<optional>: Don't zero storage in default constructor

[Casey Carter (CaseyCarter)]

Providing a minor runtime and codesize benefit.

Thanks to Ville Voutilainen for the suggestion.

[Billy O'Neal (BillyONeal)]

Given that the compiler is moving to initialize all the things does this even have an effect?

[Michael Schellenberger Costa (miscco)]

Makes sense for me given that it is invalid to access an unassigned optional anyway.

[Casey Carter (CaseyCarter)]

Given that the compiler is moving to initialize all the things does this even have an effect?

Yes, as verified by inspection of the assembly generated for:

std::optional<std::array<int, 1024>> potato() { 
    return {};
}

Before:

; Function compile flags: /Ogtpy
;	COMDAT ?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ
_TEXT	SEGMENT
___$ReturnUdt$ = 8					; size = 4
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ PROC ; potato, COMDAT
; File C:\Users\Casey\Desktop\repro.cpp
; File c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.28919\include\optional
; Line 167
	push	4100					; 00001004H
	push	0
	push	DWORD PTR ___$ReturnUdt$[esp+4]
	call	_memset                                 ; NB: memset 4100 bytes to 0
; File C:\Users\Casey\Desktop\repro.cpp
; Line 5
	mov	eax, DWORD PTR ___$ReturnUdt$[esp+8]
; File c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.28919\include\optional
; Line 167
	add	esp, 12					; 0000000cH
; File C:\Users\Casey\Desktop\repro.cpp
; Line 6
	ret	0
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ ENDP ; potato
_TEXT	ENDS

with just the change to optional's default constructor:

; Function compile flags: /Ogtpy
;	COMDAT ?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ
_TEXT	SEGMENT
___$ReturnUdt$ = 8					; size = 4
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ PROC ; potato, COMDAT
; File C:\Users\Casey\Desktop\repro.cpp
; File c:\Users\Casey\Source\Repos\STL\stl\inc\optional
; Line 68
	mov	eax, DWORD PTR ___$ReturnUdt$[esp-4]
	mov	BYTE PTR [eax], 0                       ; zero dummy char
	mov	BYTE PTR [eax+4096], 0                  ; set _Has_value to false
; File C:\Users\Casey\Desktop\repro.cpp
; Line 6
	ret	0
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ ENDP ; potato
_TEXT	ENDS

with the _Nontrivial_dummy_type change as well:

; Function compile flags: /Ogtpy
;	COMDAT ?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ
_TEXT	SEGMENT
___$ReturnUdt$ = 8					; size = 4
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ PROC ; potato, COMDAT
; File C:\Users\Casey\Desktop\repro.cpp
; File c:\Users\Casey\Source\Repos\STL\stl\inc\optional
; Line 68
	mov	eax, DWORD PTR ___$ReturnUdt$[esp-4]
	mov	BYTE PTR [eax+4096], 0                  ; set _Has_value to false
; File C:\Users\Casey\Desktop\repro.cpp
; Line 6
	ret	0
?potato@@YA?AV?$optional@V?$array@H$0EAA@@std@@@std@@XZ ENDP ; potato
_TEXT	ENDS