[PATCH] D119778: [clang] Add a note "deducing return type for 'foo'"

[Arthur O'Dwyer via Phabricator via cfe-commits]

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Emit the note when we fail to deduce a return type, or deduce conflicting return types.

It's emitted a //little// too eagerly, e.g. I wish we could avoid it in cases like

  $ bin/clang++ x.cpp
  x.cpp:1:19: error: use of undeclared identifier 'x'
  auto f() { return x; }
                    ^
  x.cpp:1:1: note: deducing return type for 'f'
  auto f() { return x; }
  ^~~~
  1 error generated.

But Clang doesn't seem to be able to distinguish "parsing and type-checking the return //expression//" from "deducing the function's actual return //type//," and I'm not inclined to dig this rabbit hole deeper than I already have. I think this is still a reasonably clean place to stop digging.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D119778

Files:
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Sema/SemaStmt.cpp
  clang/test/Sema/invalid-bitwidth-expr.mm
  clang/test/SemaCXX/cxx20-p0388-unbound-ary.cpp
  clang/test/SemaCXX/cxx2b-consteval-if.cpp
  clang/test/SemaCXX/deduced-return-type-cxx14.cpp
  clang/test/SemaCXX/deduced-return-void.cpp
  clang/test/SemaCXX/std-compare-cxx2a.cpp
  clang/test/SemaCXX/typo-correction-crash.cpp
  clang/test/SemaOpenCLCXX/template-astype.cl
  clang/test/SemaTemplate/concepts.cpp

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