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[Concepts] Placeholder constraints and abbreviated templates

Authored by saar.raz on Jan 21 2020, 4:03 PM.

Description

[Concepts] Placeholder constraints and abbreviated templates

This patch implements P1141R2 "Yet another approach for constrained declarations".

General strategy for this patch was:

  • Expand AutoType to include optional type-constraint, reflecting the wording and easing the integration of constraints.
  • Replace autos in parameter type specifiers with invented parameters in GetTypeSpecTypeForDeclarator, using the same logic previously used for generic lambdas, now unified with abbreviated templates, by:
    • Tracking the template parameter lists in the Declarator object
    • Tracking the template parameter depth before parsing function declarators (at which point we can match template parameters against scope specifiers to know if we have an explicit template parameter list to append invented parameters to or not).
  • When encountering an AutoType in a parameter context we check a stack of InventedTemplateParameterInfo structures that contain the info required to create and accumulate invented template parameters (fields that were already present in LambdaScopeInfo, which now inherits from this class and is looked up when an auto is encountered in a lambda context).

Resubmit after fixing MSAN failures caused by incomplete initialization of AutoTypeLocs in TypeSpecLocFiller.

Differential Revision: https://reviews.llvm.org/D65042

(cherry picked from commit b481f028144ca91c15d1db3649ce14f174259e7e)