fix implementation type generation for fragment on a union (#310)

before the fix

```
=== RUN   TestGenerate/ComplexNamedFragments.graphql/Build
# command-line-arguments
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2008:8: cannot use new(TopicNewestContentNewestContentArticle) (value of type *TopicNewestContentNewestContentArticle) as type TopicNewestContentNewestContentLeafContent in assignment:
	*TopicNewestContentNewestContentArticle does not implement TopicNewestContentNewestContentLeafContent (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2011:8: cannot use new(TopicNewestContentNewestContentVideo) (value of type *TopicNewestContentNewestContentVideo) as type TopicNewestContentNewestContentLeafContent in assignment:
	*TopicNewestContentNewestContentVideo does not implement TopicNewestContentNewestContentLeafContent (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2026:7: impossible type switch case: *TopicNewestContentNewestContentArticle
	(*v) (variable of type TopicNewestContentNewestContentLeafContent) cannot have dynamic type *TopicNewestContentNewestContentArticle (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2034:7: impossible type switch case: *TopicNewestContentNewestContentVideo
	(*v) (variable of type TopicNewestContentNewestContentLeafContent) cannot have dynamic type *TopicNewestContentNewestContentVideo (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2177:8: cannot use new(UserLastContentLastContentArticle) (value of type *UserLastContentLastContentArticle) as type UserLastContentLastContentLeafContent in assignment:
	*UserLastContentLastContentArticle does not implement UserLastContentLastContentLeafContent (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2180:8: cannot use new(UserLastContentLastContentVideo) (value of type *UserLastContentLastContentVideo) as type UserLastContentLastContentLeafContent in assignment:
	*UserLastContentLastContentVideo does not implement UserLastContentLastContentLeafContent (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2195:7: impossible type switch case: *UserLastContentLastContentArticle
	(*v) (variable of type UserLastContentLastContentLeafContent) cannot have dynamic type *UserLastContentLastContentArticle (missing implementsGraphQLInterfaceSimpleLeafContent method)
/genqlient/generate/testdata/tmp/ComplexNamedFragments.graphql_727791265.go:2203:7: impossible type switch case: *UserLastContentLastContentVideo
	(*v) (variable of type UserLastContentLastContentLeafContent) cannot have dynamic type *UserLastContentLastContentVideo (missing implementsGraphQLInterfaceSimpleLeafContent method)
    /genqlient/generate/generate_test.go:120: generated code does not compile: exit status 2
```

after fix

https://github.com/Khan/genqlient/pull/310/files#diff-3eaa9f1b68120a67e7558f37dd5984e995a6df5d454656180befca84c2dbf53aR2164


I also took a look at #64 which
is the real underlaying issue i wanted to fix but that seems to be
extremely complicated and probably needs multiple weeks of work. the
nested interface issues came up on a query with union spreading. this PR
fixes the problem of spreading when the union is the same fragment,
however if the spread contains different fragment we run into #64 .

docs/CHANGELOG.md

@@ -30,6 +30,7 @@ When releasing a new version:
 ### Bug fixes:
 - The presence of negative pointer directives, i.e., `# @genqlient(pointer: false)` are now respected even in the when `optional: pointer` is set in the configuration file.
 - Made name collisions between query/mutation arguments and local function variables less likely.
+- Fix generation issue related to golang type implementation of complex graphql union fragments
 
 ## v0.6.0
 

generate/convert.go

@@ -701,6 +701,17 @@ func fragmentMatches(containingTypedef, fragmentTypedef *ast.Definition) bool {
 			return true
 		}
 	}
+
+	// Handle the special case where the fragment is on a union, then the
+	// fragment can match any of the types in the union.
+	if fragmentTypedef.Kind == ast.Union {
+		for _, typeName := range fragmentTypedef.Types {
+			if typeName == containingTypedef.Name {
+				return true
+			}
+		}
+	}
+
 	return false
 }
 

generate/testdata/queries/ComplexNamedFragments.graphql

@@ -47,3 +47,34 @@ fragment ContentFields on Content {
 query ComplexNamedFragments {
   ... on Query { ...QueryFragment }
 }
+
+## two fragments of different types with fields containing the same inline named fragment of a union
+fragment SimpleLeafContent on LeafContent {
+  ... on Article {
+    id
+  }
+  ... on Video {
+    id
+  }
+}
+
+fragment UserLastContent on User {
+  lastContent {
+    ... SimpleLeafContent
+  }
+}
+
+fragment TopicNewestContent on Topic {
+  newestContent {
+    ... SimpleLeafContent
+  }
+}
+
+query ComplexNamedFragmentsWithInlineUnion {
+  user {
+    ...UserLastContent
+  }
+  root {
+    ...TopicNewestContent
+  }
+}

generate/testdata/queries/schema.graphql

@@ -69,6 +69,8 @@ type User {
   pokemon: [Pokemon!]
   greeting: Clip
   birthdate: Date
+
+  lastContent: LeafContent
 }
 
 """An audio clip, such as of a user saying hello."""
@@ -141,6 +143,8 @@ type Topic implements Content {
   schoolGrade: String
   next: Topic
   related: [Topic!]
+
+  newestContent: LeafContent
 }
 
 input RecursiveInput {