[refactoring] Wrap calls to Types.subst and Types.memberType (#1115)

This is in preparation to attempt to fix #1091. Just a refactoring, no
behavior changes.

nullaway/src/main/java/com/uber/nullaway/dataflow/CoreNullnessStoreInitializer.java

@@ -1,5 +1,6 @@
 package com.uber.nullaway.dataflow;
 
+import static com.uber.nullaway.NullabilityUtil.castToNonNull;
 import static com.uber.nullaway.Nullness.NONNULL;
 import static com.uber.nullaway.Nullness.NULLABLE;
 
@@ -16,6 +17,7 @@
 import com.uber.nullaway.Config;
 import com.uber.nullaway.NullabilityUtil;
 import com.uber.nullaway.Nullness;
+import com.uber.nullaway.generics.TypeSubstitutionUtils;
 import com.uber.nullaway.handlers.Handler;
 import java.util.List;
 import java.util.Objects;
@@ -104,7 +106,9 @@ private static NullnessStore lambdaInitialStore(
     // This obtains the types of the functional interface method parameters with preserved
     // annotations in case of generic type arguments.  Only used in JSpecify mode.
     List<Type> overridenMethodParamTypeList =
-        types.memberType(ASTHelpers.getType(code), fiMethodSymbol).getParameterTypes();
+        TypeSubstitutionUtils.memberType(
+                types, castToNonNull(ASTHelpers.getType(code)), fiMethodSymbol)
+            .getParameterTypes();
     // If fiArgumentPositionNullness[i] == null, parameter position i is unannotated
     Nullness[] fiArgumentPositionNullness = new Nullness[fiMethodParameters.size()];
     boolean isFIAnnotated =

nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java

@@ -624,7 +624,8 @@ public static void compareGenericTypeParameterNullabilityForCall(
         enclosingType = methodSymbol.owner.type;
       }
       if (enclosingType != null) {
-        invokedMethodType = state.getTypes().memberType(enclosingType, methodSymbol);
+        invokedMethodType =
+            TypeSubstitutionUtils.memberType(state.getTypes(), enclosingType, methodSymbol);
       }
     }
     // substitute type arguments for generic methods
@@ -694,7 +695,8 @@ public static void checkTypeParameterNullnessForMethodOverriding(
     // Obtain type parameters for the overridden method within the context of the overriding
     // method's class
     Type methodWithTypeParams =
-        state.getTypes().memberType(overridingMethod.owner.type, overriddenMethod);
+        TypeSubstitutionUtils.memberType(
+            state.getTypes(), overridingMethod.owner.type, overriddenMethod);
 
     checkTypeParameterNullnessForOverridingMethodReturnType(
         tree, methodWithTypeParams, analysis, state);
@@ -772,7 +774,8 @@ public static Nullness getGenericMethodReturnTypeNullness(
       // annotation should have been handled by the caller)
       return Nullness.NONNULL;
     }
-    Type overriddenMethodType = state.getTypes().memberType(enclosingType, method);
+    Type overriddenMethodType =
+        TypeSubstitutionUtils.memberType(state.getTypes(), enclosingType, method);
     verify(
         overriddenMethodType instanceof ExecutableType,
         "expected ExecutableType but instead got %s",
@@ -873,7 +876,8 @@ private static Type substituteTypeArgsInGenericMethodType(
 
     Type.ForAll forAllType = (Type.ForAll) methodSymbol.type;
     Type.MethodType underlyingMethodType = (Type.MethodType) forAllType.qtype;
-    return state.getTypes().subst(underlyingMethodType, forAllType.tvars, explicitTypeArgs);
+    return TypeSubstitutionUtils.subst(
+        state.getTypes(), underlyingMethodType, forAllType.tvars, explicitTypeArgs);
   }
 
   /**
@@ -1007,7 +1011,7 @@ public static Nullness getGenericMethodParameterNullness(
       // @Nullable annotation is handled elsewhere)
       return Nullness.NONNULL;
     }
-    Type methodType = state.getTypes().memberType(enclosingType, method);
+    Type methodType = TypeSubstitutionUtils.memberType(state.getTypes(), enclosingType, method);
     Type paramType = methodType.getParameterTypes().get(parameterIndex);
     return getTypeNullness(paramType, config);
   }

nullaway/src/main/java/com/uber/nullaway/generics/TypeSubstitutionUtils.java

@@ -0,0 +1,35 @@
+package com.uber.nullaway.generics;
+
+import com.sun.tools.javac.code.Symbol;
+import com.sun.tools.javac.code.Type;
+import com.sun.tools.javac.code.Types;
+import com.sun.tools.javac.util.List;
+
+/** Utility method related to substituting type arguments for type variables. */
+public class TypeSubstitutionUtils {
+
+  /**
+   * Returns the type of {@code sym} as a member of {@code t}.
+   *
+   * @param types the {@link Types} instance
+   * @param t the enclosing type
+   * @param sym the symbol
+   * @return the type of {@code sym} as a member of {@code t}
+   */
+  public static Type memberType(Types types, Type t, Symbol sym) {
+    return types.memberType(t, sym);
+  }
+
+  /**
+   * Substitutes the types in {@code to} for the types in {@code from} in {@code t}.
+   *
+   * @param types the {@link Types} instance
+   * @param t the type to which to perform the substitution
+   * @param from the types that will be substituted out
+   * @param to the types that will be substituted in
+   * @return the type resulting from the substitution
+   */
+  public static Type subst(Types types, Type t, List<Type> from, List<Type> to) {
+    return types.subst(t, from, to);
+  }
+}