[Bug c++/81998] New: [c++14] lambda: name from init-capture not visible in lambda-declarator

["aschepler at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81998

            Bug ID: 81998
           Summary: [c++14] lambda: name from init-capture not visible in
                    lambda-declarator
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aschepler at gmail dot com
  Target Milestone: ---

g++ 7.2.0 rejects this well-formed minimal code, whether using -std=c++14 or
-std=c++1z:

    void f() {
        [x=2] (decltype(x)) {};
    }

Errors:

    main.cpp: In function 'void f()':
    main.cpp:2:20: error: 'x' was not declared in this scope
         [x=2](decltype(x)) {};
                        ^
    main.cpp:2:20: error: 'x' was not declared in this scope

C++14 [expr.prim.lambda]/11 says: "Within the lambda-expressions
lambda-declarator and compound-statement, the identifier in the init-capture
hides any declaration of the same name in scopes enclosing the
lambda-expression."

This implies the identifier ("x") from the init-capture ("x=2") should be in
scope within the lambda-declarator ("(decltype(x))").