`val (name: T) = rhs` is mistakenly parsed the same as `val name: T = rhs`

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According to scala spec we have two following syntax endpoints that start with val keyword:

Dcl               ::=  `val' ValDcl | ...
ValDcl            ::=  ids `:' Type

PatVarDef         ::=  `val' PatDef | ...
PatDef            ::=  Pattern2 {`,' Pattern2} [`:' Type] `=' Expr

From here it's easy to see that val followed by a parenthesis can only mean a pattern which implies runtime type check rather than current compile-time one.

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Imported From: https://issues.scala-lang.org/browse/SI-8211?orig=1
Reporter: @densh
Affected Versions: 2.11.0-M8
See #8202, #900

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@densh said:
Possible fix densh/scala@3c203bc

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@xeno-by said:
@Denys, Jason. This needs a summary of our today's discussion.

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@densh said:
After discussion today the conclusion was that this behavior will not change in 2.11 but should be discussed again in 2.12 cycle.

[SethTisue]

labeling as help-wanted since ideally we would align with Scala 3 on this — under -Xsource:3, or perhaps even without