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Re: [C++ PATCH] PR c++/79133


On 8 July 2018 at 02:08, Ville Voutilainen <ville.voutilainen@gmail.com> wrote:
> On 8 July 2018 at 01:54, Paolo Carlini <paolo.carlini@oracle.com> wrote:
>>> That would make this more consistent with such a shadow warning, but I
>>> don't want
>>> to use the shadowing wording (which would be easy to do; just set
>>> 'shadowed' and do
>>> a 'goto inform'), because this isn't shadowing in the precise sense;
>>> the shadowing cases
>>> are warnings, whereas this is more like the redeclaration errors in
>>> the same function.
>>
>> ... indeed and that annoys me a bit. Not having studied at all c++/79133 so
>> far (sorry) it seems a little weird to me that according to the standard we
>> have to handle the two types of "shadowing" in different ways, one more
>> strict, one less. Thus I would suggest double checking the details of that,
>> eventually with Jason too in terms of the actual patch you would like to
>> apply.
>
> Well. The PR is about DR 2211 which, in simple terms, says that lambda
> parameters
> and captures cannot have the same name. See
> http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#2211
>
> That's stricter than -Wshadow, but otherwise equally strict as the
> other error cases already handled
> in check_local_shadow. So I'll make this error case more consistent
> with the others. We already
> handle redeclaration errors slightly differently from shadowing
> warnings in that function.

Here's an updated patch. Tested on Linux-PPC64, OK for trunk?

Backports?

2018-08-06  Ville Voutilainen  <ville.voutilainen@gmail.com>

    gcc/cp/

    PR c++/79133
    * name-lookup.c (check_local_shadow): Reject captures and parameters
    with the same name.

    testsuite/

    PR c++/79133
    * g++.dg/cpp0x/lambda/lambda-shadow3.C: New.
    * g++.dg/cpp1y/lambda-generic-variadic18.C: Likewise.
diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index 3aafb0f..72d87b3 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -2640,6 +2640,7 @@ check_local_shadow (tree decl)
 		  || TREE_CODE (decl) == TYPE_DECL)))
       && DECL_FUNCTION_SCOPE_P (old)
       && (!DECL_ARTIFICIAL (decl)
+	  || is_capture_proxy (decl)
 	  || DECL_IMPLICIT_TYPEDEF_P (decl)
 	  || (VAR_P (decl) && DECL_ANON_UNION_VAR_P (decl))))
     {
@@ -2648,7 +2649,8 @@ check_local_shadow (tree decl)
       /* Don't complain if it's from an enclosing function.  */
       if (DECL_CONTEXT (old) == current_function_decl
 	  && TREE_CODE (decl) != PARM_DECL
-	  && TREE_CODE (old) == PARM_DECL)
+	  && TREE_CODE (old) == PARM_DECL
+	  && !is_capture_proxy (decl))
 	{
 	  /* Go to where the parms should be and see if we find
 	     them there.  */
@@ -2665,6 +2667,21 @@ check_local_shadow (tree decl)
 	      return;
 	    }
 	}
+      /* DR 2211: check that captures and parameters
+	 do not have the same name. */
+      else if (is_capture_proxy (decl))
+	{
+	  if (current_lambda_expr ()
+	      && DECL_CONTEXT (old) == lambda_function (current_lambda_expr ())
+	      && TREE_CODE (old) == PARM_DECL
+	      && DECL_NAME (decl) != this_identifier)
+	    {
+	      error_at (DECL_SOURCE_LOCATION (old),
+			"lambda parameter %qD "
+			"previously declared as a capture", old);
+	    }
+	  return;
+	}
 
       /* The local structure or class can't use parameters of
 	 the containing function anyway.  */
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-shadow3.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-shadow3.C
new file mode 100644
index 0000000..8364321
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-shadow3.C
@@ -0,0 +1,6 @@
+// { dg-do compile { target c++11 } }
+
+int main() {
+  int x = 42;
+  auto lambda = [x](int x) {}; // { dg-error "previously declared as a capture" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic18.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic18.C
new file mode 100644
index 0000000..1eb9cce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic18.C
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++14 } }
+
+int main() {
+  int x = 42;
+  auto lambda2 = [x=x](int x) {}; // { dg-error "previously declared as a capture" }
+  auto lambda3 = [x](auto... x) {}; // { dg-error "previously declared as a capture" }
+  auto lambda4 = [](auto... x) {
+    auto lambda5 = [x...](auto... x) {};  // { dg-error "previously declared as a capture" }
+    auto lambda6 = [x...](int x) {};  // { dg-error "previously declared as a capture" }
+  };
+}

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