[Constraint system] Limit edge contraction to BindParam.

Edge contraction was being performed for Bind, Equal,
BindToPointerType, and BindParam constraints. However, it's behavior
on everything but BindParam is the same as what matchTypes() already
does, so only look at BindParam constraints. This simplifies the code
but shouldn't change its behavior.

Author: DougGregor

lib/Sema/ConstraintGraph.cpp

@@ -1068,10 +1068,7 @@ ConstraintGraph::computeConnectedComponents(
 /// edge in the graph.
 static bool shouldContractEdge(ConstraintKind kind) {
   switch (kind) {
-  case ConstraintKind::Bind:
   case ConstraintKind::BindParam:
-  case ConstraintKind::BindToPointerType:
-  case ConstraintKind::Equal:
     return true;
 
   default:
@@ -1103,8 +1100,6 @@ bool ConstraintGraph::contractEdges() {
     if (!(tyvar1 && tyvar2))
       continue;
 
-    auto isParamBindingConstraint = kind == ConstraintKind::BindParam;
-
     // If the argument is allowed to bind to `inout`, in general,
     // it's invalid to contract the edge between argument and parameter,
     // but if we can prove that there are no possible bindings
@@ -1114,7 +1109,7 @@ bool ConstraintGraph::contractEdges() {
     // Such action is valid because argument type variable can
     // only get its bindings from related overload, which gives
     // us enough information to decided on l-valueness.
-    if (isParamBindingConstraint && tyvar1->getImpl().canBindToInOut()) {
+    if (tyvar1->getImpl().canBindToInOut()) {
       bool isNotContractable = true;
       if (auto bindings = CS.inferBindingsFor(tyvar1)) {
         // Holes can't be contracted.
@@ -1146,26 +1141,21 @@ bool ConstraintGraph::contractEdges() {
     auto rep1 = CS.getRepresentative(tyvar1);
     auto rep2 = CS.getRepresentative(tyvar2);
 
-    if (((rep1->getImpl().canBindToLValue() ==
-          rep2->getImpl().canBindToLValue()) ||
-         // Allow l-value contractions when binding parameter types.
-         isParamBindingConstraint)) {
-      if (CS.isDebugMode()) {
-        auto &log = llvm::errs();
-        if (CS.solverState)
-          log.indent(CS.solverState->depth * 2);
-
-        log << "Contracting constraint ";
-        constraint->print(log, &CS.getASTContext().SourceMgr);
-        log << "\n";
-      }
+    if (CS.isDebugMode()) {
+      auto &log = llvm::errs();
+      if (CS.solverState)
+        log.indent(CS.solverState->depth * 2);
 
-      // Merge the edges and retire the constraint.
-      CS.retireConstraint(constraint);
-      if (rep1 != rep2)
-        CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
-      didContractEdges = true;
+      log << "Contracting constraint ";
+      constraint->print(log, &CS.getASTContext().SourceMgr);
+      log << "\n";
     }
+
+    // Merge the edges and retire the constraint.
+    CS.retireConstraint(constraint);
+    if (rep1 != rep2)
+      CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
+    didContractEdges = true;
   }
   return didContractEdges;
 }