[Issue 4302] compiler errors using startsWith in CTFE

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4302



--- Comment #1 from Rainer Schuetze <r.sagitario at gmx.de> 2010-06-13 01:23:38 PDT ---
I've tried to untangle the startsWith code, and here's the minimal test case I
could come up with so far:

///////////////////////
template binaryFunImpl(bool b)
{
       template Body()
       {
           static assert(b);
           alias bool BodyType;
       }
       alias Body!().BodyType  ReturnType;  // line 9
}

uint startsWith(A)(A a) if (is(binaryFunImpl!(true ).ReturnType)) { return 1; }
uint startsWith(A)(A a) if (is(binaryFunImpl!(false).ReturnType)) { return 0; }
 // line 13

const uint var = startsWith(1);
///////////////////////
dmd produces:

test.d(6): Error: static assert  (b) is false
test.d(9):        instantiated from here: Body!()
test.d(13):        instantiated from here: binaryFunImpl!(false)

The error does not show up if var is not const. Also, dmd 2.032 to 2.045 do not
produce this error (2.046 fails), so it seems a compiler regression being
triggered with the new implementation of startsWith.

This is what happens:

while deducing a template match,
- a template instance of binaryFunImpl!false is created to evaluate
is(binaryFunImpl!(false).ReturnType)
- the template instance is added as a member to the module (template.c(3779))
- semantic analysis fails, so the respective startsWith alternative is rejected
- compiler attempts to compile added binaryFunImpl!false and fails

maybe, the template instance should be removed from the module member list at
template.c(3975)

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