AST: Add a GenericParamList::lookUpGenericParam() method

Author: slavapestov

include/swift/AST/Decl.h

@@ -1334,6 +1334,10 @@ class GenericParamList final :
   SWIFT_DEBUG_DUMP;
 
   bool walk(ASTWalker &walker);
+
+  /// Finds a generic parameter declaration by name. This should only
+  /// be used from the SIL parser.
+  GenericTypeParamDecl *lookUpGenericParam(Identifier name) const;
 };
 
 /// A trailing where clause.

lib/AST/Decl.cpp

@@ -960,6 +960,21 @@ void GenericParamList::setDeclContext(DeclContext *dc) {
     param->setDeclContext(dc);
 }
 
+GenericTypeParamDecl *GenericParamList::lookUpGenericParam(
+    Identifier name) const {
+  for (const auto *innerParams = this;
+       innerParams != nullptr;
+       innerParams = innerParams->getOuterParameters()) {
+    for (auto *paramDecl : *innerParams) {
+      if (name == paramDecl->getName()) {
+        return const_cast<GenericTypeParamDecl *>(paramDecl);
+      }
+    }
+  }
+
+  return nullptr;
+}
+
 TrailingWhereClause::TrailingWhereClause(
                        SourceLoc whereLoc,
                        ArrayRef<RequirementRepr> requirements)