Suppress printing of implicit opaque generic parameters

When printing the generic parameters and requirements clauses of
declarations that involve opaque parameters, suppress the implicit
generic parameters created by the opaque parameters as well as the
generic requirements that involve said parameters.

Author: DougGregor

lib/AST/ASTPrinter.cpp

@@ -1658,7 +1658,42 @@ void PrintAST::printSingleDepthOfGenericSignature(
       });
   };
 
-  if (printParams) {
+  /// Separate the explicit generic parameters from the implicit, opaque
+  /// generic parameters. We only print the former.
+  TypeArrayView<GenericTypeParamType> opaqueGenericParams;
+  for (unsigned index : indices(genericParams)) {
+    auto gpDecl = genericParams[index]->getDecl();
+    if (!gpDecl)
+      continue;
+
+    if (gpDecl->isOpaqueType() && gpDecl->isImplicit()) {
+      // We found the first implicit opaque type parameter. Split the
+      // generic parameters array at this position.
+      opaqueGenericParams = genericParams.slice(index);
+      genericParams = genericParams.slice(0, index);
+      break;
+    }
+  }
+
+  // Determines whether a given type is based on one of the opaque generic
+  // parameters.
+  auto dependsOnOpaque = [&](Type type) {
+    if (opaqueGenericParams.empty())
+      return false;
+
+    if (!type->isTypeParameter())
+      return false;
+
+    auto rootGP = type->getRootGenericParam();
+    for (auto opaqueGP : opaqueGenericParams) {
+      if (rootGP->isEqual(opaqueGP))
+        return true;
+    }
+
+    return false;
+  };
+
+  if (printParams && !genericParams.empty()) {
     // Print the generic parameters.
     Printer << "<";
     llvm::interleave(
@@ -1686,10 +1721,17 @@ void PrintAST::printSingleDepthOfGenericSignature(
         continue;
 
       auto first = req.getFirstType();
+
+      if (dependsOnOpaque(first))
+        continue;
+
       Type second;
 
-      if (req.getKind() != RequirementKind::Layout)
+      if (req.getKind() != RequirementKind::Layout) {
         second = req.getSecondType();
+        if (dependsOnOpaque(second))
+          continue;
+      }
 
       if (!subMap.empty()) {
         Type subFirst = substParam(first);
@@ -1748,7 +1790,7 @@ void PrintAST::printSingleDepthOfGenericSignature(
     }
   }
 
-  if (printParams)
+  if (printParams && !genericParams.empty())
     Printer << ">";
 }