[all-commits] [llvm/llvm-project] 342e64: [Sema] Fix assertion failure when instantiating re...

[Ilya Biryukov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 342e64979afe0b3859462397c4a8abba6faa9de0
      https://github.com/llvm/llvm-project/commit/342e64979afe0b3859462397c4a8abba6faa9de0
  Author: Ilya Biryukov <ibiryukov at google.com>
  Date:   2022-06-23 (Thu, 23 Jun 2022)

  Changed paths:
    M clang/lib/Sema/SemaConcept.cpp
    M clang/lib/Sema/SemaTemplateInstantiate.cpp
    A clang/test/SemaTemplate/concepts-PR54629.cpp

  Log Message:
  -----------
  [Sema] Fix assertion failure when instantiating requires expression

Fixes #54629.
The crash is is caused by the double template instantiation.
See the added test. Here is what happens:
- Template arguments for the partial specialization get instantiated.
- This causes instantitation into the corrensponding requires
  expression.
- `TemplateInsantiator` correctly handles instantiation of parameters
  inside `RequiresExprBody` and instantiates the constraint expression
  inside the `NestedRequirement`.
- To build the substituted `NestedRequirement`, `TemplateInsantiator`
  calls `Sema::BuildNestedRequirement` calls
  `CheckConstraintSatisfaction`, which results in another template
  instantiation (with empty template arguments). This seem to be an
  implementation detail to handle constraint satisfaction and is not
  required by the standard.
- The recursive template instantiation tries to find the parameter
  inside `RequiresExprBody` and fails with the corresponding assertion.

Note that this only happens as both instantiations happen with the class
partial template specialization set as `Sema.CurContext`, which is
considered a dependent `DeclContext`.

To fix the assertion, avoid doing the recursive template instantiation
and instead evaluate resulting expressions in-place.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D127487