Proposed resolution (approved by CWG 2026-03-06):

Change in 7.5.6.3 [expr.prim.lambda.capture] paragraph 15 as follows:

When the lambda-expression is evaluated, the The entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object, and the non-static data members corresponding to the init-captures are initialized as indicated by the corresponding initializer (which may be copy- or direct-initialization). (For array members, the array elements are direct-initialized in increasing subscript order.) These initializations are performed when the lambda-expression is evaluated and in the (unspecified) order in which the non-static data members are declared. The initializations shall be well-formed even if the lambda-expression is not evaluated.

(From submission #841.)

Consider:

  struct C
  {
    C(int) = delete;
    C(){};
  };

  decltype([b = C(3)](){ return 4; }()) x; //MSVC accepts; gcc and clang reject

Arguably, 7.5.6.3 [expr.prim.lambda.capture] paragraph 15 specifies that the initialization of the non-static data members only happens when the lambda-expression is evaluated, but there is no statement when the semantic contraints of such an initialization are checked.