Application#init: avoid unnecessary implicit searches

In some situations we only call `constrainResult` for its side-effects
and ignore its result, but `constrainResult` calls
`necessarilyCompatible` which will call `viewExists` as a last-try.
Since `viewExists` doesn't have side-effects we might as well skip it,
and it turns out that using `NoViewsAllowed.constrainResult` does
exactly that.

This leads to a significant speed-up (from ~8s to sub-second with a hot
compiler) with the test case from scala#14333 (most likely this is because at
the point where we call `constrainResult` we haven't accumulated any
constraint from the arguments of the application, so implicit search is
free to go on a wild goose chase).

I also removed obsolete comments in this method.

Author: smarter

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

@@ -433,19 +433,21 @@ trait Applications extends Compatibility {
 
     protected def init(): Unit = methType match {
       case methType: MethodType =>
-        // apply the result type constraint, unless method type is dependent
-        val resultApprox = resultTypeApprox(methType)
         val obj: Compatibility = if (ctx.typerState.isCommittable) NoViewsAllowed else Applications.this
 
-        if (!obj.constrainResult(methRef.symbol, resultApprox, resultType))
-          if (ctx.typerState.isCommittable)
-            // defer the problem until after the application;
-            // it might be healed by an implicit conversion
-            ()
-          else
-            fail(TypeMismatch(methType.resultType, resultType, None))
+        val resultApprox = resultTypeApprox(methType)
+        val sym = methRef.symbol
+        if ctx.typerState.isCommittable then
+          // Here we call `resultType` only to accumulate constraints (even if
+          // it fails, we might be able to heal the expression to conform to the
+          // result type) so don't check for views since `viewExists` doesn't
+          // have any side-effect and would only slow the compiler done (cf #14333).
+          NoViewsAllowed.constrainResult(sym, resultApprox, resultType)
+        else if !constrainResult(sym, resultApprox, resultType) then
+          // Here we actually record that this alternative failed so that
+          // overloading resolution might prune it.
+          fail(TypeMismatch(methType.resultType, resultType, None))
 
-        // match all arguments with corresponding formal parameters
         matchArgs(orderedArgs, methType.paramInfos, 0)
       case _ =>
         if (methType.isError) ok = false