Date: 2008-07-15 00:12:51
Date: 2008-07-15 00:12:50 EDT (Tue, 15 Jul 2008)
New Revision: 47433
Added [core] issue 46.
sandbox/committee/concepts/issues/issues/issue46.xml (contents, props changed)
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+<?xml version='1.0' encoding='iso-8859-1' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd" [
+ <!ENTITY nbsp " ">
+<issue num="46" status="New">
+<title>Should unconstrained templates succeed at type deduction during the
+instantiation of a constrained context?</title>
+<date>14 Jul 2008</date>
+ <p>As of N2676, [temp.deduct] says:
+ If the reference to the template occurs within a constrained
+ context (14.10) and the template is an unconstrained template,
+ type deduction fails.
+ However, type deduction does <b>not</b> automatically fail for an
+ unconstrained template being considered during the instantiation of
+ the same constrained context.
+ There are differing opinions on this issue. One (call it the "anti-UT"
+ position) says that an unconstrained template (UT) should never be
+ selected within the instantiation of a constrained context, and that
+ only constrained templates (CT) and non-template functions should be
+ considered; the other (call it the "pro-UT" position) says a UT should
+ be selected during the instantiation of a constrained context if,
+ neglecting requirements, it's more specialized than the other templates
+ being considered for a given call (and if in other respects it would
+ succeed during overload resolution over non-template functions---as
+ The anti-UT argument goes something like this:
+ "If selected, a UT amounts to a late_check region. To an arbitrary
+ stack depth N from the point where a UT is selected within a constrained
+ context instantiation, there is the danger that some non-zero number of
+ templates in that stack will use operations that are not defined for the
+ given argument types---and of course, ugly instantiation errors will
+ result in that case. And of course, concept maps will be dropped, which
+ means that unintended things are likely to happen whenever a concept map
+ is 'interesting' (e.g., see the 'Semigroup' example in 6.9 stmt.late).
+ "But worse, this can happen completely without the knowledge or consent
+ of either the template author or the user of the constrained template.
+ <code>late_check</code> at least warns both the template author and the
+ template user to tread carefully---to beware of 'interesting' concept
+ maps and to take careful note of which functions and function templates
+ will be invoked within the late-checked block and within those entities.
+ But the current implicit 'pro-UT' rule has the effect of turning each
+ expression in a constrained context into a potential late-checked
+ expression. And the template found at instantiation time could belong
+ to a third-party library---which means that the finger of blame will
+ eventually point to the third-party vendor, who may remedy the situation
+ by recommending that her users revert to a previous version of the third
+ party library---one that does not use constrained templates. This
+ brings us to our next problem:
+ "The pro-UT rule is potentially a bad thing for evolving a code base:
+ part of that migration will undoubtedly involve placing both
+ unconstrained and constrained overloads of function templates within the
+ same namespace. That ensures that both CT's and UT's will be considered
+ for a given call during the instantiation of a constrained context. So
+ the implicit risk of the pro-UT rule could (rightly) scare people away
+ from placing a CT in the same namespace as a UT---thus raising the
+ barrier to making a transitional version of a library.
+ "Likewise, 'interesting' concept maps are likely to be frowned upon in
+ many shops if they could be dropped subtly as a result of the pro-UT
+ "In short, the goal of writing more type-safe generic code will become
+ somewhat incompatible with the goal of writing transitional libraries
+ that seek to bridge the gap between the 'old world' and the 'new world':
+ you can pursue one goal or the other, but in the general case, not both."
+ The pro-UT argument goes something like this:
+ [To be filled in soon]
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