Clang - Expressive Diagnostics

# Expressive Diagnostics

In addition to being fast and functional, we aim to make Clang extremely user friendly. As far as a command-line compiler goes, this basically boils down to making the diagnostics (error and warning messages) generated by the compiler be as useful as possible. There are several ways that we do this. This section talks about the experience provided by the command line compiler, contrasting Clang output to GCC 4.2's output in several examples.

## Column Numbers and Caret Diagnostics

First, all diagnostics produced by clang include full column number information. The clang command-line compiler driver uses this information to print "caret diagnostics". (IDEs can use the information to display in-line error markup.) Precise error location in the source is a feature provided by many commercial compilers, but is generally missing from open source compilers. This is nice because it makes it very easy to understand exactly what is wrong in a particular piece of code

The caret (the blue "^" character) exactly shows where the problem is, even inside of a string. This makes it really easy to jump to the problem and helps when multiple instances of the same character occur on a line. (We'll revisit this more in following examples.)

      $ gcc-4.2 -fsyntax-only -Wformat format-strings.c
      format-strings.c:91: warning: too few arguments for format
      $ clang -fsyntax-only format-strings.c
      format-strings.c:91:13: warning: '.*' specified field precision is missing a matching 'int' argument
        printf("%.*d");
                  ^

## Range Highlighting for Related Text

Clang captures and accurately tracks range information for expressions, statements, and other constructs in your program and uses this to make diagnostics highlight related information. In the following somewhat nonsensical example you can see that you don't even need to see the original source code to understand what is wrong based on the Clang error. Because clang prints a caret, you know exactly which plus it is complaining about. The range information highlights the left and right side of the plus which makes it immediately obvious what the compiler is talking about. Range information is very useful for cases involving precedence issues and many other cases.

      $ gcc-4.2 -fsyntax-only t.c
      t.c:7: error: invalid operands to binary + (have 'int' and 'struct A')
      $ clang -fsyntax-only t.c
      t.c:7:39: error: invalid operands to binary expression ('int' and 'struct A')
        return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);
                             ~~~~~~~~~~~~~~ ^ ~~~~~

## Precision in Wording

A detail is that we have tried really hard to make the diagnostics that come out of clang contain exactly the pertinent information about what is wrong and why. In the example above, we tell you what the inferred types are for the left and right hand sides, and we don't repeat what is obvious from the caret (e.g., that this is a "binary +").

Many other examples abound. In the following example, not only do we tell you that there is a problem with the * and point to it, we say exactly why and tell you what the type is (in case it is a complicated subexpression, such as a call to an overloaded function). This sort of attention to detail makes it much easier to understand and fix problems quickly.

      $ gcc-4.2 -fsyntax-only t.c
      t.c:5: error: invalid type argument of 'unary *'
      $ clang -fsyntax-only t.c
      t.c:5:11: error: indirection requires pointer operand ('int' invalid)
        int y = *SomeA.X;
                ^~~~~~~~

## No Pretty Printing of Expressions in Diagnostics

Since Clang has range highlighting, it never needs to pretty print your code back out to you. GCC can produce inscrutible error messages in some cases when it tries to do this. In this example P and Q have type "int*":

      $ gcc-4.2 -fsyntax-only t.c
      #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object  is not a function
      $ clang -fsyntax-only t.c
      t.c:12:8: error: called object type 'int' is not a function or function pointer
        (P-Q)();
        ~~~~~^

This can be particularly bad in G++, which often emits errors containing lowered vtable references. For example:

      $ cat t.cc
      struct a {
        virtual int bar();
      };

      struct foo : public virtual a {
      };

      void test(foo *P) {
        return P->bar() + *P;
      }
      $ gcc-4.2 t.cc
      t.cc: In function 'void test(foo*)':
      t.cc:9: error: no match for 'operator+' in '(((a*)P) + (*(long int*)(P->foo::<anonymous>.a::_vptr$a + -0x00000000000000020)))->a::bar() + * P'
      t.cc:9: error: return-statement with a value, in function returning 'void'
      $ clang t.cc
      t.cc:9:18: error: invalid operands to binary expression ('int' and 'foo')
        return P->bar() + *P;
               ~~~~~~~~ ^ ~~

## Typedef Preservation and Selective Unwrapping

Many programmers use high-level user defined types, typedefs, and other syntactic sugar to refer to types in their program. This is useful because they can abbreviate otherwise very long types and it is useful to preserve the typename in diagnostics. However, sometimes very simple typedefs can wrap trivial types and it is important to strip off the typedef to understand what is going on. Clang aims to handle both cases well.

The following example shows where it is important to preserve a typedef in C. Here the type printed by GCC isn't even valid, but if the error were about a very long and complicated type (as often happens in C++) the error message would be ugly just because it was long and hard to read.

      $ gcc-4.2 -fsyntax-only t.c
      t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *')
      $ clang -fsyntax-only t.c
      t.c:15:11: error: can't convert between vector values of different size ('__m128' and 'int const *')
        myvec[1]/P;
        ~~~~~~~~^~

The following example shows where it is useful for the compiler to expose underlying details of a typedef. If the user was somehow confused about how the system "pid_t" typedef is defined, Clang helpfully displays it with "aka".

