Fix Issue 22765 - Assertion failure in CppMangleVisitor.template_args

[RazvanN7]

Replace assert with an error message.

The error looks weird, but I just followed the pattern already present. Rewording the error should be the subject of a different PR.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22765	critical	Assertion failure in CppMangleVisitor.template_args

Testing this PR locally

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

dub run digger -- build "stable + dmd#15100"

[ibuclaw]

Internal compiler errors should never be reachable, so this PR introduced a regression.

[ibuclaw]

This PR introduced a regression https://issues.dlang.org/show_bug.cgi?id=23878

[dkorpel]

It's not a regression when it used to be worse (an assertion failure). Note the PR description:

The error looks weird, but I just followed the pattern already present. Rewording the error should be the subject of a different PR.

[ibuclaw]

The current understanding is that the fix should be elsewhere during semantic, not when we're generating the mangling after the point it should have been caught.

An ICE is still an ICE, so this test has to be thrown out until a proper fix is done.

FAIL: gdc.test/fail_compilation/test22765.d   (internal compiler error: C++ 'null' template value parameter is not supported)