+/* Values can be partially 'optimized out' and/or 'unavailable'.
+ These are distinct states and have different string representations
+ and related error strings.
+
+ 'unavailable' has a specific meaning in this context. It means the
+ value exists in the program (at the machine level), but GDB has no
+ means to get to it. Such a value is normally printed as
+ <unavailable>. Examples of how to end up with an unavailable value
+ would be:
+
+ - We're inspecting a traceframe, and the memory or registers the
+ debug information says the value lives on haven't been collected.
+
+ - We're inspecting a core dump, the memory or registers the debug
+ information says the value lives aren't present in the dump
+ (that is, we have a partial/trimmed core dump, or we don't fully
+ understand/handle the core dump's format).
+
+ - We're doing live debugging, but the debug API has no means to
+ get at where the value lives in the machine, like e.g., ptrace
+ not having access to some register or register set.
+
+ - Any other similar scenario.
+
+ OTOH, "optimized out" is about what the compiler decided to generate
+ (or not generate). A chunk of a value that was optimized out does
+ not actually exist in the program. There's no way to get at it
+ short of compiling the program differently.
+
+ A register that has not been saved in a frame is likewise considered
+ optimized out, except not-saved registers have a different string
+ representation and related error strings. E.g., we'll print them as
+ <not-saved> instead of <optimized out>, as in:
+
+ (gdb) p/x $rax
+ $1 = <not saved>
+ (gdb) info registers rax
+ rax <not saved>
+
+ If the debug info describes a variable as being in such a register,
+ we'll still print the variable as <optimized out>. IOW, <not saved>
+ is reserved for inspecting registers at the machine level.
+
+ When comparing value contents, optimized out chunks, unavailable
+ chunks, and valid contents data are all considered different. See
+ value_contents_eq for more info.
+*/
+
+extern bool overload_resolution;
+