Fix #20994 - Initialization of a struct-typed template parameter from…

[dkorpel]

… a different type fails

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Testing this PR locally

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dub run digger -- build "master + dmd#22921"

[thewilsonator]

Bunch of build kite failures

[dkorpel]

All from unit-threaded

../.dub/packages/unit-threaded/2.2.3/unit-threaded/subpackages/property/source/unit_threaded/randomized/gen.d(125,23): Error: cannot implicitly convert expression `0` of type `int` to `int[]`
struct Gen(T, T low = 0, T high = 6.022E23) if(from!"std.traits".isFloatingPoint!T) {
                      ^
../.dub/packages/unit-threaded/2.2.3/unit-threaded/subpackages/property/source/unit_threaded/randomized/gen.d(406,1):        while evaluating: `static assert(isGen!(Gen!(int[], 1LU, 1024LU)))`
static assert(isGen!(Gen!(int[])));
^

[dkorpel]

I think I need to gag the errors raised by initializer semantic during speculative template instantiation

[dkorpel]

This is weird, looks like also before this PR, default values are evaluated and error checked even if the template constraint fails:

int gen(T)(T a) if (true) => 1;
int gen(T, T min = ERR)(T a) if (false) => 2;
enum x = gen(3);

Results in:

Error: undefined identifier `ERR`

While I would expect the second overload to only be speculatively instantiated.