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SI-6186 TypeTags no longer supported in macros
The original idea was to support both both TypeTags and ConcreteTypeTags as context bounds on macro implementations. Back then TypeTags were the implied default flavor of type tags. Basically because "TypeTag" is shorter than "ConcreteTypeTag" everyone jumped onto them and used them everywhere. That led to problems, because at that time TypeTags could reify unresolved type parameters ("unresolved" = not having TypeTag annotations for them). This led to a series of creepy errors, when one forgets to add a context bound in the middle of a chain of methods that all pass a type tag around, and then suddenly all the tags turn into pumpkins (because that unlucky method just reifies TypeRef(NoPrefix, <type parameter symbol>, Nil and passes it down the chain). Hence we decided to rename ConcreteTypeTag => TypeTag & TypeTag => AbsTypeTag, which makes a lot of sense from a reflection point of view. Unfortunately this broke macros (in a sense), because now everyone writes TypeTag context bounds on macro implementations, which breaks in trivial situations like: "def foo[T](x: T) = identity_macro(x)" (the type of x is not concrete, so macro expansion will emit an error when trying to materialize the corresponding TypeTag). Now we restore the broken balance by banning TypeTag from macro impls. This forces anyone to use AbsTypeTags, and if someone wants to check the input for presence of abstract types, it's possible to do that manually.
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -1,4 +1,4 @@ | ||
Impls_1.scala:5: error: macro implementations cannot have implicit parameters other than TypeTag evidences | ||
def foo[U: c.TypeTag: Numeric](c: Ctx) = { | ||
Impls_1.scala:5: error: macro implementations cannot have implicit parameters other than AbsTypeTag evidences | ||
def foo[U: c.AbsTypeTag: Numeric](c: Ctx) = { | ||
^ | ||
one error found |
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -1,4 +1,4 @@ | ||
Impls_Macros_1.scala:5: error: macro implementations cannot have implicit parameters other than TypeTag evidences | ||
def foo_targs[T, U: c.TypeTag](c: Ctx)(implicit x: c.Expr[Int]) = { | ||
^ | ||
Impls_Macros_1.scala:5: error: macro implementations cannot have implicit parameters other than AbsTypeTag evidences | ||
def foo_targs[T, U: c.AbsTypeTag](c: Ctx)(implicit x: c.Expr[Int]) = { | ||
^ | ||
one error found |
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -1,4 +1,4 @@ | ||
Macros_Test_2.scala:3: error: wrong number of type parameters for method foo: [T, U, V](c: scala.reflect.macros.Context)(implicit evidence$1: c.TypeTag[T], implicit evidence$2: c.TypeTag[U], implicit V: c.TypeTag[V])c.Expr[Unit] | ||
Macros_Test_2.scala:3: error: wrong number of type parameters for method foo: [T, U, V](c: scala.reflect.macros.Context)(implicit evidence$1: c.AbsTypeTag[T], implicit evidence$2: c.AbsTypeTag[U], implicit V: c.AbsTypeTag[V])c.Expr[Unit] | ||
def foo[V] = macro Impls.foo[V] | ||
^ | ||
one error found |
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2 changes: 1 addition & 1 deletion
2
test/files/run/macro-invalidusage-partialapplication-with-tparams/Impls_Macros_1.scala
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -1,3 +1,3 @@ | ||
reflective compilation has failed: | ||
|
||
Macro expansion contains free type variable T defined by foo in Impls_Macros_1.scala:7:13. Have you forgotten to use c.TypeTag annotation for this type parameter? If you have troubles tracking free type variables, consider using -Xlog-free-types | ||
reflective compilation has failed: | ||
Macro expansion contains free type variable T defined by foo in Impls_Macros_1.scala:7:13. Have you forgotten to use c.AbsTypeTag annotation for this type parameter? If you have troubles tracking free type variables, consider using -Xlog-free-types |
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