cmd/cgo: fix for function taking pointer typedef

Fixes #19832

Change-Id: I7ce39c2c435d4716d8a42ac6784b4c87874c0e13
Reviewed-on: https://go-review.googlesource.com/49490
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/test/issue19832.go

+// Copyright 2015 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 19832. Functions taking a pointer typedef were being expanded and triggering a compiler error.
+
+package cgotest
+
+// typedef struct { int i; } *PS;
+// void T19832(PS p) {}
+import "C"
+import "testing"
+
+func test19832(t *testing.T) {
+	C.T19832(nil)
+}

src/cmd/cgo/gcc.go

@@ -2227,6 +2227,12 @@ func (c *typeConv) FuncArg(dtype dwarf.Type, pos token.Pos) *Type {
 				break
 			}
 
+			// If we already know the typedef for t just use that.
+			// See issue 19832.
+			if def := typedef[t.Go.(*ast.Ident).Name]; def != nil {
+				break
+			}
+
 			t = c.Type(ptr, pos)
 			if t == nil {
 				return nil