[dev.typeparams] cmd/compile/internal/types2: fix make with type parameter argument

For make with a type parameter argument, the structural type of
the type parameter's constraint determines what make is making.

Change-Id: I3b48f8ce3236b7624e0638b5f5be208c5915c987
Reviewed-on: https://go-review.googlesource.com/c/go/+/339899
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 12 insertions, 30 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index e9df605fd1..184cd027cb 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -472,39 +472,21 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			return
 		}
 
-		min, max := -1, 10
-		var valid func(t Type) bool
-		valid = func(t Type) bool {
-			var m int
-			switch t := under(t).(type) {
-			case *Slice:
-				m = 2
-			case *Map, *Chan:
-				m = 1
-			case *TypeParam:
-				return t.underIs(valid)
-			default:
-				return false
-			}
-			if m > min {
-				min = m
-			}
-			if m+1 < max {
-				max = m + 1
-			}
-			return true
-		}
-
-		if !valid(T) {
+		var min int // minimum number of arguments
+		switch optype(T).(type) {
+		case *Slice:
+			min = 2
+		case *Map, *Chan:
+			min = 1
+		case *top:
+			check.errorf(arg0, invalidArg+"cannot make %s; type parameter has no structural type", arg0)
+			return
+		default:
 			check.errorf(arg0, invalidArg+"cannot make %s; type must be slice, map, or channel", arg0)
 			return
 		}
-		if nargs < min || max < nargs {
-			if min == max {
-				check.errorf(call, "%v expects %d arguments; found %d", call, min, nargs)
-			} else {
-				check.errorf(call, "%v expects %d or %d arguments; found %d", call, min, max, nargs)
-			}
+		if nargs < min || min+1 < nargs {
+			check.errorf(call, invalidOp+"%v expects %d or %d arguments; found %d", call, min, min+1, nargs)
 			return
 		}