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Re: [C++1y] [PATCH 3/4] Ensure implicit template parameters have distinct canonical types.


On 20.09.2013 18:47, Jason Merrill wrote:
Why is canonical_type_parameter not doing the right thing here?  I
don't see a reason we should need to treat these differently from
normal template parms.

The issue only happens with indirect parms. The type 'auto' is given a canonical type in make_auto_1. When the parm type is plain unqualified 'auto', the implicit template code replaces it with the generated type and all's well. When the parm is 'auto&' the implicit template code only replaces the template_type_parm 'auto' with the generated type, the wrapping reference_type is left with the canonical type of 'auto&'.

Example of two parm types in the same list (chain field removed):

 <reference_type 0x7f27b2ff32a0
    type <template_type_parm 0x7f27b2ff31f8 auto VOID
       align 8 symtab 0 alias set -1
       canonical type 0x7f27b2fe0f18
       index 0 level 1 orig_level 1
       reference_to_this <reference_type 0x7f27b2ff32a0>>
    unsigned DI
    size <...> unit size <...>
    align 64 symtab 0 alias set -1
    canonical type 0x7f27b2ff3150>

 <reference_type 0x7f27b2ff33f0
    type <template_type_parm 0x7f27b2ff3348 auto VOID
       align 8 symtab 0 alias set -1
       canonical type 0x7f27b2fe0f18
       index 0 level 1 orig_level 1
       reference_to_this <reference_type 0x7f27b2ff33f0>>
    unsigned DI
    size <...> unit size <...>
    align 64 symtab 0 alias set -1
    canonical type 0x7f27b2ff3150>

After replacement:

 <reference_type 0x7f27b2ff32a0
    type <template_type_parm 0x7f27b2ff3540 <auto2> VOID
       align 8 symtab 0 alias set -1
       canonical type 0x7f27b2ff3540
       index 1 level 1 orig_level 1>>
    unsigned DI
    size <...> unit size <...>
    align 64 symtab 0 alias set -1
    canonical type 0x7f27b2ff3150>

 <reference_type 0x7f27b2ff33f0
    type <template_type_parm 0x7f27b2ff35e8 <auto3> VOID
       align 8 symtab 0 alias set -1
       canonical type 0x7f27b2ff35e8
       index 2 level 1 orig_level 1>>
    unsigned DI
    size <...> unit size <...>
    align 64 symtab 0 alias set -1
    canonical type 0x7f27b2ff3150>

Looks like I'll need to expose canonical_type_parameter and push it through from the top of the parameter rather than just replace the template_type_parm. Or leave this as is and arrange for this scenario to never occur.

I guess this wouldn't happen if we were replacing on the fly. Maybe make_auto_1 could differ in behavior when in a parameter list (would not setting a canonical type get us anywhere?) or just call a different function?

I'll have a further look when I get some more time.

Cheers,
Adam


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