http://d.puremagic.com/issues/show_bug.cgi?id=4082

           Summary: nothrow main() can throw
           Product: D
           Version: future
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-04-12 04:26:37 PDT ---
This program compiles with dmd 2.043, and the main() throws, even if it's a
nothrow function:


struct Foo {
    ~this() { throw new Exception(""); }
}
nothrow void main() {
    Foo f;
    goto NEXT;
    NEXT:;
}

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Byron Heads <bheads@emich.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bheads@emich.edu


--- Comment #1 from Byron Heads <bheads@emich.edu> 2010-06-15 14:14:19 PDT ---
Same for dmd 2.047

It seems to be a problem with the goto.

The compile errors on this code:

struct foo {
        ~this() { throw new Exception( "BAD!" ); }
}

void bar() {}

nothrow void main()
{
        foo f;
        bar();
        goto STOP;
        STOP:;
}


Error: function D main 'main' is nothrow yet may throw

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--- Comment #2 from Marco Leise <Marco.Leise@gmx.de> 2012-08-15 01:06:15 PDT ---
Since it all applies to FuncDeclaration::semantic3(...), I'll add this case:

  void b(Test t) nothrow {
  }

  struct Test {
    ~this() {
      throw new Exception("i am throwing");
    }
  }

The compiler checks for thrown exceptions or - more generally - the block exit state in two cases:

1) https://github.com/D-Programming-Language/dmd/blob/869d6dbffc4ab85576a39f19085ab7270ae2191e/src/func.c#L1301

2) https://github.com/D-Programming-Language/dmd/blob/869d6dbffc4ab85576a39f19085ab7270ae2191e/src/func.c#L1577

(2) handles the case of appending destructor calls for structs passed by value to a function. Functions without return/throw/etc. just get the destructors appended, while all others get them wrapped in a try-finally clause to ensure they are called. Since destructors may throw and the function isn't checked again after their addition, nothrow doesn't necessarily hold here. That's the case in the above example.

(1) is the normal check if a function is nothrow, but throws. blockExit()
determines the way any code block exits. This may be through 'return', 'throw',
'goto', halt (assert(0)?), 'break', 'continue' or the execution may
unconditionally succeed or 'fall through'. That's my understanding anyways.

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--- Comment #3 from Marco Leise <Marco.Leise@gmx.de> 2012-08-15 02:12:57 PDT ---
It also happens when I add a switch statement and the goto is a 'goto case ...'. It seems the BEgoto flag has a viral effect. I'll try to fix it.

    int x;
    switch(x)
    {
        case 1:
            break;
        case 0:
            goto case 1; // disables nothrow check
        default:
    }

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--- Comment #4 from Marco Leise <Marco.Leise@gmx.de> 2012-08-15 03:05:26 PDT ---
Ok I think I have fixed it. The original case was due to try-finally-statements not checking their finally section for thrown exceptions. And the goto made it so that the destructor call got wrapped in a try-finally where it was hidden from the compilers eyes.

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--- Comment #5 from github-bugzilla@puremagic.com 2012-10-22 00:46:53 PDT ---
Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/748f3509827d6e0e1c1efabddc23de24aa3873e9 fix issue 4082 - Finally-block hides thrown exceptions from nothrow attribute

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #6 from bearophile_hugs@eml.cc 2012-10-22 02:23:34 PDT ---
Closed.

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