Speculatively-instantiated templates are incorrectly cached

[dlangBugzillaToGithub]

Paul Backus (@pbackus) reported this on 2018-12-01T03:21:09Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19458

CC List

• Walter Bright (@WalterBright)

Description

Example program:

--- test.d
struct A
{
    static if (__traits(compiles, hasFoo!A)) {
        void foo() {}
    }
}

enum hasFoo(T) = __traits(hasMember, T, "foo");

void main()
{
    import std.stdio;

    writeln(hasFoo!A); // false (should be true)
    writeln(__traits(hasMember, A, "foo")); // true
}
---

Output:

$ dmd test.d && ./test
false
true

When `hasFoo!A` is instantiated in the `__traits(compiles)` expression, it evaluates to false, because the body of the `static if` statement has not been compiled yet. The same instantiation is then re-used in `main`, even though it is no longer semantically correct.

Possibly related to issue 14803.