fix #25608; improve implicit range conversion checks

[ringabout]

fix #25608

This pull request improves how the compiler handles warnings for implicit range conversions, ensuring that only non-constant values trigger downsizing warnings. It also adds new test cases to verify that assignments and function calls involving compile-time constants do not produce unnecessary warnings.

Improvements to range conversion warnings:

• Updated the logic in compiler/sempass2.nim to skip implicit range conversion warnings for compile-time constants by checking if an expression is constant with getConstExpr. Now, only non-constant values will trigger the warning.

Testing enhancements:

• Added new test cases in tests/range/timplicitrangedownsizing.nim to confirm that assignments and function calls with constant enum and integer values do not trigger downsizing warnings.

[Copilot]

Pull request overview

This PR fixes #25608 by refining the compiler’s implicit range conversion warning logic so that compile-time constants do not trigger downsizing warnings, and it extends the existing range-warning test to cover constant assignments/calls.

Changes:

• Update compiler/sempass2.nim to suppress ImplicitRangeConversion warnings when the converted expression is a compile-time constant (via getConstExpr).
• Add test coverage ensuring constant enum/int values assigned/passed to range types don’t trigger downsizing warnings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
compiler/sempass2.nim	Suppresses implicit range conversion warnings for compile-time constants during effect tracking.
tests/range/timplicitrangedownsizing.nim	Adds cases for constant enum/int values to ensure no ImplicitRangeConversion warnings are emitted.

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