[llvm-bugs] [Bug 26130] New: build error for unused local variable with incomplete type

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26130

            Bug ID: 26130
           Summary: build error for unused local variable with incomplete
                    type
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arnd at linaro.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

A .c file containing "static const struct undefined_struct object;" with no
user of 'object' and no defintion for 'struct undefined_struct' produces a
warning and an error with clang:

test.c:1:38: warning: tentative definition of variable with internal linkage
has incomplete non-array type 'const struct undefined_struct'
[-Wtentative-definition-incomplete-type]
static const struct undefined_struct object;
                                     ^
test.c:1:21: note: forward declaration of 'struct undefined_struct'
static const struct undefined_struct object;
                    ^
test.c:1:38: error: tentative definition has type 'const struct
undefined_struct' that is never completed
static const struct undefined_struct object;
                                     ^
test.c:1:21: note: forward declaration of 'struct undefined_struct'
static const struct undefined_struct object;
                    ^
1 warning and 1 error generated.

This is inconsistent with gcc, which compiles the file without issuing any
warning, and that causes a build failure in one file of the linux-kernel
(drivers/staging/wilc1000/wilc_spi.c) that incorrectly has a variable
definition remaining after the type was removed. I have submitted a patch to
the kernel to remove that obviously unused variable, but it seems that issuing
a warning without the build error would be more consistent behavior for clang.
(gcc should probably also warn about this).

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