[llvm-bugs] [Bug 39040] New: statement-expression and clang-block causes incorrect compilation error

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39040

            Bug ID: 39040
           Summary: statement-expression and clang-block causes incorrect
                    compilation error
           Product: clang
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: remi at qumulo.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 20905
  --> https://bugs.llvm.org/attachment.cgi?id=20905&action=edit
C program that causes the erroneous compilation error

In versions of clang before 7.0, it use to be the case that you could return a
clang-block from a gnu statement-expression just fine, but now in 7 it gives
this error for some reason:

block_statement_expression.c:4:42: error: returning block that lives on the
local stack
    __auto_type f = ({ const int c = 12; ^ { return c; }; });
                                         ^~~~~~~~~~~~~~~
1 error generated.

Statement-expressions as I understand them do not allocate their own stack, and
thus it should be fine to return a block from them. And indeed if we cast the
block to (void *) and back to the block type again, the error goes away and the
code works just fine.

I think this change in behavior was unintended and is breaking existing code
that is using clang-blocks.

I invoked clang like this: clang -fblocks block_statement_expression.c
I have included a c-code example and attached it to the issue.

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