Improve ternary type inference (#15399)

Resolves #15397

Worked on this as an alternative way to address #15114 (instead of
#15398). This PR does two things:

1. If a ternary has a literally-typed condition (i.e., it is definitely
true or definitely false), the type engine will use the type of the
active branch's expression. E.g., the type of `true ? 'a' : 'b'` is
`'a'`, and the type of `false ? 'a' : 'b'` is `'b'`. There was a TODO
comment in TypeAssignmentVisitor suggesting this change.
2. If the types of both branches can be combined instead of represented
as a union, the type engine will do so. For example, the type of
`unknownCondition ? 'a' : 'b'` is `'a' | 'b'` (a union), but the type of
`unknownCondition ? [stringParam] : [intParam]` is `[int | string]`
(assuming the type of `stringParam` is `string` and the type of
`intParam` is `int`). This change relies on existing type collapsing
logic, so it will handle things like combining refinements on string
types and combining objects into tagged unions if possible.

One change I made to the TypeCollapser is to collapse objects that
*can't* be combined into a tagged union into an object whose properties
are a union of the possible property types of the inputs. This is
similar to how we collapse tuple types. Given a template like the
following:

```bicep
param a {
  foo: string
}

param b {
  bar: string
  *: int
}

param condition bool

var value = condition ? a : b
```

`value` would have a type of `{ bar: string, foo: int | string, *: int
}`, with all properties flagged as optional.
###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/Azure/bicep/pull/15399)

src/Bicep.Core.IntegrationTests/ScenarioTests.cs

@@ -1985,10 +1985,11 @@ param isProdLike bool
     // https://github.com/Azure/bicep/issues/2248
     public void Test_Issue2248_UnionTypeInArrayAccessBaseExpression_NegativeCase()
     {
-        var result = CompilationHelper.Compile(@"
-var foos = true ? true : []
-var primaryFoo = foos[0]
-");
+        var result = CompilationHelper.Compile("""
+            param condition bool
+            var foos = condition ? true : []
+            var primaryFoo = foos[0]
+            """);
         result.ExcludingLinterDiagnostics().Should().HaveDiagnostics(new[]
         {
             ("BCP076", DiagnosticLevel.Error, "Cannot index over expression of type \"<empty array> | true\". Arrays or objects are required.")
@@ -2009,7 +2010,7 @@ param which bool
 
 var chosenOne = which ? input : default
 
-var p = chosenOne.foo
+var p = chosenOne.?foo
 ");
         result.ExcludingLinterDiagnostics().Should().NotHaveAnyDiagnostics();
     }
@@ -3341,8 +3342,8 @@ public void Test_Issue_9736_property_access_works_with_object_union_types()
 """);
 
         result.ExcludingLinterDiagnostics().Should().HaveDiagnostics([
-            ("BCP070", DiagnosticLevel.Error, """Argument of type "(object | object) => (1 | 2)" is not assignable to parameter of type "any => string"."""),
-            ("BCP070", DiagnosticLevel.Error, """Argument of type "(object | object) => (2 | 3)" is not assignable to parameter of type "any => string"."""),
+            ("BCP070", DiagnosticLevel.Error, """Argument of type "(object | object) => int" is not assignable to parameter of type "any => string"."""),
+            ("BCP070", DiagnosticLevel.Error, """Argument of type "(object | object) => int" is not assignable to parameter of type "any => string"."""),
         ]);
     }
 
@@ -6211,7 +6212,7 @@ public void Test_Issue14839()
 ");
 
         result.ExcludingLinterDiagnostics().Should().HaveDiagnostics([
-            ("BCP053", DiagnosticLevel.Error, """The type "object" does not contain property "foo". Available properties include "bar"."""),
+            ("BCP053", DiagnosticLevel.Error, """The type "object | object" does not contain property "foo". Available properties include "bar"."""),
         ]);
     }
 
@@ -6230,6 +6231,26 @@ param firstItem object
 output foo string[] = [for item in items: item.foo]
 ");
 
