cmd/cgo: always use a function literal for pointer checking

The pointer checking code needs to know the exact type of the parameter
expected by the C function, so that it can use a type assertion to
convert the empty interface returned by cgoCheckPointer to the correct
type. Previously this was done by using a type conversion, but that
meant that the code accepted arguments that were convertible to the
parameter type, rather than arguments that were assignable as in a
normal function call. In other words, some code that should not have
passed type checking was accepted.

This CL changes cgo to always use a function literal for pointer
checking. Now the argument is passed to the function literal, which has
the correct argument type, so type checking is performed just as for a
function call as it should be.

Since we now always use a function literal, simplify the checking code
to run as a statement by itself. It now no longer needs to return a
value, and we no longer need a type assertion.

This does have the cost of introducing another function call into any
call to a C function that requires pointer checking, but the cost of the
additional call should be minimal compared to the cost of pointer
checking.

Fixes #16591.

Change-Id: I220165564cf69db9fd5f746532d7f977a5b2c989
Reviewed-on: https://go-review.googlesource.com/31233
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

2 files changed, 18 insertions, 0 deletions

diff --git a/misc/cgo/errors/issue16591.go b/misc/cgo/errors/issue16591.go
new file mode 100644
index 0000000000..10eb8403cf
--- /dev/null
+++ b/misc/cgo/errors/issue16591.go
@@ -0,0 +1,17 @@
+// Copyright 2016 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.
+
+// Issue 16591: Test that we detect an invalid call that was being
+// hidden by a type conversion inserted by cgo checking.
+
+package p
+
+// void f(int** p) { }
+import "C"
+
+type x *C.int
+
+func F(p *x) {
+	C.f(p) // ERROR HERE
+}
diff --git a/misc/cgo/errors/test.bash b/misc/cgo/errors/test.bash
index 84d44d8a33..cb442507a6 100755
--- a/misc/cgo/errors/test.bash
+++ b/misc/cgo/errors/test.bash
@@ -46,6 +46,7 @@ check issue13423.go
 expect issue13635.go C.uchar C.schar C.ushort C.uint C.ulong C.longlong C.ulonglong C.complexfloat C.complexdouble
 check issue13830.go
 check issue16116.go
+check issue16591.go
 
 if ! go build issue14669.go; then
 	exit 1