C++: suggest missing headers for implicit use of "std" (PR c++/85021)

We provide fix-it hints for the most common "std" names when an explicit
"std::" prefix is present, however we don't yet provide fix-it hints for
this implicit case:

  using namespace std;
  void f() {  cout << "test"; }

for which we emit:

  t.cc: In function 'void f()':
  t.cc:2:13: error: 'cout' was not declared in this scope
  void f() {  cout << "test"; }
              ^~~~

This patch detects if a "using namespace std;" directive is present
in the current namespace, and if so, offers a suggestion for
unrecognized names that are in our list of common "std" names:

  t.cc: In function 'void f()':
  t.cc:2:13: error: 'cout' was not declared in this scope
   void f() {  cout << "test"; }
               ^~~~
  t.cc:2:13: note: 'std::cout' is defined in header '<iostream>'; did you forget to '#include <iostream>'?
  +#include <iostream>
   using namespace std;
   void f() {  cout << "test"; }
               ^~~~

gcc/cp/ChangeLog:
	PR c++/85021
	* name-lookup.c (using_directives_contain_std_p): New function.
	(has_using_namespace_std_directive_p): New function.
	(suggest_alternatives_for): Simplify if/else logic using early
	returns.  If no candidates were found, and there's a
	"using namespace std;" directive, call
	maybe_suggest_missing_std_header.
	(maybe_suggest_missing_header): Split later part of the function
	into..
	(maybe_suggest_missing_std_header): New.

gcc/testsuite/ChangeLog:
	PR c++/85021
	* g++.dg/lookup/missing-std-include-7.C: New test.

From-SVN: r259179

gcc/cp/ChangeLog

+2018-04-06  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/85021
+	* name-lookup.c (using_directives_contain_std_p): New function.
+	(has_using_namespace_std_directive_p): New function.
+	(suggest_alternatives_for): Simplify if/else logic using early
+	returns.  If no candidates were found, and there's a
+	"using namespace std;" directive, call
+	maybe_suggest_missing_std_header.
+	(maybe_suggest_missing_header): Split later part of the function
+	into..
+	(maybe_suggest_missing_std_header): New.
+
 2018-04-06  Jason Merrill  <jason@redhat.com>
 
 	PR c++/85242 - ICE with class definition in template parm.

gcc/cp/name-lookup.c

@@ -41,6 +41,7 @@ static cxx_binding *cxx_binding_make (tree value, tree type);
 static cp_binding_level *innermost_nonclass_level (void);
 static void set_identifier_type_value_with_scope (tree id, tree decl,
 						  cp_binding_level *b);
+static bool maybe_suggest_missing_std_header (location_t location, tree name);
 
 /* Create an overload suitable for recording an artificial TYPE_DECL
    and another decl.  We use this machanism to implement the struct
@@ -5330,6 +5331,48 @@ qualify_lookup (tree val, int flags)
   return true;
 }
 
+/* Is there a "using namespace std;" directive within USINGS?  */
+
+static bool
+using_directives_contain_std_p (vec<tree, va_gc> *usings)
+{
+  if (!usings)
+    return false;
+
+  for (unsigned ix = usings->length (); ix--;)
+    if ((*usings)[ix] == std_node)
+      return true;
+
+  return false;
+}
+
+/* Is there a "using namespace std;" directive within the current
+   namespace (or its ancestors)?
+   Compare with name_lookup::search_unqualified.  */
+
+static bool
+has_using_namespace_std_directive_p ()
+{
+  /* Look at local using-directives.  */
+  for (cp_binding_level *level = current_binding_level;
+       level->kind != sk_namespace;
+       level = level->level_chain)
+    if (using_directives_contain_std_p (level->using_directives))
+      return true;
+
+  /* Look at this namespace and its ancestors.  */
+  for (tree scope = current_namespace; scope; scope = CP_DECL_CONTEXT (scope))
+    {
+      if (using_directives_contain_std_p (DECL_NAMESPACE_USING (scope)))
+	return true;
+
+      if (scope == global_namespace)
+	break;
+    }
+
+  return false;
+}
+
 /* Suggest alternatives for NAME, an IDENTIFIER_NODE for which name
    lookup failed.  Search through all available namespaces and print out
    possible candidates.  If no exact matches are found, and
@@ -5400,11 +5443,23 @@ suggest_alternatives_for (location_t location, tree name,
 	  inform (location_of (val), "  %qE", val);
 	}
       candidates.release ();
+      return;
     }
-  else if (!suggest_misspellings)
-    ;
-  else if (name_hint hint = lookup_name_fuzzy (name, FUZZY_LOOKUP_NAME,
-					       location))
+
+  /* No candidates were found in the available namespaces.  */
+
+  /* If there's a "using namespace std;" active, and this
+     is one of the most common "std::" names, then it's probably a
+     missing #include.  */
+  if (has_using_namespace_std_directive_p ())
+    if (maybe_suggest_missing_std_header (location, name))
+      return;
+
+  /* Otherwise, consider misspellings.  */
+  if (!suggest_misspellings)
+    return;
+  if (name_hint hint = lookup_name_fuzzy (name, FUZZY_LOOKUP_NAME,
+					  location))
     {
       /* Show a spelling correction.  */
       gcc_rich_location richloc (location);
@@ -5512,20 +5567,13 @@ get_std_name_hint (const char *name)
   return NULL;
 }
 
