When simplifying match types, ensure fully defined before reducing

Fixes #11977

Previously the match types were more often fully defined because inlining at Typer
caused more situations where type variables were instantiated. Now that inlining
is done later, we need to compensate. I believe it's a good idea anyway since it
means we can reduce more match types.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -139,6 +139,10 @@ object TypeOps:
           case tp1 => tp1
         }
       case tp: AppliedType =>
+        tp.tycon match
+          case tycon: TypeRef if tycon.info.isInstanceOf[MatchAlias] =>
+            isFullyDefined(tp, ForceDegree.all)
+          case _ =>
         val normed = tp.tryNormalize
         if normed.exists then normed else tp.map(simplify(_, theMap))
       case tp: TypeParamRef =>

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1816,10 +1816,12 @@ object Types {
     /** A simplified version of this type which is equivalent wrt =:= to this type.
      *  This applies a typemap to the type which (as all typemaps) follows type
      *  variable instances and reduces typerefs over refined types. It also
-     *  re-evaluates all occurrences of And/OrType with &/| because
-     *  what was a union or intersection of type variables might be a simpler type
-     *  after the type variables are instantiated. Finally, it
-     *  maps poly params in the current constraint set back to their type vars.
+     *
+     *   - re-evaluates all occurrences of And/OrType with &/| because
+     *     what was a union or intersection of type variables might be a simpler type
+     *     after the type variables are instantiated.
+     *   - maps poly params in the current constraint set back to their type vars.
+     *   - forces match types to be fully defined and tries to normalize them.
      *
      *  NOTE: Simplifying an intersection type might change its erasure (for
      *  example, the Java erasure of `Object & Serializable` is `Object`,

tests/pos/i11977.scala

@@ -0,0 +1,7 @@
+object Test:
+
+  def add0(a : (Int, String), b : (Int, String)): Int =
+    val x = 3
+    x + b(0)
+
+  def add(a : (Int, String), b : (Int, String)) : (Int, String) = (a(0) + b(0), a(1) + b(1))