fix issue 20044 - Compiler crash when using an opaque struct as template parameter

[ghost]

Just an unchecked access

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20044	critical	Compiler crash when using an opaque struct as template parameter

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + dmd#10173"