Make --no_fwd_decls respect local forward decls

Before this, the --no_fwd_decls mode would force-remove existing forward
decls:

    class Foo;
    void Bar(const Foo*);

Add an additional rule that we avoid promoting uses of forward-
declarations in the same file to full uses, to retain forward-decls like
the above.

This makes the behavior of --no_fwd_decls simpler and more consistent:
optimize for fewer redeclarations instead of fewer includes. Whenever
there's a choice between a forward-decl and an include, choose the
include.

Add test coverage for more variants of included forward decls.

iwyu_output.cc

@@ -1105,8 +1105,12 @@ void ProcessForwardDeclare(OneUse* use,
     }
   }
 
-  // (A7) If --no_fwd_decls has been passed, recategorize as a full use.
-  if (GlobalFlags().no_fwd_decls) {
+  // (A7) If --no_fwd_decls has been passed, recategorize as a full use unless
+  // the decl is in this file (in which case it must be a self-sufficient decl
+  // being used, so we can just let IWYU do its work).
+  if (!use->ignore_use() &&
+      GlobalFlags().no_fwd_decls &&
+      GetFileEntry(use->decl_loc()) != GetFileEntry(use->use_loc())) {
     use->set_full_use();
   }
 }

tests/cxx/no_fwd_decls-fwd.h

@@ -0,0 +1,15 @@
+//===--- no_fwd_decls-fwd.h - test input file for iwyu --------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS_FWD_H_
+#define INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS_FWD_H_
+
+class Fwd;
+
+#endif  // INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS-FWD_H_

tests/cxx/no_fwd_decls-nameonly.h

@@ -0,0 +1,19 @@
+//===--- no_fwd_decls-nameonly.h - test input file for iwyu ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS_NAMEONLY_H_
+#define INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS_NAMEONLY_H_
+
+class NameOnly;
+
+const void* AddressOf(const NameOnly& a) {
+  return reinterpret_cast<const void*>(&a);
+}
+
+#endif  // INCLUDE_WHAT_YOU_USE_TESTS_CXX_NO_FWD_DECLS-NAMEONLY_H_

tests/cxx/no_fwd_decls.cc

@@ -9,12 +9,38 @@
 
 // Test that passing the --no_fwd_decls switch to IWYU suggests including the
 // corresponding header file even when the use is not a full use.
+//
+// The normal IWYU policy is to minimize the number of #include directives
+// without introducing unnecessary coupling.
+
+// The --no_fwd_decls policy is to minimize the number of redeclarations to
+// avoid over-loose coupling, which might lead to errors and busy-work when a
+// type eventually changes.
 
 #include "tests/cxx/direct.h"
+#include "tests/cxx/no_fwd_decls-fwd.h"
+#include "tests/cxx/no_fwd_decls-nameonly.h"
 
 // IWYU: IndirectClass is...*indirect.h
 IndirectClass* global;
 
+// Existing forward-declares that don't exist anywhere else should stay,
+// because we don't know where a declaration would come from otherwise.
+class LocalFwd;
+void ForwardDeclareUse(const LocalFwd*);
+
+// A forward-declare that also exists in an included header can be removed.
+// Normally IWYU would optimize for fewer includes, but in --no_fwd_decls mode
+// we optimize for fewer redeclarations instead.
+class Fwd;
+void ForwardDeclareUse(const Fwd&);
+
+// Make sure a forward-declare included from a desired header is not repeated
+// here.
+bool AreSame(const NameOnly& a, const NameOnly& b) {
+  return AddressOf(a) == AddressOf(b);
+}
+
 /**** IWYU_SUMMARY
 
 tests/cxx/no_fwd_decls.cc should add these lines:
@@ -22,8 +48,12 @@ tests/cxx/no_fwd_decls.cc should add these lines:
 
 tests/cxx/no_fwd_decls.cc should remove these lines:
 - #include "tests/cxx/direct.h"  // lines XX-XX
+- class Fwd;  // lines XX-XX
 
 The full include-list for tests/cxx/no_fwd_decls.cc:
 #include "tests/cxx/indirect.h"  // for IndirectClass
+#include "tests/cxx/no_fwd_decls-fwd.h"  // for Fwd
+#include "tests/cxx/no_fwd_decls-nameonly.h"  // for AddressOf, NameOnly
+class LocalFwd;  // lines XX-XX
 
 ***** IWYU_SUMMARY */