cmd/cgo: given typedef struct S T, make C.T and C.struct_S interchangeable
For incomplete struct S, C.T and C.struct_S were interchangeable in Go 1.2
and earlier, because all incomplete types were interchangeable
(even C.struct_S1 and C.struct_S2).
CL 76450043, which fixed issue 7409, made different incomplete types
different from Go's point of view, so that they were no longer completely
interchangeable.
However, imprecision about C.T and C.struct_S - really the same
underlying C type - is the one behavior enabled by the bug that
is most likely to be depended on by existing cgo code.
Explicitly allow it, to keep that code working.
Fixes #7786.
LGTM=iant, r
R=golang-codereviews, iant, r
CC=golang-codereviews
https://golang.org/cl/98580046
diff --git a/doc/go1.3.html b/doc/go1.3.html
index 9a9f9f8..f4e055a 100644
--- a/doc/go1.3.html
+++ b/doc/go1.3.html
@@ -253,7 +253,13 @@
to a function expecting another.
Go 1.3 corrects this mistake by translating each different
incomplete struct to a different named type.
-However, some Go code took advantage of this bug to pass (for example) a <code>*C.FILE</code>
+</p>
+
+<p>
+Given the C declaration <code>typedef struct S T</code> for an incomplete <code>struct S</code>,
+some Go code used this bug to refer to the types <code>C.struct_S</code> and <code>C.T</code> interchangeably.
+Cgo now explicitly allows this use, even for completed struct types.
+However, some Go code also used this bug to pass (for example) a <code>*C.FILE</code>
from one package to another.
This is not legal and no longer works: in general Go packages
should avoid exposing C types and names in their APIs.