+        result.ExcludingLinterDiagnostics().Should().HaveDiagnostics([
+            ("BCP187", DiagnosticLevel.Warning, """The property "foo" does not exist in the resource or type definition, although it might still be valid. If this is a resource type definition inaccuracy, report it using https://aka.ms/bicep-type-issues."""),
+        ]);
+    }
+
+    [TestMethod]
+    // https://github.com/azure/bicep/issues/14839
+    public void Test_Issue14839_3()
+    {
+        var result = CompilationHelper.Compile("""
+            param firstItem object
+
+            var items = [
+              firstItem
+              { bar: 'def' }
+            ]
+
+            output foo string[] = [for item in items: item.?foo]
+            """);
+
         result.ExcludingLinterDiagnostics().Should().NotHaveAnyDiagnostics();
     }
 

src/Bicep.Core.Samples/Files/baselines/InvalidLambdas_LF/main.symbols.bicep

@@ -95,7 +95,7 @@ var toObject7 = toObject(range(0, 10), i => '${i}', () => null)
 var ternary = map([123], true ? i => '${i}' : i => 'hello!')
 //@[32:33) Local i. Type: any. Declaration start char: 32, length: 1
 //@[46:47) Local i. Type: any. Declaration start char: 46, length: 1
-//@[04:11) Variable ternary. Type: any. Declaration start char: 0, length: 60
+//@[04:11) Variable ternary. Type: string[]. Declaration start char: 0, length: 60
 
 var outsideArgs = i => 123
 //@[18:19) Local i. Type: any. Declaration start char: 18, length: 1

src/Bicep.Core.Samples/Files/baselines/InvalidOutputs_CRLF/main.diagnostics.bicep

@@ -179,7 +179,6 @@ output o object = 'str'
 output exp string = 2 + 3
 //@[20:25) [BCP033 (Error)] Expected a value of type "string" but the provided value is of type "5". (bicep https://aka.ms/bicep/core-diagnostics#BCP033) |2 + 3|
 output union string = true ? 's' : 1
-//@[22:36) [BCP033 (Error)] Expected a value of type "string" but the provided value is of type "'s' | 1". (bicep https://aka.ms/bicep/core-diagnostics#BCP033) |true ? 's' : 1|
 output bad int = true && !4
 //@[25:27) [BCP044 (Error)] Cannot apply operator "!" to operand of type "4". (bicep https://aka.ms/bicep/core-diagnostics#BCP044) |!4|
 output deeper bool = true ? -true : (14 && 's') + 10

src/Bicep.Core.Samples/Files/baselines/InvalidResources_CRLF/main.diagnostics.bicep

@@ -2241,7 +2241,7 @@ resource dataCollectionRuleRes2 'Microsoft.Insights/dataCollectionRules@2021-04-
   properties: {
     description: dataCollectionRule.description
     destinations: empty([]) ? [for x in []: {}] : [for x in []: {}]
-//@[018:067) [BCP036 (Warning)] The property "destinations" expected a value of type "DataCollectionRuleDestinations | null" but the provided value is of type "object[] | object[]". If this is a resource type definition inaccuracy, report it using https://aka.ms/bicep-type-issues. (bicep https://aka.ms/bicep/core-diagnostics#BCP036) |empty([]) ? [for x in []: {}] : [for x in []: {}]|
+//@[018:067) [BCP036 (Warning)] The property "destinations" expected a value of type "DataCollectionRuleDestinations | null" but the provided value is of type "object[]". If this is a resource type definition inaccuracy, report it using https://aka.ms/bicep-type-issues. (bicep https://aka.ms/bicep/core-diagnostics#BCP036) |empty([]) ? [for x in []: {}] : [for x in []: {}]|
 //@[031:034) [BCP138 (Error)] For-expressions are not supported in this context. For-expressions may be used as values of resource, module, variable, and output declarations, or values of resource and module properties. (bicep https://aka.ms/bicep/core-diagnostics#BCP138) |for|
 //@[051:054) [BCP138 (Error)] For-expressions are not supported in this context. For-expressions may be used as values of resource, module, variable, and output declarations, or values of resource and module properties. (bicep https://aka.ms/bicep/core-diagnostics#BCP138) |for|
     dataSources: dataCollectionRule.dataSources

