[C++ Patch/RFC] PR 80145

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

this ICE on invalid code isn't a regression, thus a patch probably 
doesn't qualify for Stage 4, but IMHO I made good progress on it and I'm 
sending what I have now anyway... The ICE happens during error recovery 
after a sensible diagnostic for the first declaration in:

auto* foo() { return 0; }
auto* foo();

After the error, finish_function does:

apply_deduced_return_type (fndecl, void_type_node);
fntype = TREE_TYPE (fndecl);

which then is inconsistent with the auto* return type of the second 
declaration and leads to an ICE in merge_types (which duplicate_decls 
thought was safe to call because types_match is true (evidently: 
decls_match uses fndecl_declared_return_type)). Thus, in terms of error 
recovery, I think that in cases like the one at issue it makes sense not 
to replace auto* after the error and leave the return type untouched: 
certainly the below passes testing and avoids ICEing on the testcase at 
issue and a variant of it.

Thanks,
Paolo.

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