diff --git a/src/type/__tests__/predicate-test.js b/src/type/__tests__/predicate-test.js
index a825db0e6c..ed9fba43d0 100644
--- a/src/type/__tests__/predicate-test.js
+++ b/src/type/__tests__/predicate-test.js
@@ -157,35 +157,6 @@ describe('Type predicates', () => {
     it('returns false for custom scalar', () => {
       expect(isSpecifiedScalarType(ScalarType)).to.equal(false);
     });
-
-    it('returns false for scalar class (rather than specified instance)', () => {
-      expect(isSpecifiedScalarType(GraphQLScalarType)).to.equal(false);
-    });
-
-    it('returns false for wrapped specified scalar', () => {
-      expect(isSpecifiedScalarType(GraphQLList(GraphQLString))).to.equal(false);
-    });
-
-    it('returns false for non-scalar', () => {
-      expect(isSpecifiedScalarType(EnumType)).to.equal(false);
-      expect(isSpecifiedScalarType(Directive)).to.equal(false);
-    });
-
-    it('returns false for spec defined directive', () => {
-      expect(isSpecifiedScalarType(GraphQLSkipDirective)).to.equal(false);
-    });
-
-    it('returns false for object type named like specified scalar', () => {
-      const ObjectNamedLikeScalar = new GraphQLObjectType({
-        name: 'String',
-        fields: { serialize: { type: GraphQLString } },
-      });
-      expect(isSpecifiedScalarType(ObjectNamedLikeScalar)).to.equal(false);
-    });
-
-    it('returns false for random garbage', () => {
-      expect(isSpecifiedScalarType({ what: 'is this' })).to.equal(false);
-    });
   });
 
   describe('isObjectType', () => {
@@ -700,29 +671,5 @@ describe('Directive predicates', () => {
     it('returns false for custom directive', () => {
       expect(isSpecifiedDirective(Directive)).to.equal(false);
     });
-
-    it('returns false for directive class (rather than specified instance)', () => {
-      expect(isSpecifiedDirective(GraphQLDirective)).to.equal(false);
-    });
-
-    it('returns false for non-directive', () => {
-      expect(isSpecifiedDirective(EnumType)).to.equal(false);
-      expect(isSpecifiedDirective(ScalarType)).to.equal(false);
-    });
-
-    it('returns false for spec defined scalar type', () => {
-      expect(isSpecifiedDirective(GraphQLString)).to.equal(false);
-    });
-
-    it('returns false for scalar type named like specified directive', () => {
-      const ScalarNamedLikeDirective = new GraphQLScalarType({
-        name: 'deprecated',
-      });
-      expect(isSpecifiedDirective(ScalarNamedLikeDirective)).to.equal(false);
-    });
-
-    it('returns false for random garbage', () => {
-      expect(isSpecifiedDirective({ what: 'is this' })).to.equal(false);
-    });
   });
 });
diff --git a/src/type/directives.js b/src/type/directives.js
index bd11613dac..6b42a5e870 100644
--- a/src/type/directives.js
+++ b/src/type/directives.js
@@ -198,9 +198,8 @@ export const specifiedDirectives = Object.freeze([
   GraphQLDeprecatedDirective,
 ]);
 
-export function isSpecifiedDirective(directive: mixed): boolean %checks {
-  return (
-    isDirective(directive) &&
-    specifiedDirectives.some(({ name }) => name === directive.name)
-  );
+export function isSpecifiedDirective(
+  directive: GraphQLDirective,
+): boolean %checks {
+  return specifiedDirectives.some(({ name }) => name === directive.name);
 }
diff --git a/src/type/introspection.js b/src/type/introspection.js
index d899648bac..2a6ddd2173 100644
--- a/src/type/introspection.js
+++ b/src/type/introspection.js
@@ -14,6 +14,7 @@ import { type GraphQLDirective } from './directives';
 import { GraphQLString, GraphQLBoolean } from './scalars';
 import {
   type GraphQLType,
+  type GraphQLNamedType,
   type GraphQLInputField,
   type GraphQLEnumValue,
   type GraphQLField,
@@ -31,7 +32,6 @@ import {
   isListType,
   isNonNullType,
   isAbstractType,
-  isNamedType,
 } from './definition';
 
 export const __Schema = new GraphQLObjectType({
@@ -485,9 +485,6 @@ export const introspectionTypes = Object.freeze([
   __TypeKind,
 ]);
 
