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Ping @rsmith @dblaikie @MaskRay @haowei @Xiangling_L @lebedev.ri

Ping @rsmith @dblaikie @MaskRay @haowei @Xiangling_L @lebedev.ri

In D90448#2504022, @condy wrote:

The tests failed though

In D90448#2500829, @lebedev.ri wrote:

Tests missing

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Some more format fixes.

Some format fixes.

In this update I delete some extra test code, move several comments and split the new type comparing code into a new function to make sure that the commonly used old type comparing function not affected by it.

In D90448#2376710, @dblaikie wrote:

How's this compare to the similar checks for variable templates? Is there some code/checking we could share here?

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Change some format.

In D87962#2312617, @aaron.ballman wrote:

LGTM! Do you need someone to commit on your behalf?

In this revision I delete the useless main function in the new test case and rename ext_multichar_character_literal & ext_four_char_character_literal to warn_multichar_character_literal & warn_four_char_character_literal.

I restore some needless modification in the previous patch.

I change the four-char constants back to "Warning" in this revision, but set it to be ignored by default.

In D87962#2298021, @aaron.ballman wrote:

Multichar literals are implementation-defined in C and conditionally supported with implementation-defined semantics in C++. I agree that it may make sense to warn about their use for portability reasons, but I'm not certain whether it makes sense to promote their use to be always-on diagnostics. I'm curious to know if this change causes any issues with system headers (which may or may not still define four char codes) or popular libraries.

I was curious as to why this was an extension in the first place and found the original commits (https://github.com/llvm/llvm-project/commit/74c95e20af4838152a63010292d1063835176711 and https://github.com/llvm/llvm-project/commit/8577f62622d50183c7413d7507ec783d3c1486fc) but there's no justification as to why this was picked as an extension.

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