[llvm-bugs] [Bug 48446] New: Incorrect source location for parameters of instantiated member functions and member function templates

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48446

            Bug ID: 48446
           Summary: Incorrect source location for parameters of
                    instantiated member functions and member function
                    templates
           Product: clang
           Version: 11.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: thonerma at synopsys.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Clang 11 and earlier (I checked back as far as Clang 3.6.0) associate the
source location of (explicit) parameters of instantiated member function and
member function template specializations with the first declaration of those
entities rather than their definition.  This behavior is inconsistent with the
source locations used for the implicit 'this' parameter and with parameters of
instantiated specializations of (non-member) function templates.

The following test case illustrates the inconsistency (with exception for
implicit 'this' parameters since the AST dump doesn't include those).  The test
case places each parameter declaration on its own line to ensure the AST dump
explicitly includes the line number (line numbers are otherwise omitted in some
cases).  For the 'ft<int>' specialization, the source locations displayed for
the ParmVarDecl nodes corresponds to the location of the function template
definition from which the specialization was instantiated.  But for the
'ct<int>::mf' and 'ct<int>::mft<int>' specializations, those source locations
correspond to the location of the first declaration of the templates they were
instantiated from rather than the definitions of those templates.

Note also that, in the selected AST dump output for the class specialization
members below, the displayed parameter names do not correspond to the parameter
name at the displayed location.  The 'bad_X' names were chosen for the
declarations in order to highlight this discrepancy; the member specializations
use the "good" names associated with the member definitions, but the "bad"
source location associated with their initial declaration.

The incorrect source location is used regardless of whether the parameter has a
dependent or non-dependent type.

The following output was produced using Clang 11 and can also be viewed at
https://godbolt.org/z/v5aWzr.

$ cat t.cpp
template<typename T>
void ft(
        T bad_t,                         // Line 3
                 int bad_i               // Line 4
                          );
template<typename T>
struct ct {
  void mf(
          T bad_t,                       // Line 9
                   int bad_i             // Line 10
                            );
  template<typename U>
  void mft(
           T bad_t,                      // Line 14
                    U bad_u,             // Line 15
                             int bad_i   // Line 16
                                      );
};
template<typename T>
void ft(
        T t,                             // Line 21
             int i                       // Line 22
                  ) {}
template<typename T>
void ct<T>::mf(
               T t,                      // Line 26
                    int i                // Line 27
                         ) {}
template<typename T>
template<typename U>
void ct<T>::mft(
                T t,                     // Line 32
                     U u,                // Line 33
                          int i          // Line 34
                               ) {}
void f(ct<int> p) {
  ft(0, 1);
  p.mf(0, 1);
  p.mft(0, 1, 2);
}

$ clang -c -Xclang -ast-dump t.cpp
TranslationUnitDecl
|-FunctionTemplateDecl <line:1:1, line:5:27> line:2:6 ft
...
| `-FunctionDecl <line:20:1, line:23:22> line:20:6 used ft 'void (int, int)'
|   |-TemplateArgument type 'int'
|   | `-BuiltinType 'int'
|   |-ParmVarDecl <line:21:9, col:11> col:11 t 'int':'int'
|   |-ParmVarDecl <line:22:14, col:18> col:18 i 'int'
|   `-CompoundStmt <line:23:21, col:22>
...
|-ClassTemplateDecl <line:6:1, line:18:1> line:7:8 ct
...
| `-ClassTemplateSpecializationDecl <line:6:1, line:18:1> line:7:8 struct ct
definition
...
|   |-CXXMethodDecl <line:25:1, line:28:29> line:8:8 used mf 'void (int, int)'
|   | |-ParmVarDecl <line:9:11, col:13> col:13 t 'int':'int'
|   | |-ParmVarDecl <line:10:20, col:24> col:24 i 'int'
|   | `-CompoundStmt <line:28:28, col:29>
|   `-FunctionTemplateDecl <line:12:3, line:17:39> line:13:8 mft
...
|     `-CXXMethodDecl <line:31:1, line:35:35> line:13:8 used mft 'void (int,
int, int)'
|       |-TemplateArgument type 'int'
|       | `-BuiltinType 'int'
|       |-ParmVarDecl <line:14:12, col:14> col:14 t 'int':'int'
|       |-ParmVarDecl <line:15:21, col:23> col:23 u 'int':'int'
|       |-ParmVarDecl <line:16:30, col:34> col:34 i 'int'
|       `-CompoundStmt <line:35:34, col:35>
...

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