src/Bicep.Core.Samples/Files/baselines/Lambdas_LF/main.symbols.bicep

@@ -107,7 +107,7 @@ var reduceStringConcatEven = reduce(['abc', 'def', 'ghi'], '', (cur, next, i) =>
 //@[064:067) Local cur. Type: 'abc' | 'def' | 'ghi'. Declaration start char: 64, length: 3
 //@[069:073) Local next. Type: 'abc' | 'def' | 'ghi'. Declaration start char: 69, length: 4
 //@[075:076) Local i. Type: int. Declaration start char: 75, length: 1
-//@[004:026) Variable reduceStringConcatEven. Type: 'abc' | 'def' | 'ghi' | string. Declaration start char: 0, length: 117
+//@[004:026) Variable reduceStringConcatEven. Type: string. Declaration start char: 0, length: 117
 var reduceFactorial = reduce(range(1, 5), 1, (cur, next) => cur * next)
 //@[046:049) Local cur. Type: int. Declaration start char: 46, length: 3
 //@[051:055) Local next. Type: int. Declaration start char: 51, length: 4

src/Bicep.Core.Samples/Files/baselines/PrettyPrint_LF/main.symbols.bicep

@@ -43,16 +43,16 @@ var w42__ = concat('xxxxx', 'xxxxxxxxxxx')
 //@[04:09) Variable w42__. Type: string. Declaration start char: 0, length: 42
 
 var w38___ = true? 'xxxxx' : 'xxxxxx'
-//@[04:10) Variable w38___. Type: 'xxxxx' | 'xxxxxx'. Declaration start char: 0, length: 37
+//@[04:10) Variable w38___. Type: 'xxxxx'. Declaration start char: 0, length: 37
 var w39___ = true
 //@[04:10) Variable w39___. Type: 'xxxxxx'. Declaration start char: 0, length: 39
 ? 'xxxxxx' : 'xxxxxx' // suffix
 var w40___ = true ?'xxxxxx' : 'xxxxxxx'
-//@[04:10) Variable w40___. Type: 'xxxxxx' | 'xxxxxxx'. Declaration start char: 0, length: 39
+//@[04:10) Variable w40___. Type: 'xxxxxx'. Declaration start char: 0, length: 39
 var w41___ = true ? 'xxxxxxx' :         'xxxxxxx'
 //@[04:10) Variable w41___. Type: 'xxxxxxx'. Declaration start char: 0, length: 49
 var w42___ = true ? 'xxxxxxx':'xxxxxxxx'
-//@[04:10) Variable w42___. Type: 'xxxxxxx' | 'xxxxxxxx'. Declaration start char: 0, length: 40
+//@[04:10) Variable w42___. Type: 'xxxxxxx'. Declaration start char: 0, length: 40
 
 ////////////////////////////////////////////////////////////////////////////////
 //////////////////////////// Baselines for width 80 ////////////////////////////
@@ -103,9 +103,9 @@ var w78___ = /* xxxxxxxxxxxxxxxxxxxxxxxxxxxx */ true
 ? 1234567890
 : 1234567890
 var w79___ = /* xxxxxxxxxxxxxxxxxxxxxxxxxxxxx */ true ? { foo: 1 } : [12345678]
-//@[04:10) Variable w79___. Type: [12345678] | object. Declaration start char: 0, length: 79
+//@[04:10) Variable w79___. Type: object. Declaration start char: 0, length: 79
 var w80___ = true ? { foo: true, bar: false } : [123, 234, 456, { xyz: 'xxxx' }]
-//@[04:10) Variable w80___. Type: [123, 234, 456, object] | object. Declaration start char: 0, length: 80
+//@[04:10) Variable w80___. Type: object. Declaration start char: 0, length: 80
 var w81___ = /* xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */ true ? 1234567890 : 1234567890
 //@[04:10) Variable w81___. Type: 1234567890. Declaration start char: 0, length: 81
 var w82___ = /* xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx */ true ? 1234567890 : 1234567890
@@ -185,22 +185,22 @@ var forceBreak10 = [1, 2, intersection({ foo: true, bar: false }, {
   foo: true})]
 
