Expression.Default and Expression.New demonstrate incorrect behavior when given structs with parameterless ctors

[GrabYourPitchforks]

When given a struct type with an explicit parameterless ctor, the APIs Expression.New and Expression.Default may incorrectly invoke (or fail to invoke) the ctor.

Scenario	Expected behavior	Actual behavior
Expression.Default(Type) [compiled]	ctor not invoked	ctor not invoked
Expression.Default(Type) [interpreted]	ctor not invoked	ctor invoked
Expression.New(Type) [compiled]	ctor invoked	ctor not invoked
Expression.New(Type) [interpreted]	ctor invoked	ctor invoked
Expression.New(ConstructorInfo) [compiled]	ctor invoked	ctor invoked
Expression.New(ConstructorInfo) [interpreted]	ctor invoked	ctor invoked

Full unit tests demonstrating the issue at GrabYourPitchforks@1a22b6a.

Found while prototyping #36194.

[ghost]

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Issue Details

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When given a struct type with an explicit parameterless ctor, the APIs Expression.New and Expression.Default may incorrectly invoke (or fail to invoke) the ctor.

Scenario	Expected behavior	Actual behavior
Expression.Default(Type) [compiled]	ctor not invoked	ctor not invoked
Expression.Default(Type) [interpreted]	ctor not invoked	ctor invoked
Expression.New(Type) [compiled]	ctor invoked	ctor not invoked
Expression.New(Type) [interpreted]	ctor invoked	ctor invoked
Expression.New(ConstructorInfo) [compiled]	ctor invoked	ctor invoked
Expression.New(ConstructorInfo) [interpreted]	ctor invoked	ctor invoked

Full unit tests demonstrating the issue at GrabYourPitchforks@1a22b6a.

Found while prototyping #36194.

Author:	GrabYourPitchforks
Assignees:	-
Labels:	area-System.Linq.Expressions, bug
Milestone:	-

[GrabYourPitchforks]

/cc @333fred as co-area owner

[eerhardt]

Adding a few more scenarios to this issue, as I found the same problem in more places while annotating System.Linq.Expressions for ILLink warnings (#45623).

Scenario	Expected behavior	Actual behavior	Link To Code
Nullable<>.GetValueOrDefault() [interpreted]	ctor not invoked	ctor invoked	link
BlockExpression Initialize Variable [interpreted]	ctor not invoked	ctor invoked	link 1 link 2

If these calls aren't changed to GetUninitializedObject, we need to revisit the suppressions being added in these places. See

https://github.com/dotnet/runtime/pull/47585/files/07c0fc14fbab9b092a2270083bd933cbee8b2248#r567222530