Re: [PATCH] C++: show namespaces for enum values (PR c++/88121)

[Jason Merrill]

On 11/21/18 8:35 AM, David Malcolm wrote:
Consider this test case:
namespace json
{
   enum { JSON_OBJECT };
}

void test ()
{
   JSON_OBJECT;
}

which erroneously accesses an enum value in another namespace without
qualifying the access.

GCC 6 through 8 issue a suggestion that doesn't mention the namespace:

<source>: In function 'void test()':
<source>:8:3: error: 'JSON_OBJECT' was not declared in this scope
    JSON_OBJECT;
    ^~~~~~~~~~~
<source>:8:3: note: suggested alternative:
<source>:3:10: note:   'JSON_OBJECT'
    enum { JSON_OBJECT };
           ^~~~~~~~~~~

which is suboptimal.

I made the problem worse with r265610, as gcc 9 now consolidates
the single suggestion into the error, and emits:

<source>: In function 'void test()':
<source>:8:3: error: 'JSON_OBJECT' was not declared in this scope; did
    you mean 'JSON_OBJECT'?
     8 |   JSON_OBJECT;
       |   ^~~~~~~~~~~
       |   JSON_OBJECT
<source>:3:10: note: 'JSON_OBJECT' declared here
     3 |   enum { JSON_OBJECT };
       |          ^~~~~~~~~~~

where the message:
   'JSON_OBJECT' was not declared in this scope; did you mean 'JSON_OBJECT'?
is nonsensical.

The root cause is that dump_scope doesn't print anything when called for
CONST_DECL in a namespace: the scope is an ENUMERAL_TYPE, rather than
a namespace.

Although that's only true for unscoped enums.

This patch tweaks dump_scope to detect ENUMERAL_TYPE, and to use the
enclosing namespace, so that the CONST_DECL is dumped as
"json::JSON_OBJECT".

@@ -182,6 +182,12 @@ dump_scope (cxx_pretty_printer *pp, tree scope, int flags)
    if (scope == NULL_TREE)
      return;
  
+  /* Enum values will be CONST_DECL with an ENUMERAL_TYPE as their
+     "scope".  Use CP_TYPE_CONTEXT of the ENUMERAL_TYPE, so as to
+     print the enclosing namespace.  */
+  if (TREE_CODE (scope) == ENUMERAL_TYPE)
+    scope = CP_TYPE_CONTEXT (scope);

This needs to handle scoped enums differently.

diff --git a/gcc/testsuite/g++.dg/lookup/suggestions-scoped-enums.C 
b/gcc/testsuite/g++.dg/lookup/suggestions-scoped-enums.C
new file mode 100644
index 0000000..2bf3ed6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/suggestions-scoped-enums.C
@@ -0,0 +1,13 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-fdiagnostics-show-caret" }
+
+enum class vegetable { CARROT, TURNIP };
+
+void misspelled_value_in_scoped_enum ()
+{
+  vegetable::TURNUP; // { dg-error "'TURNUP' is not a member of 'vegetable'" }
+  /* { dg-begin-multiline-output "" }
+   vegetable::TURNUP;
+              ^~~~~~
+     { dg-end-multiline-output "" } */
+}

I don't see any suggestion in the expected output, and would hope for it 
to suggest vegetable::TURNIP.

Jason