Check for mismatched argument type length in PatternMatcher due to Named Tuple :* syntax

[aherlihy]

Fixes #23155

After discussing at the compiler meeting, it was decided that the Named Tuple behavior should be the same as the Tuple behavior with respect to pattern matching with the :* syntax. To error in the exact same way, it should be caught here:

scala3/compiler/src/dotty/tools/dotc/typer/Applications.scala

Line 291 in 00d19df

report.error(UnapplyInvalidNumberOfArguments(qual, argTypes), pos)

For regular tuples, argTypes is an AppliedType of :* and is not yet reduced, so has length 1, so the arity check will fail. However, at this point Named Tuple types are fully reduced by tupleElementTypes here:

scala3/compiler/src/dotty/tools/dotc/typer/Applications.scala

Line 249 in 00d19df

tt.tupleElementTypes

So for Named Tuples, argTypes at the time of the check will be (Int, Int) so the arity check will pass, causing a later crash in PatternMatcher here:

scala3/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

Line 270 in 6146b90

val sym :: syms1 = syms: @unchecked

We cannot un-reduce or fail to reduce for Named Tuple types because it will cause a host of other tests to fail that rely on the fully reduced Named Tuple-value types. For now I have added a check in PatternMatcher because it’s better to fix the compiler crash ASAP, but I think we could revisit if we want something like Int :* Int :* EmptyTuple to really behave differently from (Int, Int) here, since pattern matching currently works for both Tuples and Named Tuples using (Int, Int) or Tuple2[Int, Int] , just not :*.