Fix problem where gold fails to issue an undefined symbol error during LTO.

During LTO, if (1) an IR file contains a COMDAT group that is kept,
(2) a later non-claimed file contains the same group, which we discard,
and (3) the plugin fails to provide a definition of the symbols in that
COMDAT group, gold silently resolves any references to those symbols
to 0.

This patch adds a check for a placeholder symbol when deciding
whether to issue an undefined symbol error. It also adds an extra
note after any undefined placeholder symbol error that explains
that a definition was expected from the plugin.

2016-03-21  Cary Coutant  <ccoutant@gmail.com>

gold/
	PR gold/19842
	* errors.cc (Errors::undefined_symbol): Add info message when
	symbol should have been provided by a plugin.
	* target-reloc.h (issue_undefined_symbol_error): Check for
	placeholder symbols defined in discarded sections.
	* testsuite/Makefile.am (plugin_test_9b): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/plugin_test_9b_elf.cc: New test source file.
	* testsuite/plugin_test_9b_ir.cc: New test source file.

diff --git a/gold/errors.cc b/gold/errors.cc
index f90e53f..27392cc 100644
--- a/gold/errors.cc
+++ b/gold/errors.cc
@@ -198,6 +198,9 @@ Errors::undefined_symbol(const Symbol* sym, const std::string& location)
     gold_info(_("%s: the vtable symbol may be undefined because "
 		"the class is missing its key function"),
 	      program_name);
+  if (sym->is_placeholder())
+    gold_info(_("%s: the symbol should have been defined by a plugin"),
+	      program_name);
 }
 
 // Issue a debugging message.
diff --git a/gold/target-reloc.h b/gold/target-reloc.h
index bdf673d..e275d33 100644
--- a/gold/target-reloc.h
+++ b/gold/target-reloc.h
@@ -186,8 +186,10 @@ issue_undefined_symbol_error(const Symbol* sym)
   if (sym->is_weak_undefined())
     return false;
 
-  // We don't report symbols defined in discarded sections.
-  if (sym->is_defined_in_discarded_section())
+  // We don't report symbols defined in discarded sections,
+  // unless they're placeholder symbols that should have been
+  // provided by a plugin.
+  if (sym->is_defined_in_discarded_section() && !sym->is_placeholder())
     return false;
 
   // If the target defines this symbol, don't report it here.
diff --git a/gold/testsuite/Makefile.am b/gold/testsuite/Makefile.am
index fbaee16..bf222c3 100644
--- a/gold/testsuite/Makefile.am
+++ b/gold/testsuite/Makefile.am
@@ -2015,6 +2015,25 @@ MOSTLYCLEANFILES += two_file_test_1c.o
 two_file_test_1c.o: two_file_test_1.o
 	cp two_file_test_1.o $@
 
+# As above, but check COMDAT case, where a non-IR file contains a duplicate
+# of a COMDAT group in an IR file.
+check_DATA += plugin_test_9b.err
+MOSTLYCLEANFILES += plugin_test_9b.err
+plugin_test_9b.err: plugin_test_9b_ir.o.syms plugin_test_9b_ir.o plugin_test_9b_elf.o gcctestdir/ld plugin_test.so
+	@echo $(CXXLINK) -Bgcctestdir/ -o plugin_test_9b -Wl,--no-demangle,--plugin,"./plugin_test.so",--plugin-opt,"_ZN1A5printEv" plugin_test_9b_ir.o plugin_test_9b_elf.o "2>$@"
+	@if $(CXXLINK) -Bgcctestdir/ -o plugin_test_9b -Wl,--no-demangle,--plugin,"./plugin_test.so",--plugin-opt,"_ZN1A5printEv" plugin_test_9b_ir.o plugin_test_9b_elf.o 2>$@; then \
+	  echo 1>&2 "Link of plugin_test_9b should have failed"; \
+	  rm -f $@; \
+	  exit 1; \
+	fi
+# Make a .syms file that claims to define a method in class A in a COMDAT group.
+# The real plugin_test_9b_ir.o will be compiled without the -D, and will not
+# define any methods in class A.
+plugin_test_9b_ir.o.syms: plugin_test_9b_ir_witha.o
+	$(TEST_READELF) -sW $< >$@ 2>/dev/null
+plugin_test_9b_ir_witha.o: plugin_test_9b_ir.cc
+	$(CXXCOMPILE) -c -DUSE_CLASS_A -o $@ $<
+
 check_PROGRAMS += plugin_test_10
 check_SCRIPTS += plugin_test_10.sh
 check_DATA += plugin_test_10.sections