[LLVMbugs] [Bug 20461] New: Rejects valid for alleged duplicate case label when proper conversion yields different results

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20461

            Bug ID: 20461
           Summary: Rejects valid for alleged duplicate case label when
                    proper conversion yields different results
           Product: clang
           Version: trunk
          Hardware: Other
                OS: other
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hstong at ca.ibm.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The constraint on duplicate case labels is for having the same value after
conversion to the promoted type of the switch condition (C++03 subclause 6.4.2
[stmt.switch]).

In the following program, the expression val + 0 has the type which is the
promoted type of char (char must promote to int or unsigned int).
Both switches ought to compile; however, Clang compiles the first but not the
second.

GCC and EDG accept both switch statements.

### SOURCE:
$ cat casedup.cc 
void foo(unsigned char val) {
   switch (val + 0) {
      case (unsigned char)-1:
      case (unsigned short)-1:
         ;
   }
   switch (val) {
      case (unsigned char)-1:
      case (unsigned short)-1:
         ;
   }
}


### COMPILER INVOCATION AND OUTPUT:
$ clang++ casedup.cc -c
casedup.cc:9:12: warning: overflow converting case value to switch condition
type (65535 to 255) [-Wswitch]
      case (unsigned short)-1:
           ^
casedup.cc:9:12: error: duplicate case value '255'
casedup.cc:8:12: note: previous case defined here
      case (unsigned char)-1:
           ^
1 warning and 1 error generated.


### EXPECTED BEHAVIOUR:
Clean compile.


### COMPILER VERSION INFO:
$ clang++ -v
clang version 3.5.0 (trunk 212631)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Selected multilib: .;@m64

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