-/* If SCOPE is the "std" namespace, then suggest pertinent header
-   files for NAME at LOCATION.
+/* Suggest pertinent header files for NAME at LOCATION, for common
+   names within the "std" namespace.
    Return true iff a suggestion was offered.  */
 
 static bool
-maybe_suggest_missing_header (location_t location, tree name, tree scope)
+maybe_suggest_missing_std_header (location_t location, tree name)
 {
-  if (scope == NULL_TREE)
-    return false;
-  if (TREE_CODE (scope) != NAMESPACE_DECL)
-    return false;
-  /* We only offer suggestions for the "std" namespace.  */
-  if (scope != std_node)
-    return false;
   gcc_assert (TREE_CODE (name) == IDENTIFIER_NODE);
 
   const char *name_str = IDENTIFIER_POINTER (name);
@@ -5542,6 +5590,23 @@ maybe_suggest_missing_header (location_t location, tree name, tree scope)
   return true;
 }
 
+/* If SCOPE is the "std" namespace, then suggest pertinent header
+   files for NAME at LOCATION.
+   Return true iff a suggestion was offered.  */
+
+static bool
+maybe_suggest_missing_header (location_t location, tree name, tree scope)
+{
+  if (scope == NULL_TREE)
+    return false;
+  if (TREE_CODE (scope) != NAMESPACE_DECL)
+    return false;
+  /* We only offer suggestions for the "std" namespace.  */
+  if (scope != std_node)
+    return false;
+  return maybe_suggest_missing_std_header (location, name);
+}
+
 /* Look for alternatives for NAME, an IDENTIFIER_NODE for which name
    lookup failed within the explicitly provided SCOPE.  Suggest the
    the best meaningful candidates (if any) as a fix-it hint.

gcc/testsuite/ChangeLog

+2018-04-06  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/85021
+	* g++.dg/lookup/missing-std-include-7.C: New test.
+
 2018-04-06  Tamar Christina  <tamar.christina@arm.com>
 
 	* gcc.dg/struct-simple.c: Revert r254862.

gcc/testsuite/g++.dg/lookup/missing-std-include-7.C

+/* PR c++/85021: Verify that we suggest missing headers for common names in std::
+   if there's a "using namespace std;" active.  */
+
+/* No using-directive.  */
+
+void test_1 ()
+{
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-bogus "'<iostream>'" "" { target *-*-* } .-1 }
+}
+
+/* Local using-directive.  */
+
+void test_2 ()
+{
+  using namespace std;
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-message "'std::cout' is defined in header '<iostream>'" "" { target *-*-* } .-1 }
+}
+
+/* Local using-directive, but not of "std".  */
+
+namespace not_std {}
+void test_3 ()
+{
+  using namespace not_std;
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-bogus "'<iostream>'" "" { target *-*-* } .-1 }
+}
+
+/* Local using-directive in wrong block.  */
+
+void test_4 ()
+{
+  {
+    using namespace std;
+  }
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-bogus "'<iostream>'" "" { target *-*-* } .-1 }
+}
+
+/* Local using-directive used from nested block.  */
+
+void test_5 ()
+{
+  using namespace std;
+
+  for (int i = 0; i < 10; i++)
+    {
+      cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+      // { dg-message "'std::cout' is defined in header '<iostream>'" "" { target *-*-* } .-1 }
+    }
+}
+
+namespace ns_1 {
+
+namespace ns_2 {
+
+using namespace std;
+
+/* using-directive within the same namespace.  */
+
+void test_6 ()
+{
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-message "'std::cout' is defined in header '<iostream>'" "" { target *-*-* } .-1 }
+}
+
+namespace ns_3 {
+
+/* Locate the using-directive within ns_2, the parent namespace.  */
+
+void test_7 ()
+{
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-message "'std::cout' is defined in header '<iostream>'" "" { target *-*-* } .-1 }
+}
+
+} // namespace ns_3
+} // namespace ns_2
+
+/* Back in ns_1, should not locate the using-directive.  */
+
+void test_8 ()
+{
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-bogus "'<iostream>'" "" { target *-*-* } .-1 }
+}
+
+} // namespace ns_1
+
+/* using-directive in global namespace.  */
+using namespace std;
+
+void test_9 ()
+{
+  cout << "test"; // { dg-error "'cout' was not declared in this scope" }
+  // { dg-message "'std::cout' is defined in header '<iostream>'" "" { target *-*-* } .-1 }
+}
+