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cppdraft/utility/undefined.md

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+[utility.undefined]
+
+# 22 General utilities library [[utilities]](./#utilities)
+
+## 22.2 Utility components [[utility]](utility#undefined)
+
+### 22.2.9 Undefined behavior [utility.undefined]
+
+[🔗](#lib:unreachable)
+
+`[[noreturn]] void unreachable();
+`
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L696)
+
+*Preconditions*: false is true[.](#1.sentence-1)
+
+[*Note [1](#note-1)*:
+
+This precondition cannot be satisfied, thus the behavior
+of calling unreachable is undefined[.](#1.sentence-2)
+
+— *end note*]
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L704)
+
+[*Example [1](#example-1)*: int f(int x) {switch (x) {case 0:case 1:return x; default: std::unreachable(); }}int a = f(1); // OK, a has value 1int b = f(3); // undefined behavior — *end example*]
+
+[🔗](#lib:observable_checkpoint)
+
+`void observable_checkpoint() noexcept;
+`
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L728)
+
+*Effects*: Establishes an observable checkpoint ([[intro.abstract]](intro.abstract "4.1.2 Abstract machine"))[.](#3.sentence-1)