Bug 5896 – const overload matching is succumb to template parameter one

This code doesn't compile when -version=expect. ---- struct X { version(expect) { T opCast(T)() { return 10; } // 1a T opCast(T)()const { return 11; } // 1b } else { T opCast(T:int)() { return 10; } // 2a T opCast(T:int)()const { return 11; } // 2b } } void main() { auto xm = X(); auto xc = const(X)(); assert(cast(int)xm == 10); assert(cast(int)xc == 11); } ---- test.d(18): Error: template test.X.opCast(T) opCast(T) matches more than one template declaration, test.d(5):opCast(T) and test.d(6):opCast(T) ---- Template function matching processed by template.c TemplateDeclaration::deduceFunctionTemplateMatch(), and const overload matching is more priority than template parameter one. T:int(2a,2b) is more specialized from T(1a,1b), then first priority is MATCHexact and second is MATCHconvert. Next, currently const overload matching returns MATCHconst or not, but the priority of T(MATCHconvert) is less than MATCHconst, so both 1a and 1b return MATCHconvert, then make ambiguous. It seems to me that more priority level is need: Priority name 0 MATCHnomatch 1 MATCHconvertconst ... const conversion + template parameter conversion (new) 2 MATCHconvert ... template parameter conversion 3 MATCHconst ... const conversion 4 MATCHexact ... exact match

I don't understand why case 2a,2b works. It seems to me that case 1 is identical to: struct X{} T foo(T)(const(X) _this) { return 10; } T foo(T)(X _this) { return 11; } void main(){ auto xm = X(); assert(foo!int(xm)==11); } Which fails in exactly the same way.

(In reply to comment #1) > I don't understand why case 2a,2b works. > It seems to me that case 1 is identical to: > > struct X{} > > T foo(T)(const(X) _this) { return 10; } > > T foo(T)(X _this) { return 11; } > > void main(){ > auto xm = X(); > assert(foo!int(xm)==11); > } > > Which fails in exactly the same way. I have just started reading the code around the template, template function matching may have three steps: 1. Tempate parameter vs explicit template arguments. ex. opCast(T)() vs opCast!X() -> T vs X 2. const overload matching on ethis void f(T)() vs const void f(T)() -> (mutable) vs const 3. function parameters type matching (and may inference of implict template parameters) void f(X x) vs void f(const(X) x) -> X vs const(X) The cause of this issue may be step 1 and 2, and cause of your test case may be 3. ---- Re-explain of my test case. on xm (type is X) lookup... 1a matching: step1: opCast(T) vs opCast!(int) -> MATCHconvert step2: through -> MATCHconst -> afterward result: MATCHconvert 1b matching: step1: opCast(T) vs opCast!(int) -> MATCHconvert step2: MATCHconst if can -> !!! MATCHconst is succomb to MATCHconvert -> afterward result: MATCHconvert Both result of 1a and 1b are MATCHconvert, so make ambiguous. 2a matching: step1: opCast(T:int) vs opCast!(int) -> MATCHexact step2: through -> afterward result: MATCHexact 2b matching: step1: opCast(T:int) vs opCast!(int) -> MATCHexact step2: MATCHconst if can -> MATCHconst -> afterward result: MATCHconvert 2a is more specialized than 2b, then 2a is selected.

Patch and discussions: https://github.com/D-Programming-Language/dmd/pull/45

The 2 examples compile with 2.059

More complicated test from my pull is still broken. ---- struct X5896 { T opCast(T)(){ return 1; } // L3 const T opCast(T)(){ return 2; } // L4 immutable T opCast(T)(){ return 3; } shared T opCast(T)(){ return 4; } // L6 const shared T opCast(T)(){ return 5; } // L7 } void test5896() { auto xm = X5896 (); auto xc = const(X5896) (); auto xi = immutable(X5896) (); auto xs = shared(X5896) (); auto xcs= const(shared(X5896))(); assert(cast(int)xm == 1); // L16 assert(cast(int)xc == 2); assert(cast(int)xi == 3); // L18 assert(cast(int)xs == 4); // L19 assert(cast(int)xcs== 5); } Errors: ---- test.d(16): Error: template test.X5896.opCast matches more than one template declaration, test.d(3):opCast(T) and test.d(4):opCast(T) test.d(18): Error: template test.X5896.opCast matches more than one template declaration, test.d(4):opCast(T) and test.d(7):opCast(T) test.d(19): Error: template test.X5896.opCast matches more than one template declaration, test.d(6):opCast(T) and test.d(7):opCast(T)

Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/60d7fadd7100c52746a29fade983c5644a4c8cfb fix Issue 5896 - const overload matching is succumb to template parameter one Treat matching levels separately based on initial template arguments and inferred from function arguments. https://github.com/D-Programming-Language/dmd/commit/f21d0cc53e2af192c8441ee8de786981d3bbca57 Merge pull request #45 from 9rnsr/fix5896 Issue 5896 - const overload matching is succumb to template parameter one