Constraints on declaration after definition of same function are lost

[mattmccutchen-cci]

When a declaration of a function comes after a definition of the same function, any constraints placed on the declaration are lost. This seems to be due to an asymmetry in the declaration merging code.

Example:

defn_then_decl.c:

void foo(int *p) {}

#define MYDECL void foo(int *p);

MYDECL

3c -output-postfix=checked defn_then_decl.c writes the following to defn_then_decl.checked.c:

void foo(_Ptr<int> p) {}

#define MYDECL void foo(int *p);

MYDECL

Then clang defn_then_decl.checked.c fails with:

defn_then_decl.checked.c:5:1: error: conflicting types for 'foo'
MYDECL
^
defn_then_decl.checked.c:3:21: note: expanded from macro 'MYDECL'
#define MYDECL void foo(int *p);
                    ^
defn_then_decl.checked.c:1:6: note: previous definition is here
void foo(_Ptr<int> p) {}
     ^
1 error generated.

Compare to decl_then_defn.c:

#define MYDECL void foo(int *p);

MYDECL

void foo(int *p) {}

3C correctly does not change this because of the macro.

@john-h-kastner expressed interest in fixing this if it ends up being a small amount of work.