Don't drop member mixins from multiple mixins

This fixes a bug where a member that sources traits from multiple
mixins would have all but the last of those mixins dropped from its
list, *if* it also happened to apply new traits locally.

smithy-model/src/main/java/software/amazon/smithy/model/loader/ApplyMixin.java

@@ -99,22 +99,25 @@ public void modifyShape(
                     }
                 }
             }
-            if (introducedMember == null && memberBuilders.containsKey(member.getMemberName())) {
-                MemberShape.Builder original = memberBuilders.get(member.getMemberName());
-                introducedMember = original.addMixin(member).build();
-                if (!introducedMember.getTarget().equals(member.getTarget())) {
-                    mixinMemberConflict(original, member);
+
+            if (introducedMember == null) {
+                if (memberBuilders.containsKey(member.getMemberName())) {
+                    MemberShape.Builder original = memberBuilders.get(member.getMemberName());
+                    introducedMember = original.addMixin(member).build();
+                    if (!introducedMember.getTarget().equals(member.getTarget())) {
+                        mixinMemberConflict(original, member);
+                    }
+                } else if (!introducedTraits.isEmpty()) {
+                    // Build local member copies before adding mixins if traits
+                    // were introduced to inherited mixin members.
+                    introducedMember = MemberShape.builder()
+                            .id(targetId)
+                            .target(member.getTarget())
+                            .source(member.getSourceLocation())
+                            .addTraits(introducedTraits.values())
+                            .addMixin(member)
+                            .build();
                 }
-            } else if (!introducedTraits.isEmpty()) {
-                // Build local member copies before adding mixins if traits
-                // were introduced to inherited mixin members.
-                introducedMember = MemberShape.builder()
-                        .id(targetId)
-                        .target(member.getTarget())
-                        .source(member.getSourceLocation())
-                        .addTraits(introducedTraits.values())
-                        .addMixin(member)
-                        .build();
             }
 
             if (introducedMember != null) {

smithy-model/src/main/java/software/amazon/smithy/model/shapes/NamedMemberUtils.java

@@ -48,11 +48,11 @@ static Map<String, MemberShape> computeMixinMembers(
                 String name = member.getMemberName();
                 if (members.get().containsKey(name)) {
                     MemberShape localMember = members.get().get(name);
-                    // Rebuild the member with the proper inherited mixin if needed.
+                    // Rebuild the member with the proper inherited mixins if needed.
                     // This catches errant cases where a member is added to a structure/union
                     // but omits the mixin members of parent shapes. Arguably, that's way too
                     // nuanced and error-prone to _not_ try to smooth over.
-                    if (localMember.getMixins().isEmpty() || !mixins.containsKey(member.getId())) {
+                    if (localMember.getMixins().isEmpty() || !localMember.getMixins().contains(member.getId())) {
                         localMember = localMember.toBuilder().clearMixins().addMixin(member).build();
                     }
                     computedMembers.put(name, localMember);

smithy-model/src/test/java/software/amazon/smithy/model/loader/IdlModelLoaderTest.java

@@ -491,4 +491,11 @@ public void throwsWhenTooNested() {
         assertThat(e.getMessage(),
                    startsWith("Syntax error at line 1, column 1: Parser exceeded maximum allowed depth of 64"));
     }
+
+    @Test
+    public void handlesMultipleMemberInheritance() {
+        ValidatedResult<Model> result = Model.assembler()
+                .addImport(getClass().getResource("error-recovery/to-dollar.smithy"))
+                .assemble();
+    }
 }

smithy-model/src/test/java/software/amazon/smithy/model/loader/ModelAssemblerTest.java

@@ -18,6 +18,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.contains;
 import static org.hamcrest.Matchers.containsInAnyOrder;
+import static org.hamcrest.Matchers.containsInRelativeOrder;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.equalTo;
@@ -44,6 +45,7 @@
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -77,6 +79,8 @@
 import software.amazon.smithy.model.traits.InternalTrait;
 import software.amazon.smithy.model.traits.MediaTypeTrait;
 import software.amazon.smithy.model.traits.MixinTrait;
+import software.amazon.smithy.model.traits.PatternTrait;
+import software.amazon.smithy.model.traits.RequiredTrait;
 import software.amazon.smithy.model.traits.SensitiveTrait;
 import software.amazon.smithy.model.traits.SuppressTrait;
 import software.amazon.smithy.model.traits.TagsTrait;
@@ -1368,4 +1372,23 @@ public void resolvesDuplicateTraitApplicationsToSameMixedInMember() throws Excep
                 .assemble()
                 .unwrap();
     }
+
+    @Test
+    public void handlesMultipleInheritanceForMixinMembers() {
+        Model model = Model.assembler()
+                .addImport(getClass().getResource("mixins/multiple-inheritance-with-introduction.smithy"))
+                .assemble()
+                .unwrap();
+
+        MemberShape shape = model.expectShape(ShapeId.from("com.example#FinalStructure$member"), MemberShape.class);
+
+        assertThat(shape.getMixins(), contains(
+                ShapeId.from("com.example#MixinA$member"),
+                ShapeId.from("com.example#MixinB$member")
+        ));
+        assertThat(shape.getAllTraits().keySet(),
+                containsInAnyOrder(PatternTrait.ID, RequiredTrait.ID, InternalTrait.ID));
+        String actualPattern = shape.expectTrait(PatternTrait.class).getValue();
+        assertThat(actualPattern, equalTo("baz"));
+    }
 }

smithy-model/src/test/resources/software/amazon/smithy/model/loader/mixins/multiple-inheritance-with-introduction.smithy

@@ -0,0 +1,22 @@
+$version: "2"
+
+namespace com.example
+
+@mixin
+structure MixinA {
+    @pattern("foo")
+    @required
+    member: String
+}
+
+@mixin
+structure MixinB {
+    @pattern("bar")
+    @internal
+    member: String
+}
+
+structure FinalStructure with [MixinA, MixinB] {
+    @pattern("baz")
+    member: String
+}