[C++ Patch] Avoid redundant error messages from build_x_arrow

[Paolo Carlini]

Hi,

while working on improving the locations of cp_build_indirect_ref_1 & 
co, I noticed this nit which seems a separate issue.

In a nutshell, at variance with many other cases, in build_x_arrow we 
don't immediately check for error_mark_node the return value of 
decay_conversion. Then, for the testcase, after a sensible:

error: invalid use of member function ‘C* C::f()’ (did you forget the 
‘()’ ?)

we also issue:

error: base operand of ‘->’ is not a pointer

which is certainly redundant and a bit misleading, is talking about the 
'f' mentioned in the first message. The amended behavior is also 
consistent with EDG and CLANG.

Tested x86_64-linux, as usual.

Thanks, Paolo.

//////////////////////////

/gcc
2019-11-20  Paolo Carlini  <paolo.carl...@oracle.com>

        * typeck2.c (build_x_arrow): Early return if decay_conversion
        returns error_mark_node.

/testsuite
2019-11-20  Paolo Carlini  <paolo.carl...@oracle.com>

        * g++.dg/parse/error43.C: Adjust expected error.
Index: cp/typeck2.c
===================================================================
--- cp/typeck2.c        (revision 278499)
+++ cp/typeck2.c        (working copy)
@@ -2044,7 +2044,11 @@ build_x_arrow (location_t loc, tree expr, tsubst_f
        last_rval = convert_from_reference (last_rval);
     }
   else
-    last_rval = decay_conversion (expr, complain);
+    {
+      last_rval = decay_conversion (expr, complain);
+      if (last_rval == error_mark_node)
+       return error_mark_node;
+    }
 
   if (TYPE_PTR_P (TREE_TYPE (last_rval)))
     {
Index: testsuite/g++.dg/parse/error43.C
===================================================================
--- testsuite/g++.dg/parse/error43.C    (revision 278499)
+++ testsuite/g++.dg/parse/error43.C    (working copy)
@@ -2,4 +2,4 @@
 // { dg-options "" }
 
 class C { public: C* f(); int get(); };
-int f(C* p) { return p->f->get(); }  // { dg-error "forget the '\\(\\)'|base 
operand" }
+int f(C* p) { return p->f->get(); }  // { dg-error "25:invalid use of member 
function" }