go.tools/go/types: more robust handling of ... errors with built-ins

Catch ... errors earlier and in case of error, type-check all
arguments anyway for better type reporting and fewer "declared
but not used" errors.

R=adonovan
CC=golang-dev
https://golang.org/cl/14600043

go/types/builtins.go

@@ -19,6 +19,14 @@ import (
 // false, and *x is undefined.
 //
 func (check *checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ bool) {
+	// append is the only built-in that permits the use of ... for the last argument
+	bin := predeclaredFuncs[id]
+	if call.Ellipsis.IsValid() && id != _Append {
+		check.invalidOp(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
+		check.use(call.Args)
+		return
+	}
+
 	// determine actual arguments
 	var arg getter
 	nargs := len(call.Args)
@@ -34,14 +42,7 @@ func (check *checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			}
 		}
 	case _Make, _New, _Offsetof, _Trace:
-		// arguments requires special handling
-	}
-
-	// append is the only built-in that permits the use of ... for the last argument
-	bin := predeclaredFuncs[id]
-	if call.Ellipsis.IsValid() && id != _Append {
-		check.invalidOp(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
-		return
+		// arguments require special handling
 	}
 
 	// check argument count

go/types/call.go

@@ -16,12 +16,7 @@ func (check *checker) call(x *operand, e *ast.CallExpr) exprKind {
 
 	switch x.mode {
 	case invalid:
-		// We don't have a valid call or conversion but we have a list of arguments.
-		// Typecheck them independently for better partial type information in
-		// the presence of type errors.
-		for _, arg := range e.Args {
-			check.expr(x, arg)
-		}
+		check.use(e.Args)
 		x.mode = invalid
 		x.expr = e
 		return statement
@@ -85,6 +80,16 @@ func (check *checker) call(x *operand, e *ast.CallExpr) exprKind {
 	}
 }
 
+// use type-checks each list element.
+// Useful to make sure a list of expressions is evaluated
+// (and variables are "used") in the presence of other errors.
+func (check *checker) use(list []ast.Expr) {
+	var x operand
+	for _, e := range list {
+		check.rawExpr(&x, e, nil)
+	}
+}
+
 // TODO(gri) use unpack for assignment checking as well.
 
 // A getter sets x as the i'th operand, where 0 <= i < n and n is the total

go/types/testdata/builtins.src

@@ -629,7 +629,6 @@ func Offsetof1() {
 	_ = unsafe.Offsetof(y2 /* ERROR method value */ .m)
 
 	var s []byte
-	_ = s
 	_ = unsafe.Offsetof(s... /* ERROR invalid use of \.\.\. */ )
 }
 
@@ -731,7 +730,6 @@ func trace1() {
 	// _ = trace(true, 1.2, '\'', "foo", 42i, "foo" <= "bar")
 
 	var s []byte
-	_ = s
 	trace(s... /* ERROR invalid use of \.\.\. */ )
 }