{PATCH] New C++ warning -Wcatch-value

[Volker Reichelt <v.reichelt@netcologne.de>]

Hi,

catching exceptions by value is a bad thing, as it may cause slicing, i.e.
a) a superfluous copy
b) which is only partial.
See also https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#e15-catch-exceptions-from-a-hierarchy-by-reference

To warn the user about catch handlers of non-reference type,
the following patch adds a new C++/ObjC++ warning option "-Wcatch-value".

Bootstrapped and regtested on x86_64-pc-linux-gnu.
OK for trunk?

Regards,
Volker


2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>

	* doc/invoke.texi (-Wcatch-value): Document new warning option.

Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 247416)
+++ gcc/doc/invoke.texi	(working copy)
@@ -265,7 +265,7 @@
 -Wno-builtin-declaration-mismatch @gol
 -Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat @gol
 -Wc++-compat  -Wc++11-compat  -Wc++14-compat  -Wcast-align  -Wcast-qual  @gol
--Wchar-subscripts -Wchkp  -Wclobbered  -Wcomment  @gol
+-Wchar-subscripts  -Wchkp  -Wcatch-value  -Wclobbered  -Wcomment  @gol
 -Wconditionally-supported  @gol
 -Wconversion  -Wcoverage-mismatch  -Wno-cpp  -Wdangling-else  -Wdate-time @gol
 -Wdelete-incomplete @gol
@@ -5827,6 +5827,11 @@
 literals to @code{char *}.  This warning is enabled by default for C++
 programs.
 
+@item -Wcatch-value @r{(C++ and Objective-C++ only)}
+@opindex Wcatch-value
+@opindex Wno-catch-value
+Warn about catch handler of non-reference type.
+
 @item -Wclobbered
 @opindex Wclobbered
 @opindex Wno-clobbered
===================================================================

2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>

	* c.opt (Wcatch-value): New C++ warning flag.

Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt	(revision 247416)
+++ gcc/c-family/c.opt	(working copy)
@@ -388,6 +388,10 @@
 C ObjC C++ ObjC++ Var(warn_cast_qual) Warning
 Warn about casts which discard qualifiers.
 
+Wcatch-value
+C++ ObjC++ Var(warn_catch_value) Warning
+Warn about catch handlers of non-reference type.
+
 Wchar-subscripts
 C ObjC C++ ObjC++ Var(warn_char_subscripts) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about subscripts whose type is \"char\".
===================================================================

2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>

	* semantics.c (finish_handler_parms): Warn about non-reference type
	catch handlers.

Index: gcc/cp/semantics.c
===================================================================
--- gcc/cp/semantics.c	(revision 247416)
+++ gcc/cp/semantics.c	(working copy)
@@ -1321,7 +1321,15 @@
 	}
     }
   else
-    type = expand_start_catch_block (decl);
+    {
+      type = expand_start_catch_block (decl);
+      if (warn_catch_value
+	  && type != NULL_TREE
+	  && type != error_mark_node
+	  && TREE_CODE (TREE_TYPE (decl)) != REFERENCE_TYPE)
+	warning (OPT_Wcatch_value,
+		 "catching non-reference type %qT", TREE_TYPE (decl));
+    }
   HANDLER_TYPE (handler) = type;
 }
 
===================================================================
 
2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>

	* g++.dg/warn/Wcatch-value-1.C: New test.

Index: gcc/testsuite/g++.dg/warn/Wcatch-value-1.C
===================================================================
--- gcc/testsuite/g++.dg/warn/Wcatch-value-1.C	2017-05-01
+++ gcc/testsuite/g++.dg/warn/Wcatch-value-1.C	2017-05-01
@@ -0,0 +1,45 @@
+// { dg-options "-Wcatch-value" }
+
+struct A {};
+struct B {};
+struct C {};
+
+void foo()
+{
+  try {}
+  catch (int)      {}  // { dg-warning "catching non-reference type" }
+  catch (double*)  {}  // { dg-warning "catching non-reference type" }
+  catch (float&)   {}
+  catch (A)        {}  // { dg-warning "catching non-reference type" }
+  catch (A[2])     {}  // { dg-warning "catching non-reference type" }
+  catch (B*)       {}  // { dg-warning "catching non-reference type" }
+  catch (B&)       {}
+  catch (const C&) {}
+}
+
+template<typename T> void foo1()
+{
+  try {}
+  catch (T) {}
+}
+
+void bar1()
+{
+  foo1<int&>();
+  foo1<const A&>();
+}
+
+template<typename T> void foo2()
+{
+  try {}
+  catch (T) {}  // { dg-warning "catching non-reference type" }
+
+  try {}
+  catch (T&) {}
+}
+
+void bar2()
+{
+  foo2<int*>();      // { dg-message "required" }
+  foo2<A>();         // { dg-message "required" }
+}
===================================================================