 var forceBreak11 = true // comment
-//@[04:16) Variable forceBreak11. Type: false | true. Declaration start char: 0, length: 57
+//@[04:16) Variable forceBreak11. Type: true. Declaration start char: 0, length: 57
     ? true
     : false
 var forceBreak12 = true ? true // comment
-//@[04:16) Variable forceBreak12. Type: false | true. Declaration start char: 0, length: 53
+//@[04:16) Variable forceBreak12. Type: true. Declaration start char: 0, length: 53
     : false
 var forceBreak13 = true
-//@[04:16) Variable forceBreak13. Type: false | true. Declaration start char: 0, length: 57
+//@[04:16) Variable forceBreak13. Type: true. Declaration start char: 0, length: 57
     ? true // comment
     : false
 var forceBreak14 = true ? {
-//@[04:16) Variable forceBreak14. Type: false | object. Declaration start char: 0, length: 49
+//@[04:16) Variable forceBreak14. Type: object. Declaration start char: 0, length: 49
     foo: 42
 } : false
 var forceBreak15 = true ? { foo: 0 } : {
-//@[04:16) Variable forceBreak15. Type: object | object. Declaration start char: 0, length: 52
+//@[04:16) Variable forceBreak15. Type: object. Declaration start char: 0, length: 52
     bar: 1}
 
 var forceBreak16 = union({ foo: 0 }, {

src/Bicep.Core.Samples/Files/baselines/Resources_CRLF/main.diagnostics.bicep

@@ -45,7 +45,6 @@ resource withExpressions 'Microsoft.Storage/storageAccounts@2017-10-01' = {
   properties: {
     supportsHttpsTrafficOnly: !false
     accessTier: true ? 'Hot' : 'Cold'
-//@[16:37) [BCP036 (Warning)] The property "accessTier" expected a value of type "'Cool' | 'Hot' | null" but the provided value is of type "'Cold' | 'Hot'". If this is a resource type definition inaccuracy, report it using https://aka.ms/bicep-type-issues. (bicep https://aka.ms/bicep/core-diagnostics#BCP036) |true ? 'Hot' : 'Cold'|
     encryption: {
       keySource: 'Microsoft.Storage'
       services: {

src/Bicep.Core.Samples/Files/baselines/Variables_LF/main.symbols.bicep

@@ -300,7 +300,7 @@ var isTrue = sys.max(1, 2) == 3
 var isFalse = !isTrue
 //@[04:11) Variable isFalse. Type: true. Declaration start char: 0, length: 21
 var someText = isTrue ? sys.concat('a', sys.concat('b', 'c')) : 'someText'
-//@[04:12) Variable someText. Type: 'someText' | string. Declaration start char: 0, length: 74
+//@[04:12) Variable someText. Type: 'someText'. Declaration start char: 0, length: 74
 
 // Bicep functions that cannot be converted into ARM functions
 var scopesWithoutArmRepresentation = {
@@ -320,7 +320,7 @@ var scopesWithArmRepresentation = {
 var issue1332_propname = 'ptest'
 //@[04:22) Variable issue1332_propname. Type: 'ptest'. Declaration start char: 0, length: 32
 var issue1332 = true ? {
-//@[04:13) Variable issue1332. Type: object | object. Declaration start char: 0, length: 86
+//@[04:13) Variable issue1332. Type: object. Declaration start char: 0, length: 86
     prop1: {
         '${issue1332_propname}': {}
     }

src/Bicep.Core.Samples/Files/baselines_bicepparam/Expressions/parameters.symbols.bicepparam

