C++: provide macro used-before-defined hint (PR c++/72786)

This patch uses the name_hint/deferred_diagnostic to provide
a message in the C++ frontend if a macro is used before it is defined
e.g.:

test.c:6:24: error: expected ';' at end of member declaration
   virtual void clone() const OVERRIDE { }
                        ^~~~~
                             ;
test.c:6:30: error: 'OVERRIDE' does not name a type
   virtual void clone() const OVERRIDE { }
                              ^~~~~~~~
test.c:6:30: note: the macro 'OVERRIDE' had not yet been defined
test.c:15:0: note: it was later defined here
 #define OVERRIDE override

It's possible to do it from the C++ frontend as tokenization happens
up-front (and hence the macro already exists when the above is parsed);
I attempted to do it from the C frontend, but because the C frontend only
tokenizes on-demand during parsing, the macro isn't known about until
later.

gcc/cp/ChangeLog:
	PR c++/72786
	* name-lookup.c (class macro_use_before_def): New class.
	(lookup_name_fuzzy): Detect macro that were used before being
	defined, and report them as such.

gcc/ChangeLog:
	PR c++/72786
	* spellcheck.h (best_match::blithely_get_best_candidate): New
	accessor.

gcc/testsuite/ChangeLog:
	PR c++/72786
	* g++.dg/spellcheck-macro-ordering-2.C: New test case.
	* g++.dg/spellcheck-macro-ordering.C: Add dg-message directives
	for macro used-before-defined.

libcpp/ChangeLog:
	PR c++/72786
	* include/cpplib.h (cpp_macro_definition_location): New decl.
	* macro.c (cpp_macro_definition): New function.

From-SVN: r254978

2 files changed, 55 insertions, 2 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 5de732f..908b48e 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,10 @@
+2017-11-20  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/72786
+	* name-lookup.c (class macro_use_before_def): New class.
+	(lookup_name_fuzzy): Detect macro that were used before being
+	defined, and report them as such.
+
 2017-11-20  Jason Merrill  <jason@redhat.com>
 
 	* decl2.c (constrain_class_visibility): Don't warn about artificial
diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index 9d97da3..fc317b1 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -5636,12 +5636,49 @@ consider_binding_level (tree name, best_match <tree, const char *> &bm,
     }
 }
 
+/* Subclass of deferred_diagnostic.  Notify the user that the
+   given macro was used before it was defined.
+   This can be done in the C++ frontend since tokenization happens
+   upfront.  */
+
+class macro_use_before_def : public deferred_diagnostic
+{
+ public:
+  /* Ctor.  LOC is the location of the usage.  MACRO is the
+     macro that was used.  */
+  macro_use_before_def (location_t loc, cpp_hashnode *macro)
+  : deferred_diagnostic (loc), m_macro (macro)
+  {
+    gcc_assert (macro);
+  }
+
+  ~macro_use_before_def ()
+  {
+    if (is_suppressed_p ())
+      return;
+
+    source_location def_loc = cpp_macro_definition_location (m_macro);
+    if (def_loc != UNKNOWN_LOCATION)
+      {
+	inform (get_location (), "the macro %qs had not yet been defined",
+		(const char *)m_macro->ident.str);
+	inform (def_loc, "it was later defined here");
+      }
+  }
+
+ private:
+  cpp_hashnode *m_macro;
+};
+
+
 /* Search for near-matches for NAME within the current bindings, and within
    macro names, returning the best match as a const char *, or NULL if
-   no reasonable match is found.  */
+   no reasonable match is found.
+
+   Use LOC for any deferred diagnostics.  */
 
 name_hint
-lookup_name_fuzzy (tree name, enum lookup_name_fuzzy_kind kind, location_t)
+lookup_name_fuzzy (tree name, enum lookup_name_fuzzy_kind kind, location_t loc)
 {
   gcc_assert (TREE_CODE (name) == IDENTIFIER_NODE);
 
@@ -5671,6 +5708,15 @@ lookup_name_fuzzy (tree name, enum lookup_name_fuzzy_kind kind, location_t)
   /* If a macro is the closest so far to NAME, consider it.  */
   if (best_macro)
     bm.consider ((const char *)best_macro->ident.str);
+  else if (bmm.get_best_distance () == 0)
+    {
+      /* If we have an exact match for a macro name, then the
+	 macro has been used before it was defined.  */
+      cpp_hashnode *macro = bmm.blithely_get_best_candidate ();
+      if (macro)
+	return name_hint (NULL,
+			  new macro_use_before_def (loc, macro));
+    }
 
   /* Try the "starts_decl_specifier_p" keywords to detect
      "singed" vs "signed" typos.  */