[dev.typeparams] cmd/compile/internal/types2: better error message for invalid ... use

This partially addresses the issue below: In many (all) cases we want to
handle invalid ... use in the parser as a syntax error; but this ensures
that we get a decent error if we get here anyway.

Updates #43680.

Change-Id: I93af43a5f5741d8bc76e7a13c0db75e6edf43111
Reviewed-on: https://go-review.googlesource.com/c/go/+/283475
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/examples/types.go2

@@ -113,6 +113,9 @@ type I1[T any] interface{
 	m1(T)
 }
 
+// There is no such thing as a variadic generic type.
+type _[T ... /* ERROR invalid use of ... */ interface{}] struct{}
+
 // Generic interfaces may be embedded as one would expect.
 type I2 interface {
 	I1(int)     // method!

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

@@ -1181,7 +1181,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		check.ident(x, e, nil, false)
 
 	case *syntax.DotsType:
-		// ellipses are handled explicitly where they are legal
+		// dots are handled explicitly where they are legal
 		// (array composite literals and parameter lists)
 		check.error(e, "invalid use of '...'")
 		goto Error

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

@@ -521,6 +521,12 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 		typ.elem = check.varType(e.Elem)
 		return typ
 
+	case *syntax.DotsType:
+		// dots are handled explicitly where they are legal
+		// (array composite literals and parameter lists)
+		check.error(e, "invalid use of '...'")
+		check.use(e.Elem)
+
 	case *syntax.StructType:
 		typ := new(Struct)
 		def.setUnderlying(typ)