NuGet package: CppWinRTPrecompiledHeader should be "." when not using PCH

[davidmatson]

In Microsoft.Windows.CppWinRT.targets, in this part of the code:

        <PropertyGroup Condition="('$(CppWinRTOverrideSDKReferences)' == 'true') or ('$(TargetPlatformVersion)' &gt; '10.0.17708.0')">
            <CppWinRTUsePrefixes Condition="'$(CppWinRTUsePrefixes)' == ''">true</CppWinRTUsePrefixes>
            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == ''">$(_PCH)</CppWinRTPrecompiledHeader>
        </PropertyGroup>

Should likely be something like this:

            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '$(UsePCH)' == 'true'">$(_PCH)</CppWinRTPrecompiledHeader>
            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '$(UsePCH)' == 'false'">.</CppWinRTPrecompiledHeader>

(Otherwise, when the project has PCH disabled, the generated code fails to compile.)

[kennykerr]

@jlaanstra

[jlaanstra]

        <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '$(UsePCH)' == 'true'">$(_PCH)</CppWinRTPrecompiledHeader>
        <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '$(UsePCH)' == 'false'">.</CppWinRTPrecompiledHeader>

Why the second line? Not sure why we would set it to a single dot?

[jlaanstra]

Actually I can't find any docs about a UsePCH flag? Where is that coming from? @davidmatson

[kennykerr]

On the command line, the . means that no precompiled header should be used since the default is pch.h.

cppwinrt/cppwinrt/main.cpp

Line 132 in 0be01db

settings.component_pch = args.value("pch", "pch.h");

cppwinrt/cppwinrt/main.cpp

Lines 138 to 141 in 0be01db

	if (settings.component_pch == ".")
	{
	settings.component_pch.clear();
	}

[kennykerr]

But I have no idea what UsePCH is. 😜

[davidmatson]

I think that was just pseudo-syntax for "checking whether this project has precompiled headers enabled or not".

Here's the syntax added to the project file when enabling them in VS 2019. (They're off by default, at least for some templates.)

<PrecompiledHeader>Use</PrecompiledHeader>

I believe the valid values are Use, Create, or NotUsing. (I'm not sure what Create should do in this case; haven't checked exactly what it means.)

[kennykerr]

"Create" means create the PCH whereas "Use" means depend on the previously created PCH. Only the "pch.cpp" would have "Create" while everything else (that depend on the PCH) would have "Use".

[kennykerr]

Feel free to contribute a fix, otherwise I'll have to close this issue. Realistically, most projects use a PCH so this isn't a major blocking issue.