      $ gcc-4.2 -fsyntax-only t.c
      t.c:13: error: request for member 'x' in something not a structure or union
      $ clang -fsyntax-only t.c
      t.c:13:9: error: member reference base type 'pid_t' (aka 'int') is not a structure or union
        myvar = myvar.x;
                ~~~~~ ^

In C++, type preservation includes retaining any qualification written into type names. For example, if we take a small snippet of code such as:

>     namespace services {
>       struct WebService {  };
>     }
>     namespace myapp {
>       namespace servers {
>         struct Server {  };
>       }
>     }

>     using namespace myapp;
>     void addHTTPService(servers::Server const &server, ::services::WebService const *http) {
>       server += http;
>     }

and then compile it, we see that Clang is both providing more accurate information and is retaining the types as written by the user (e.g., "servers::Server", "::services::WebService"):

      $ g++-4.2 -fsyntax-only t.cpp
      t.cpp:9: error: no match for 'operator+=' in 'server += http'
      $ clang -fsyntax-only t.cpp
      t.cpp:9:10: error: invalid operands to binary expression ('servers::Server const' and '::services::WebService const *')
        server += http;
        ~~~~~~ ^  ~~~~

Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like `std::vector<Real>`) was spelled within the source code. For example:

      $ g++-4.2 -fsyntax-only t.cpp
      t.cpp:12: error: no match for 'operator=' in 'str = vec'
      $ clang -fsyntax-only t.cpp
      t.cpp:12:7: error: incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>')
        str = vec;
            ^ ~~~

## Fix-it Hints

"Fix-it" hints provide advice for fixing small, localized problems in source code. When Clang produces a diagnostic about a particular problem that it can work around (e.g., non-standard or redundant syntax, missing keywords, common mistakes, etc.), it may also provide specific guidance in the form of a code transformation to correct the problem. In the following example, Clang warns about the use of a GCC extension that has been considered obsolete since 1993. The underlined code should be removed, then replaced with the code below the caret line (".x =" or ".y =", respectively).

      $ clang t.c
      t.c:5:28: warning: use of GNU old-style field designator extension
      struct point origin = { x: 0.0, y: 0.0 };
                              ~~ ^
                              .x =
      t.c:5:36: warning: use of GNU old-style field designator extension
      struct point origin = { x: 0.0, y: 0.0 };
                                      ~~ ^
                                      .y =

"Fix-it" hints are most useful for working around common user errors and misconceptions. For example, C++ users commonly forget the syntax for explicit specialization of class templates, as in the error in the following example. Again, after describing the problem, Clang provides the fix--add `template<>`--as part of the diagnostic.

      $ clang t.cpp
      t.cpp:9:3: error: template specialization requires 'template<>'
        struct iterator_traits<file_iterator> {
        ^
        template<>

## Automatic Macro Expansion

Many errors happen in macros that are sometimes deeply nested. With traditional compilers, you need to dig deep into the definition of the macro to understand how you got into trouble. The following simple example shows how Clang helps you out by automatically printing instantiation information and nested range information for diagnostics as they are instantiated through macros and also shows how some of the other pieces work in a bigger example.

      $ gcc-4.2 -fsyntax-only t.c
      t.c: In function 'test':
      t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float')
      $ clang -fsyntax-only t.c
      t.c:80:3: error: invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float'))
        X = MYMAX(P, F);
            ^~~~~~~~~~~
      t.c:76:94: note: instantiated from:
      #define MYMAX(A,B)    __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })
                                                                                               ~~~ ^ ~~~

Here's another real world warning that occurs in the "window" Unix package (which implements the "wwopen" class of APIs):

      $ clang -fsyntax-only t.c
      t.c:22:2: warning: type specifier missing, defaults to 'int'
              ILPAD();
              ^
      t.c:17:17: note: instantiated from:
      #define ILPAD() PAD((NROW - tt.tt_row) * 10)    /* 1 ms per char */
                      ^
      t.c:14:2: note: instantiated from:
              register i; \
              ^

In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested macros that in simple ones.

## Quality of Implementation and Attention to Detail

Finally, we have put a lot of work polishing the little things, because little things add up over time and contribute to a great user experience.

The following example shows a trivial little tweak, where we tell you to put the semicolon at the end of the line that is missing it (line 4) instead of at the beginning of the following line (line 5). This is particularly important with fixit hints and caret diagnostics, because otherwise you don't get the important context.

      $ gcc-4.2 t.c
      t.c: In function 'foo':
      t.c:5: error: expected ';' before '}' token
      $ clang t.c
      t.c:4:8: error: expected ';' after expression
        bar()
             ^
             ;

The following example shows much better error recovery than GCC. The message coming out of GCC is completely useless for diagnosing the problem. Clang tries much harder and produces a much more useful diagnosis of the problem.

      $ gcc-4.2 t.c
      t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
      $ clang t.c
      t.c:3:1: error: unknown type name 'foo_t'
      foo_t *P = 0;
      ^

The following example shows that we recover from the simple case of forgetting a ; after a struct definition much better than GCC.

      $ cat t.cc
      template<class T>
      class a {}
      class temp {};
      a<temp> b;
      struct b {
      }
      $ gcc-4.2 t.cc
      t.cc:3: error: multiple types in one declaration
      t.cc:4: error: non-template type 'a' used as a template
      t.cc:4: error: invalid type in declaration before ';' token
      t.cc:6: error: expected unqualified-id at end of input
      $ clang t.cc
      t.cc:2:11: error: expected ';' after class
      class a {}
                ^
                ;
      t.cc:6:2: error: expected ';' after struct
      }
       ^
       ;

While each of these details is minor, we feel that they all add up to provide a much more polished experience.