[Issue 14018] New: Treat CTFE-able functions as pure in CTFE-only contexts

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14018

          Issue ID: 14018
           Summary: Treat CTFE-able functions as pure in CTFE-only
                    contexts
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: DMD
          Assignee: nobody at puremagic.com
          Reporter: verylonglogin.reg at gmail.com

If a function is executed during CTFE it's effectively `pure` for given
parameters. This enhancement proposes to treat a function as a `pure` one in
such contexts.

E.g. it will allow implicit conversion of a function result to `immutable`:
---
int g = 0;
int[] f(int i)
{
    if(i)
        ++g;
    return new int[1];
}

static immutable s1 = f(0); // OK

void main()
{
    static immutable s2 = f(0); // OK
    immutable s3 = f(0); // error, the context doesn't require CTFE
}
---

Pros:
Allow previously rejected but logically valid code to compile.

Cons:
Function return type is treated differently depending on a calling context
(CTFE or not) which complicates language type system.

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