Patchwork C++ PATCH for c++/52026 (use of enclosing constant var in lambda)

login
register
mail settings
Submitter Jason Merrill
Date Feb. 15, 2013, 4:09 a.m.
Message ID <511DB4FC.9050600@redhat.com>
Download mbox | patch
Permalink /patch/220621/
State New
Headers show

Comments

Jason Merrill - Feb. 15, 2013, 4:09 a.m.
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.

Patch

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 ();
+}