@@ -61,7 +61,7 @@ param myInt = sys.int(myBool ? 123 : 456)
 //@[6:11) ParameterAssignment myInt. Type: int. Declaration start char: 0, length: 41
 
 param myArray = [
-//@[6:13) ParameterAssignment myArray. Type: ['a' | 'b', false, 579, 333, 6, 5, true, false, false, true]. Declaration start char: 0, length: 123
+//@[6:13) ParameterAssignment myArray. Type: ['a', false, 579, 333, 6, 5, true, false, false, true]. Declaration start char: 0, length: 123
   (true ? 'a' : 'b')
   !true
   123 + 456

src/Bicep.Core.Samples/Files/user_submitted/201/wvd-create-hostpool/main.bicep

@@ -221,7 +221,7 @@ var applicationGroupReferencesArr = (('' == allApplicationGroupReferences)
 var hostpoolRequiredProps = {
   friendlyName: hostpoolFriendlyName
   description: hostpoolDescription
-  hostpoolType: hostpoolType
+  hostPoolType: hostpoolType
   personalDesktopAssignmentType: personalDesktopAssignmentType
   maxSessionLimit: maxSessionLimit
   loadBalancerType: loadBalancerType

src/Bicep.Core.Samples/Files/user_submitted/201/wvd-create-hostpool/main.json

@@ -526,7 +526,7 @@
     "hostpoolRequiredProps": {
       "friendlyName": "[parameters('hostpoolFriendlyName')]",
       "description": "[parameters('hostpoolDescription')]",
-      "hostpoolType": "[parameters('hostpoolType')]",
+      "hostPoolType": "[parameters('hostpoolType')]",
       "personalDesktopAssignmentType": "[parameters('personalDesktopAssignmentType')]",
       "maxSessionLimit": "[parameters('maxSessionLimit')]",
       "loadBalancerType": "[parameters('loadBalancerType')]",

src/Bicep.Core.Samples/Files/user_submitted/201/wvd-create-hostpool/main.symbolicnames.json

@@ -527,7 +527,7 @@
     "hostpoolRequiredProps": {
       "friendlyName": "[parameters('hostpoolFriendlyName')]",
       "description": "[parameters('hostpoolDescription')]",
-      "hostpoolType": "[parameters('hostpoolType')]",
+      "hostPoolType": "[parameters('hostpoolType')]",
       "personalDesktopAssignmentType": "[parameters('personalDesktopAssignmentType')]",
       "maxSessionLimit": "[parameters('maxSessionLimit')]",
       "loadBalancerType": "[parameters('loadBalancerType')]",

src/Bicep.Core.UnitTests/TypeSystem/TypeHelperTests.cs

@@ -183,7 +183,19 @@ public void Tagged_union_should_not_be_formed_when_discriminator_is_not_required
                 null),
         };
 
-        TypeHelper.TryCollapseTypes(prospectiveTaggedUnionMembers).Should().BeNull();
+        var collapsed = TypeHelper.TryCollapseTypes(prospectiveTaggedUnionMembers).Should().BeAssignableTo<ObjectType>().Subject;
+        collapsed.Properties.Should().HaveCount(3);
+        collapsed.Properties.ContainsKey("type").Should().BeTrue();
+        collapsed.Properties["type"].TypeReference.Type.Name.Should().Be("'a' | 'b'");
+        collapsed.Properties["type"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.Properties.ContainsKey("foo").Should().BeTrue();
+        collapsed.Properties["foo"].TypeReference.Type.Name.Should().Be("string");
+        collapsed.Properties["foo"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.Properties.ContainsKey("bar").Should().BeTrue();
+        collapsed.Properties["bar"].TypeReference.Type.Name.Should().Be("int");
+        collapsed.Properties["bar"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
     }
 
     [TestMethod]
@@ -203,20 +215,63 @@ public void Tagged_union_should_not_be_formed_when_multiple_members_use_same_dis
                 default,
                 new TypeProperty[]
                 {
-                    new("type", TypeFactory.CreateStringLiteralType("b"), default),
+                    new("type", TypeFactory.CreateStringLiteralType("b"), TypePropertyFlags.Required),
                     new("bar", LanguageConstants.Int, TypePropertyFlags.Required),
                 },
                 null),
             new("{type: 'a', baz: int}",
                 default,
                 new TypeProperty[]
                 {
-                    new("type", TypeFactory.CreateStringLiteralType("a"), default),
+                    new("type", TypeFactory.CreateStringLiteralType("a"), TypePropertyFlags.Required),
                     new("baz", LanguageConstants.Int, TypePropertyFlags.Required),
                 },
                 null),
         };
 
