Fix TimeSpan constructor overflow

[asad24-dev]

Fixes an overflow in the day-based TimeSpan constructors where intermediate microsecond arithmetic could wrap before the existing range check.

The calculation now uses widened Int128 arithmetic before validating against the supported TimeSpan range and narrowing to long.

Regression coverage has been added for positive and negative overflow, including the reported wraparound case and the affected constructor overloads.

Local validation:

• System.Private.CoreLib compilation succeeded.
• git diff --check HEAD^ HEAD succeeded.

Resolves #134109

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[asad24-dev]

@dotnet-policy-service agree

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[Copilot]

🔵 Needs a closer look

Address the moderate performance concern regarding widened multiplication overhead in TimeSpan.cs.

Pull request overview

Fixes overflow in day-based TimeSpan constructors by widening intermediate arithmetic before range validation.

Changes:

• Uses Int128 for intermediate calculations.
• Adds regression tests for overflow and wraparound cases.

File summaries

File	Summary
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/TimeSpanTests.cs	Adds constructor overflow regression coverage.
src/libraries/System.Private.CoreLib/src/System/TimeSpan.cs	Prevents intermediate arithmetic overflow.

Review details

Suppressed comments (1)

src/libraries/System.Private.CoreLib/src/System/TimeSpan.cs:298

• Because all constructor inputs are int, only the day product needs a widened multiplication: even int.MaxValue * MicrosecondsPerHour fits in long, and the smaller-unit products are narrower. Calling the full 64-bit Math.BigMul for each of these four terms adds unnecessary high-half multiplications to every normal constructor call; keep the Int128 accumulator but use ordinary long products here (or provide benchmark evidence for the added cost).

                                     + Math.BigMul(hours, MicrosecondsPerHour)
                                     + Math.BigMul(minutes, MicrosecondsPerMinute)
                                     + Math.BigMul(seconds, MicrosecondsPerSecond)
                                     + Math.BigMul(milliseconds, MicrosecondsPerMillisecond)

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

The remaining performance comment is a non-blocking nit.

Review details

Suppressed comments (1)

src/libraries/System.Private.CoreLib/src/System/TimeSpan.cs:298

• Because all four non-day constructor components are int, each product fits in long (even int.MaxValue * MicrosecondsPerHour is below long.MaxValue), while the Int128 accumulator already prevents overflow in the additions. Calling Math.BigMul for hours/minutes/seconds/milliseconds therefore performs four unnecessary full-width multiplications on every constructor call (including the 4- and 5-argument overloads, which delegate here). Keep the widened day product, but use ordinary long products for these terms.

                                     + Math.BigMul(hours, MicrosecondsPerHour)
                                     + Math.BigMul(minutes, MicrosecondsPerMinute)
                                     + Math.BigMul(seconds, MicrosecondsPerSecond)
                                     + Math.BigMul(milliseconds, MicrosecondsPerMillisecond)

• Files reviewed: 2/2 changed files
• Comments generated: 0 new
• Review effort level: Lite

[tarekgh]

LGTM, thanks @asad24-dev