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Ensure null-coalescing LHS is evaluated only once #12667

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Merged
iSazonov merged 4 commits into from
May 16, 2020
Merged

Ensure null-coalescing LHS is evaluated only once #12667

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43faebd
Ensure null-coalescing LHS is evaluated only once
rjmholt May 14, 2020
823517d
Optimise test state creation
rjmholt May 14, 2020
fddb646
Pander to CodeFactor's brainless opinions
rjmholt May 14, 2020
7170f35
Address @iSazonov's feedback
rjmholt May 15, 2020
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24 changes: 14 additions & 10 deletions src/System.Management.Automation/engine/parser/Compiler.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -982,19 +982,23 @@ private static Expression Coalesce(Expression left, Expression right)
{
return left;
}
else if (leftType == typeof(AutomationNull))
{
return right;
}
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else
{
Expression lhs = left.Cast(typeof(object));
Expression rhs = right.Cast(typeof(object));
ParameterExpression lhsStoreVar = Expression.Variable(typeof(object));
var blockParameters = new ParameterExpression[] { lhsStoreVar };
var blockStatements = new Expression[]
{
Expression.Assign(lhsStoreVar, left.Cast(typeof(object))),
Expression.Condition(
Expression.Call(CachedReflectionInfo.LanguagePrimitives_IsNull, lhsStoreVar),
right.Cast(typeof(object)),
lhsStoreVar),
};

return Expression.Condition(
Expression.Call(CachedReflectionInfo.LanguagePrimitives_IsNull, lhs),
rhs,
lhs);
return Expression.Block(
typeof(object),
blockParameters,
blockStatements);
}
}

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12 changes: 12 additions & 0 deletions test/powershell/Language/Operators/NullConditional.Tests.ps1
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Original file line number Diff line number Diff line change
Expand Up @@ -172,6 +172,18 @@ Describe 'NullCoalesceOperations' -Tags 'CI' {
It 'Lhs is $?' {
{$???$false} | Should -BeTrue
}

It 'Should only evaluate LHS once when it IS null' {
$testState = [pscustomobject]@{ Value = 0 }
(& { [void]$testState.Value++ }) ?? 'Nothing' | Should -BeExactly 'Nothing'
$testState.Value | Should -Be 1
}

It 'Should only evaluate LHS once when it is NOT null' {
$testState = [pscustomobject]@{ Value = 0 }
(& { 'Test'; [void]$testState.Value++ }) ?? 'Nothing' | Should -BeExactly 'Test'
$testState.Value | Should -Be 1
}
}

Context 'Null Coalesce ?? operator precedence' {
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