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C++ PATCH for c++/52026 (use of enclosing constant var in lambda)
- From: Jason Merrill <jason at redhat dot com>
- To: gcc-patches List <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 14 Feb 2013 23:09:32 -0500
- Subject: C++ PATCH for c++/52026 (use of enclosing constant var in lambda)
In this testcase, we see a use of a constant variable in a lambda
(though the same thing can happen with a local class member function).
We try to pull out the constant value in finish_id_expression, but
because the initializer isn't a reduced constant yet, that fails and we
end up just returning the variable. At instantiation time tsubst
instantiates the use of the variable by creating a new one in the lambda
context, which is obviously broken. A better solution is to defer
deciding how to handle the variable until instantiation time, at which
point we will know. Since the logic for this is in
finish_id_expression, it's easy to just return the name and look it up
again at that point.
Tested x86_64-pc-linux-gnu, applying to trunk and 4.7.
commit 38e1d69672c4028124637e3b78430f4c45dff6a8
Author: Jason Merrill <jason@redhat.com>
Date: Thu Feb 14 22:11:56 2013 -0500
PR c++/52026
* semantics.c (finish_id_expression): In a template, return
the identifier for a constant variable.
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 28b4b79..0e09d04 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3015,7 +3015,14 @@ finish_id_expression (tree id_expression,
FIXME update for final resolution of core issue 696. */
if (decl_constant_var_p (decl))
- return integral_constant_value (decl);
+ {
+ if (processing_template_decl)
+ /* In a template, the constant value may not be in a usable
+ form, so look it up again at instantiation time. */
+ return id_expression;
+ else
+ return integral_constant_value (decl);
+ }
/* If we are in a lambda function, we can move out until we hit
1. the context,
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const2.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const2.C
new file mode 100644
index 0000000..d2457d6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const2.C
@@ -0,0 +1,15 @@
+// PR c++/52026
+// { dg-options "-std=c++11 -O" }
+// { dg-do run }
+
+template<bool B>
+int func() {
+ const int constVal1 = B ? 100 : -100;
+ const int constVal = constVal1;
+ return [] { return constVal; }();
+}
+
+int main() {
+ if (func<true>() != 100)
+ __builtin_abort ();
+}