+        var collapsed = TypeHelper.TryCollapseTypes(prospectiveTaggedUnionMembers).Should().BeAssignableTo<ObjectType>().Subject;
+        collapsed.Properties.Should().HaveCount(4);
+        collapsed.Properties.ContainsKey("type").Should().BeTrue();
+        collapsed.Properties["type"].TypeReference.Type.Name.Should().Be("'a' | 'b'");
+        collapsed.Properties["type"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeTrue();
+
+        collapsed.Properties.ContainsKey("foo").Should().BeTrue();
+        collapsed.Properties["foo"].TypeReference.Type.Name.Should().Be("string");
+        collapsed.Properties["foo"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.Properties.ContainsKey("bar").Should().BeTrue();
+        collapsed.Properties["bar"].TypeReference.Type.Name.Should().Be("int");
+        collapsed.Properties["bar"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.Properties.ContainsKey("baz").Should().BeTrue();
+        collapsed.Properties["baz"].TypeReference.Type.Name.Should().Be("int");
+        collapsed.Properties["baz"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+    }
+
+    [TestMethod]
+    public void Union_should_not_be_collapsed_when_some_members_are_not_objects()
+    {
+        var prospectiveTaggedUnionMembers = new TypeSymbol[]
+        {
+            new ObjectType("{type: 'a', foo: string}",
+                default,
+                new TypeProperty[]
+                {
+                    new("type", TypeFactory.CreateStringLiteralType("a"), TypePropertyFlags.Required),
+                    new("foo", LanguageConstants.String, TypePropertyFlags.Required),
+                },
+                null),
+            new ObjectType("{type: 'b', bar: int}",
+                default,
+                new TypeProperty[]
+                {
+                    new("type", TypeFactory.CreateStringLiteralType("b"), TypePropertyFlags.Required),
+                    new("bar", LanguageConstants.Int, TypePropertyFlags.Required),
+                },
+                null),
+            LanguageConstants.String,
+        };
+
         TypeHelper.TryCollapseTypes(prospectiveTaggedUnionMembers).Should().BeNull();
     }
 
@@ -312,4 +367,40 @@ public void Tagged_union_formation_should_prefer_alpha_sorted_discriminator_when
         collapsed.Should().BeOfType<DiscriminatedObjectType>();
         collapsed.As<DiscriminatedObjectType>().DiscriminatorKey.Should().Be("fizz");
     }
+
+    [TestMethod]
+    public void Object_collapse_should_incorporate_additionalProperties_types()
+    {
+        var toCollapse = new ObjectType[]
+        {
+            new("{foo: string}",
+                default,
+                new TypeProperty[]
+                {
+                    new("foo", LanguageConstants.String, TypePropertyFlags.Required),
+                },
+                null),
+            new("{bar: string, *: int}",
+                default,
+                new TypeProperty[]
+                {
+                    new("bar", LanguageConstants.String, TypePropertyFlags.Required),
+                },
+                LanguageConstants.Int),
+        };
+
+        var collapsed = TypeHelper.TryCollapseTypes(toCollapse).Should().BeAssignableTo<ObjectType>().Subject;
+        collapsed.Properties.Should().HaveCount(2);
+        collapsed.Properties.ContainsKey("foo").Should().BeTrue();
+        collapsed.Properties["foo"].TypeReference.Type.Name.Should().Be("int | string");
+        collapsed.Properties["foo"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.Properties.ContainsKey("bar").Should().BeTrue();
+        collapsed.Properties["bar"].TypeReference.Type.Name.Should().Be("string");
+        collapsed.Properties["bar"].Flags.HasFlag(TypePropertyFlags.Required).Should().BeFalse();
+
+        collapsed.AdditionalPropertiesType.Should().NotBeNull();
+        collapsed.AdditionalPropertiesType!.Type.Name.Should().Be("int");
+        collapsed.AdditionalPropertiesFlags.HasFlag(TypePropertyFlags.FallbackProperty).Should().BeTrue();
+    }
 }