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Problem with templates in type definitions #4228

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vegansk opened this issue May 27, 2016 · 1 comment · Fixed by #24005
Closed

Problem with templates in type definitions #4228

vegansk opened this issue May 27, 2016 · 1 comment · Fixed by #24005

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@vegansk
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vegansk commented May 27, 2016

The code:

template seqType(t: typedesc): typedesc =
  when t is int:
    seq[int]
  else:
    seq[string]

proc mkSeq[T: int|string](v: T): seqType(T) =
  result = newSeq[T](1)
  result[0] = v

echo mkSeq("a")
# Fails here
echo mkSeq(1)

fails to compile with the message:

typetest.nim(13, 11) template/generic instantiation from here
typetest.nim(8, 21) Error: type mismatch: got (seq[int]) but expected 'seq[string]'
@vegansk
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vegansk commented May 27, 2016

The problem is that t is tyTypeDesc(tyGenericParam T) here: https://github.com/nim-lang/Nim/blob/devel/compiler/vm.nim#L1083. That's why VM chooses else branch

@zah zah self-assigned this May 13, 2017
@ghost ghost added the Generics label Oct 15, 2017
@metagn metagn self-assigned this Aug 19, 2024
@Araq Araq closed this as completed in 69ea133 Aug 26, 2024
narimiran pushed a commit that referenced this issue Dec 12, 2024
…n fixes (#24005)

fixes #4228, fixes #4990, fixes #7006, fixes #7008, fixes #8406, fixes
(remaining issue fixed), refs #8545 (works properly now with
`cast[static[bool]]` changed to `cast[bool]`), refs #22342 and #22607
(disabled tests added), succeeds #23194

Parameter and return type nodes in generic procs now undergo the same
`inGenericContext` treatment that nodes in generic type bodies do. This
allows many of the fixes in #22029 and followups to also apply to
generic proc signatures. Like #23983 however this needs some more
compiler fixes, but this time mostly in `sigmatch` and type
instantiations.

1. `tryReadingGenericParam` no longer treats `tyCompositeTypeClass` like
a concrete type anymore, so expressions like `Foo.T` where `Foo` is a
generic type don't look for a parameter of `Foo` in non-generic code
anymore. It also doesn't generate `tyFromExpr` in non-generic code for
any generic LHS. This is to handle a very specific case in `asyncmacro`
which used `FutureVar.astToStr` where `FutureVar` is generic.
2. The `tryResolvingStaticExpr` call when matching `tyFromExpr` in
sigmatch now doesn't consider call nodes in general unresolved, only
nodes with `tyFromExpr` type, which is emitted on unresolved expressions
by increasing `c.inGenericContext`. `c.inGenericContext == 0` is also
now required to attempt instantiating `tyFromExpr`. So matching against
`tyFromExpr` in proc signatures works in general now, but I'm
speculating it depends on constant folding in `semExpr` for statics to
match against it properly.
3. `paramTypesMatch` now doesn't try to change nodes with `tyFromExpr`
type into `tyStatic` type when fitting to a static type, because it
doesn't need to, they'll be handled the same way (this was a workaround
in place of the static type instantiation changes, only one of the
fields in the #22647 test doesn't work with it).
4. `tyStatic` matching now uses `inferStaticParam` instead of just range
type matching, so `Foo[N div 2]` can infer `N` in the same way `array[N
div 2, int]` can. `inferStaticParam` also disabled itself if the
inferred static param type already had a node, but `makeStaticExpr`
generates static types with unresolved nodes, so we only disable it if
it also doesn't have a binding. This might not work very well but the
static type instantiation changes should really lower the amount of
cases where it's encountered.
5. Static types now undergo type instantiation. Previously the branch
for `tyStatic` in `semtypinst` was a no-op, now it acts similarly to
instantiating any other type with the following differences:
- Other types only need instantiation if `containsGenericType` is true,
static types also get instantiated if their value node isn't a literal
node. Ideally any value node that is "already evaluated" should be
ignored, but I'm not sure of a better way to check this, maybe if
`evalConstExpr` emitted a flag. This is purely for optimization though.
- After instantiation, `semConstExpr` is called on the value node if
`not cl.allowMetaTypes` and the type isn't literally a `static` type.
Then the type of the node is set to the base type of the static type to
deal with `semConstExpr` stripping abstract types.
We need to do this because calls like `foo(N)` where `N` is `static int`
and `foo`'s first parameter is just `int` do not generate `tyFromExpr`,
they are fully typed and so `makeStaticExpr` is called on them, giving a
static type with an unresolved node.

(cherry picked from commit 69ea133)
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