-export function isIntrospectionType(type: mixed): boolean %checks {
-  return (
-    isNamedType(type) &&
-    introspectionTypes.some(({ name }) => type.name === name)
-  );
+export function isIntrospectionType(type: GraphQLNamedType): boolean %checks {
+  return introspectionTypes.some(({ name }) => type.name === name);
 }
diff --git a/src/type/scalars.js b/src/type/scalars.js
index 7505827923..17d418c746 100644
--- a/src/type/scalars.js
+++ b/src/type/scalars.js
@@ -11,7 +11,7 @@ import { print } from '../language/printer';
 
 import { GraphQLError } from '../error/GraphQLError';
 
-import { GraphQLScalarType, isScalarType } from './definition';
+import { type GraphQLNamedType, GraphQLScalarType } from './definition';
 
 // As per the GraphQL Spec, Integers are only treated as valid when a valid
 // 32-bit signed integer, providing the broadest support across platforms.
@@ -271,9 +271,6 @@ export const specifiedScalarTypes = Object.freeze([
   GraphQLID,
 ]);
 
-export function isSpecifiedScalarType(type: mixed): boolean %checks {
-  return (
-    isScalarType(type) &&
-    specifiedScalarTypes.some(({ name }) => type.name === name)
-  );
+export function isSpecifiedScalarType(type: GraphQLNamedType): boolean %checks {
+  return specifiedScalarTypes.some(({ name }) => type.name === name);
 }
diff --git a/tstypes/type/directives.d.ts b/tstypes/type/directives.d.ts
index 7608cd8884..c6b8689810 100644
--- a/tstypes/type/directives.d.ts
+++ b/tstypes/type/directives.d.ts
@@ -69,6 +69,4 @@ export const GraphQLDeprecatedDirective: GraphQLDirective;
  */
 export const specifiedDirectives: ReadonlyArray<GraphQLDirective>;
 
-export function isSpecifiedDirective(
-  directive: any,
-): directive is GraphQLDirective;
+export function isSpecifiedDirective(directive: GraphQLDirective): boolean;
diff --git a/tstypes/type/introspection.d.ts b/tstypes/type/introspection.d.ts
index 3acfe6e210..72ccb75937 100644
--- a/tstypes/type/introspection.d.ts
+++ b/tstypes/type/introspection.d.ts
@@ -37,4 +37,4 @@ export const TypeNameMetaFieldDef: GraphQLField<any, any>;
 
 export const introspectionTypes: ReadonlyArray<GraphQLType>;
 
-export function isIntrospectionType(type: any): boolean;
+export function isIntrospectionType(type: GraphQLType): boolean;
diff --git a/tstypes/type/scalars.d.ts b/tstypes/type/scalars.d.ts
index fe16fd5cad..71593d10c2 100644
--- a/tstypes/type/scalars.d.ts
+++ b/tstypes/type/scalars.d.ts
@@ -1,4 +1,4 @@
-import { GraphQLScalarType } from './definition';
+import { GraphQLScalarType, GraphQLNamedType } from './definition';
 
 export const GraphQLInt: GraphQLScalarType;
 export const GraphQLFloat: GraphQLScalarType;
@@ -8,4 +8,4 @@ export const GraphQLID: GraphQLScalarType;
 
 export const specifiedScalarTypes: ReadonlyArray<GraphQLScalarType>;
 
-export function isSpecifiedScalarType(type: any): type is GraphQLScalarType;
+export function isSpecifiedScalarType(type: GraphQLNamedType): boolean;