Xcode 12.2 doesn't include 10.15 SDK, causing builds to fail

[jdv85]

Description
In #2177 the default Xcode version was updated from 12.0.1 to 12.2. Unfortunately, Xcode 12.2 doesn't include the macOS 10.15 SDK. So now we need to manually select the Xcode version in order to build software targeting macOS 10.15 with the macos-10.15 image.

Area for Triage: Apple

Question, Bug, or Feature?: Bug

Virtual environments affected

• Ubuntu 16.04
• Ubuntu 18.04
• Ubuntu 20.04
• macOS 10.15
• macOS 11.0
• Windows Server 2016 R2
• Windows Server 2019

Expected behavior
The macos-10.15 image should be able to build software targetting macOS 10.15 using Xcode without manually setting the Xcode version.

Actual behavior
Builds are failing because Xcode 12.2 does not know the macOS 10.15 SDK.

Repro steps
In a pipeline job using the macos-10.15 image, add an Xcode task like this:

- task: Xcode@5
  inputs:
    actions: 'test'
    scheme: 'scheme'
    sdk: 'macosx10.15'
    configuration: 'Debug'
    xcWorkspacePath: 'project.xcodeproj'
    xcodeVersion: 'default'

[jdv85]

Here's the Xcode version that's failing (on the pipeline):

==============================================================================
Generating script.
Script contents:
xcodebuild -version
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/work/_temp/af4a1758-7e1f-49c9-a528-ecb69ecc3ff2.sh
Xcode 12.2
Build version 12B45b

And here's the list of known SDKs:

Script contents:
xcodebuild -showsdks
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/work/_temp/8ab21599-a0b6-4f19-b016-868de08bbbfb.sh
iOS SDKs:
	iOS 14.2                      	-sdk iphoneos14.2

iOS Simulator SDKs:
	Simulator - iOS 14.2          	-sdk iphonesimulator14.2

macOS SDKs:
	DriverKit 20.0                	-sdk driverkit.macosx20.0
	macOS 11.0                    	-sdk macosx11.0

tvOS SDKs:
	tvOS 14.2                     	-sdk appletvos14.2

tvOS Simulator SDKs:
	Simulator - tvOS 14.2         	-sdk appletvsimulator14.2

watchOS SDKs:
	watchOS 7.1                   	-sdk watchos7.1

watchOS Simulator SDKs:
	Simulator - watchOS 7.1       	-sdk watchsimulator7.1

The same commands after using xcode-select to go back to Xcode 12.0.1:

xcodebuild -version
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/work/_temp/a726c128-4886-4a55-81ef-b3b3a9ccd714.sh
Xcode 12.0.1
Build version 12A7300

And:

Script contents:
xcodebuild -showsdks
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc /Users/runner/work/_temp/d5adfa0f-b68a-41cb-befb-faf0678394b7.sh
iOS SDKs:
	iOS 14.0                      	-sdk iphoneos14.0

iOS Simulator SDKs:
	Simulator - iOS 14.0          	-sdk iphonesimulator14.0

macOS SDKs:
	DriverKit 19.0                	-sdk driverkit.macosx19.0
	macOS 10.15                   	-sdk macosx10.15

tvOS SDKs:
	tvOS 14.0                     	-sdk appletvos14.0

tvOS Simulator SDKs:
	Simulator - tvOS 14.0         	-sdk appletvsimulator14.0

watchOS SDKs:
	watchOS 7.0                   	-sdk watchos7.0

watchOS Simulator SDKs:
	Simulator - watchOS 7.0       	-sdk watchsimulator7.0

[al-cheb]

Hello, @jdv85
Xcode 12.2 doesn't contain MacOSX10.15.sdk - #2211 (comment)

If you still need to use Xcode 12.0:

- bash: '/bin/bash -c "sudo xcode-select -s /Applications/Xcode_12.0.app/Contents/Developer"'
  displayName: 'Set Xcode 12.0 as default'

or specifyPath for the task:
#xcodeVersion: 'default' # Optional. Options: 8, 9, 10, default, specifyPath

Duplicate: #2211