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cppdraft/declval.md

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+[declval]
+
+# 22 General utilities library [[utilities]](./#utilities)
+
+## 22.2 Utility components [[utility]](utility#declval)
+
+### 22.2.6 Function template declval [declval]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L508)
+
+The library provides the function template declval to simplify the definition of
+expressions which occur as [unevaluated operands](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]")[.](#1.sentence-1)
+
+[🔗](#lib:declval)
+
+`template<class T> add_rvalue_reference_t<T> declval() noexcept;    // as unevaluated operand
+`
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L518)
+
+*Mandates*: This function is not odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule"))[.](#2.sentence-1)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L522)
+
+*Remarks*: The template parameter T of declval may be an incomplete type[.](#3.sentence-1)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L526)
+
+[*Example [1](#example-1)*:
+
+template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&); declares a function template convert which only participates in overload resolution if the
+type From can be explicitly converted to type To[.](#4.sentence-1)
+
+For another example see class
+template common_type ([[meta.trans.other]](meta.trans.other "21.3.9.7 Other transformations"))[.](#4.sentence-2)
+
+— *end example*]