[string]int
 	_ = make(map[string]int)
@@ -156,7 +156,7 @@ func _[
 	_ = make(M1)
 	_ = make(M1, 10)
 	_ = make/* ERROR expects 1 or 2 arguments */(M1, 10, 20)
-	_ = make(M2 /* ERROR cannot make .* no single underlying type */ )
+	_ = make(M2 /* ERROR cannot make M2: no structural type */ )
 
 	type C0 chan int
 	_ = make(chan int)
@@ -164,7 +164,7 @@ func _[
 	_ = make(C1)
 	_ = make(C1, 10)
 	_ = make/* ERROR expects 1 or 2 arguments */(C1, 10, 20)
-	_ = make(C2 /* ERROR cannot make .* no single underlying type */ )
+	_ = make(C2 /* ERROR cannot make C2: no structural type */ )
 	_ = make(C3)
 }
 

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

@@ -134,7 +134,7 @@ func _[T interface{ ~string }] (x T, i, j, k int) { var _ T = x /* ERROR 3-index
 type myByte1 []byte
 type myByte2 []byte
 func _[T interface{ []byte | myByte1 | myByte2 }] (x T, i, j, k int) { var _ T = x[i:j:k] }
-func _[T interface{ []byte | myByte1 | []int }] (x T, i, j, k int) { var _ T = x /* ERROR no single underlying type */ [i:j:k] }
+func _[T interface{ []byte | myByte1 | []int }] (x T, i, j, k int) { var _ T = x /* ERROR no structural type */ [i:j:k] }
 
 // len/cap built-ins
 
@@ -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 single underlying type */ {}
+        for range c2 /* ERROR cannot range over c2.*no structural type */ {}
 
         var c3 C3
         for range c3 /* ERROR receive from send-only channel */ {}
@@ -226,7 +226,7 @@ func _[
         for _, _ = range s1 {}
 
         var s2 S2
-        for range s2 /* ERROR cannot range over s2.*no single underlying type */ {}
+        for range s2 /* ERROR cannot range over s2.*no structural type */ {}
 
         var a0 []int
         for range a0 {}
@@ -239,7 +239,7 @@ func _[
         for _, _ = range a1 {}
 
         var a2 A2
-        for range a2 /* ERROR cannot range over a2.*no single underlying type */ {}
+        for range a2 /* ERROR cannot range over a2.*no structural type */ {}
 
         var p0 *[10]int
         for range p0 {}
@@ -252,7 +252,7 @@ func _[
         for _, _ = range p1 {}
 
         var p2 P2
-        for range p2 /* ERROR cannot range over p2.*no single underlying type */ {}
+        for range p2 /* ERROR cannot range over p2.*no structural type */ {}
 
         var m0 map[string]int
         for range m0 {}
@@ -265,7 +265,7 @@ func _[
         for _, _ = range m1 {}
 
         var m2 M2
-        for range m2 /* ERROR cannot range over m2.*no single underlying type */ {}
+        for range m2 /* ERROR cannot range over m2.*no structural type */ {}
 }
 
 // type inference checks