cmd/cgo: recognize known C typedefs as types

Fixes #14483.

Change-Id: I0cddfe27fd8d00ba85659d0b618410e39ebf45cb
Reviewed-on: https://go-review.googlesource.com/19860Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/22041Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/errors/ptr.go

@@ -122,6 +122,16 @@ var ptrTests = []ptrTest{
 		body:    `i := 0; p := &S{p:&i}; s := p.a[:]; C.f(unsafe.Pointer(&s[0]))`,
 		fail:    false,
 	},
+	{
+		// Passing the address of a slice of an array that is
+		// an element in a struct, with a type conversion.
+		name:    "slice-ok-4",
+		c:       `typedef void* PV; void f(PV p) {}`,
+		imports: []string{"unsafe"},
+		support: `type S struct { p *int; a [4]byte }`,
+		body:    `i := 0; p := &S{p:&i}; C.f(C.PV(unsafe.Pointer(&p.a[0])))`,
+		fail:    false,
+	},
 	{
 		// Passing the address of a static variable with no
 		// pointers doesn't matter.

src/cmd/cgo/gcc.go

@@ -819,14 +819,17 @@ func (p *Package) hasSideEffects(f *File, x ast.Expr) bool {
 func (p *Package) isType(t ast.Expr) bool {
 	switch t := t.(type) {
 	case *ast.SelectorExpr:
-		if t.Sel.Name != "Pointer" {
-			return false
-		}
 		id, ok := t.X.(*ast.Ident)
 		if !ok {
 			return false
 		}
-		return id.Name == "unsafe"
+		if id.Name == "unsafe" && t.Sel.Name == "Pointer" {
+			return true
+		}
+		if id.Name == "C" && typedef["_Ctype_"+t.Sel.Name] != nil {
+			return true
+		}
+		return false
 	case *ast.Ident:
 		// TODO: This ignores shadowing.
 		switch t.Name {