.NET 11 preview SDK handling double-literal to decimal conversions differently than .NET 10

[nr-ahemsath]

Describe the bug

Casting a double floating-point literal to decimal in a compile-time constant expression
produces an incorrect, overly-precise decimal value when compiled with the C# compiler
(Roslyn 5.10.0) bundled in the .NET 11 Preview 7 SDK. The same expression compiles correctly
with the Roslyn 5.9.0 compiler bundled in the .NET 10 GA SDK.

This is a compile-time issue, not a runtime one. With both the .NET 10 GA SDK and the .NET 11
Preview SDK installed side by side:

TargetFramework	Active SDK (compiler)	Runtime that executes it	Result
net11.0	11.0.100-preview.7 (unpinned)	.NET 11 preview	wrong (123.45669999999999788542481838)
net10.0	11.0.100-preview.7 (unpinned, still resolves highest)	.NET 10.0.1	wrong (123.45669999999999788542481838)
net10.0	10.0.400 (pinned via global.json)	.NET 10.0.1	correct (123.4567)

Only pinning the compiling SDK via global.json fixes the output. Changing only the
TargetFramework (while the .NET 11 preview SDK remains the highest-installed/active SDK)
has no effect — confirming the miscompilation happens at the (decimal) cast's constant-folding
step in the compiler, before the TFM-specific reference assemblies or runtime are ever involved.

Steps to reproduce

Minimal repro

decimal fromCast = (decimal) 123.4567;   // 123.4567 is a double literal
decimal fromLiteral = 123.4567m;         // true decimal literal, for comparison

Console.WriteLine(fromCast);      // expected: 123.4567
Console.WriteLine(fromLiteral);   // 123.4567
Console.WriteLine(decimal.GetBits(fromCast).SequenceEqual(decimal.GetBits(fromLiteral)));

Expected behavior

Actual output (compiled with .NET 11.0.100-preview.7.26381.103, Roslyn 5.10.0-1.26381.103):
123.45669999999999788542481838
123.4567
False

Actual behavior

Expected output (and actual output when compiled with .NET 10.0.400 GA, Roslyn 5.9.0-1.26379.115):
123.4567
123.4567
True

Is this a regression?

Yes, it worked in .NET 10.x

Are there any workarounds?

Specify decimal literals as decimal literals (e.g. 123.4567m) instead of double literals (123.4567)

dotnet --info output

.NET SDK:
 Version:           11.0.100-preview.7.26381.103
 Commit:            e2c1e00b3d
 Workload version:  11.0.100-manifests.2ce71a7f
 MSBuild version:   18.10.0-1.26381.103+e2c1e00b3

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.26200
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\11.0.100-preview.7.26381.103

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
  Version:      11.0.0-preview.7.26381.103
  Architecture: x64
  Commit:       e2c1e00b3d

.NET SDKs installed:
  10.0.400 [C:\Program Files\dotnet\sdk]
  11.0.100-preview.7.26381.103 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 11.0.0-preview.7.26381.103 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 11.0.0-preview.7.26381.103 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.30 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.19 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 10.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 11.0.0-preview.7.26381.103 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

Learn more:
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  https://aka.ms/dotnet/download

IDE version

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Other details

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