Fix gocognit output when a function has generic receiver type

recvString func in gocognit returns BADRECV when the function's receiver is a
generic type.

Implement recvString in two variants in order to keep the code
compatible with go versions less than 1.18

gocognit.go

@@ -49,18 +49,6 @@ func funcName(fn *ast.FuncDecl) string {
 	return fn.Name.Name
 }
 
-// recvString returns a string representation of recv of the
-// form "T", "*T", or "BADRECV" (if not a proper receiver type).
-func recvString(recv ast.Expr) string {
-	switch t := recv.(type) {
-	case *ast.Ident:
-		return t.Name
-	case *ast.StarExpr:
-		return "*" + recvString(t.X)
-	}
-	return "BADRECV"
-}
-
 // Complexity calculates the cognitive complexity of a function.
 func Complexity(fn *ast.FuncDecl) int {
 	v := complexityVisitor{

recv.go

@@ -0,0 +1,30 @@
+//go:build go1.18
+// +build go1.18
+
+package gocognit
+
+import (
+	"go/ast"
+	"strings"
+)
+
+// recvString returns a string representation of recv of the
+// form "T", "*T", Type[T], Type[T, V], or "BADRECV" (if not a proper receiver type).
+func recvString(recv ast.Expr) string {
+	switch t := recv.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + recvString(t.X)
+	case *ast.IndexExpr:
+		return recvString(t.X) + "[" + recvString(t.Index) + "]"
+	case *ast.IndexListExpr:
+		targs := make([]string, len(t.Indices))
+		for i, exp := range t.Indices {
+			targs[i] = recvString(exp)
+		}
+
+		return recvString(t.X) + "[" + strings.Join(targs, ", ") + "]"
+	}
+	return "BADRECV"
+}

recv_pre118.go

@@ -0,0 +1,20 @@
+//go:build !go1.18
+// +build !go1.18
+
+package gocognit
+
+import (
+	"go/ast"
+)
+
+// recvString returns a string representation of recv of the
+// form "T", "*T", or "BADRECV" (if not a proper receiver type).
+func recvString(recv ast.Expr) string {
+	switch t := recv.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + recvString(t.X)
+	}
+	return "BADRECV"
+}