[core] Do not select template instances if their template arguments are invalid decls in the AST

[dpiparo]

for example, when they are only forward declared.
Side effect: less lookups are carried out during the selection, therefore less memory and runtime is used for dictionary generation.

Fixes #18982

[vgvassilev]

Any chance for writing a test?

[dpiparo]

You are right @vgvassilev , we need one and it's not too hard to write.

[github-actions]

Test Results

    19 files      19 suites   3d 10h 46m 31s ⏱️
 2 809 tests  2 808 ✅ 0 💤 1 ❌
51 871 runs  51 870 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit e5b6bee.

[vgvassilev]

@smuzaffar, can we give this a try on cmssw scale?

[smuzaffar]

sure, CMSSW tests are running via cms-sw#223

[dpiparo]

@smuzaffar thanks a lot. I see some failures but can't judge if they are caused by these changes: what would be your verdict?

[smuzaffar]

CMSSW tests look good for this. The unit tests failure during PR tests are not related to this change ( we already have those unit tests failing for cmssw IBs)

[dpiparo]

Thanks!