Validate that type expressions that will be compiled to ARM schema nodes can be expressed in ARM's type system prior to compilation (#15901)

Resolves #15277
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Author: jeskew

src/Bicep.Core.IntegrationTests/UserDefinedTypeTests.cs

@@ -1838,4 +1838,28 @@ public void Parameterized_type_recursion_raises_diagnostic(string template)
             ("BCP298", DiagnosticLevel.Error, "This type definition includes itself as required component, which creates a constraint that cannot be fulfilled."),
         });
     }
+
+    // https://www.github.com/Azure/bicep/issues/15277
+    [DataTestMethod]
+    [DataRow("type resourceDerived = resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings")]
+    [DataRow("param resourceDerived resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings")]
+    [DataRow("output resourceDerived resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings = 'foo'")]
+    [DataRow("type t = { property: resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings }")]
+    [DataRow("type t = { *: resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings }")]
+    [DataRow("type t = [ resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings ]")]
+    [DataRow("type t = resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings[]")]
+    [DataRow("func f() resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings => 'foo'")]
+    [DataRow("func f(p resourceInput<'Microsoft.Compute/virtualMachines/extensions@2019-12-01'>.properties.settings) string => 'foo'")]
+    public void Type_expressions_that_will_become_ARM_schema_nodes_are_checked_for_ARM_type_system_compatibility_prior_to_compilation(string template)
+    {
+        var result = CompilationHelper.Compile(
+            new ServiceBuilder().WithFeatureOverrides(new(TestContext, ResourceDerivedTypesEnabled: true)),
+            template);
+
+        result.Template.Should().BeNull();
+        result.ExcludingLinterDiagnostics().Should().HaveDiagnostics(new[]
+        {
+            ("BCP411", DiagnosticLevel.Error, """The type "any" cannot be used in a type assignment because it does not fit within one of ARM's primitive type categories (string, int, bool, array, object). If this is a resource type definition inaccuracy, report it using https://aka.ms/bicep-type-issues."""),
+        });
+    }
 }

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

@@ -38,6 +38,7 @@ type nullLiteral = null
 //@[19:023) [BCP289 (Error)] The type definition is not valid. (bicep https://aka.ms/bicep/core-diagnostics#BCP289) |null|
 
 type unionOfNulls = null|null
+//@[20:029) [BCP411 (Error)] The type "null" cannot be used in a type assignment because it does not fit within one of ARM's primitive type categories (string, int, bool, array, 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#BCP411) |null|null|
 //@[20:029) [BCP294 (Error)] Type unions must be reducible to a single ARM type (such as 'string', 'int', or 'bool'). (bicep https://aka.ms/bicep/core-diagnostics#BCP294) |null|null|
 
 @minLength(3)

src/Bicep.Core/Diagnostics/DiagnosticBuilder.cs

@@ -1882,6 +1882,10 @@ public Diagnostic ResourceParameterizedTypeIsDeprecated(ParameterizedTypeInstant
             }
 
             public Diagnostic AttemptToDivideByZero() => CoreError("BCP410", "Division by zero is not supported.");
+
+            public Diagnostic TypeExpressionResolvesToUnassignableType(TypeSymbol type) => CoreError(
+                "BCP411",
+                $"The type \"{type}\" cannot be used in a type assignment because it does not fit within one of ARM's primitive type categories (string, int, bool, array, object).{TypeInaccuracyClause}");
         }
 
         public static DiagnosticBuilderInternal ForPosition(TextSpan span)

src/Bicep.Core/TypeSystem/TypeAssignmentVisitor.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Concurrent;
 using System.Collections.Immutable;
+using Azure.Deployments.Core.Diagnostics;
 using Azure.Deployments.Templates.Export;
 using Bicep.Core.Diagnostics;
 using Bicep.Core.Emit;
@@ -216,13 +217,15 @@ VariableBlockSyntax block when this.binder.GetParent(block) is LambdaSyntax lamb
             });
 
         public override void VisitTypedLocalVariableSyntax(TypedLocalVariableSyntax syntax)
-            => AssignType(syntax, () =>
+            => AssignTypeWithDiagnostics(syntax, diagnostics =>
             {
                 if (typeManager.GetDeclaredType(syntax) is not { } declaredType)
                 {
                     return ErrorType.Empty();
                 }
 
+                diagnostics.WriteMultiple(ValidateTypeAssignability(syntax.Type, declaredType));
+
                 base.VisitTypedLocalVariableSyntax(syntax);
 
                 return declaredType;
@@ -521,9 +524,10 @@ public override void VisitTypeDeclarationSyntax(TypeDeclarationSyntax syntax)
 
                 if (declaredType is not null)
                 {
-                    ValidateDecorators(syntax.Decorators,
-                        declaredType is TypeType wrapped ? wrapped.Unwrapped : declaredType,
-                        diagnostics);
+                    var unwrapped = declaredType is TypeType wrapped ? wrapped.Unwrapped : declaredType;
+                    ValidateDecorators(syntax.Decorators, unwrapped, diagnostics);
+
+                    diagnostics.WriteMultiple(ValidateTypeAssignability(syntax.Value, unwrapped));
                 }
 
                 return declaredType ?? ErrorType.Empty();
@@ -600,6 +604,8 @@ public override void VisitArrayTypeMemberSyntax(ArrayTypeMemberSyntax syntax)
 
                 base.VisitArrayTypeMemberSyntax(syntax);
 
+                diagnostics.WriteMultiple(ValidateTypeAssignability(syntax.Value, declaredType));
+
                 return declaredType;
             });
 
@@ -878,6 +884,7 @@ private TypeSymbol GetDeclaredTypeAndValidateDecorators(DecorableSyntax targetSy
             }
 
             this.ValidateDecorators(targetSyntax.Decorators, declaredType, diagnostics);
+            diagnostics.WriteMultiple(ValidateTypeAssignability(typeSyntax, declaredType));
 
             return declaredType;
         }
@@ -1958,6 +1965,8 @@ public override void VisitTypedLambdaSyntax(TypedLambdaSyntax syntax)
                     CollectErrors(errors, argumentType.Type);
                 }
 
+                diagnostics.WriteMultiple(ValidateTypeAssignability(syntax.ReturnType, declaredLambdaType.ReturnType.Type));
+
                 var returnType = TypeValidator.NarrowTypeAndCollectDiagnostics(typeManager, binder, this.parsingErrorLookup, diagnostics, syntax.Body, declaredLambdaType.ReturnType.Type);
                 CollectErrors(errors, returnType);
 
@@ -2396,6 +2405,43 @@ private IEnumerable<IDiagnostic> ValidateDefaultValue(ParameterDefaultValueSynta
             return diagnosticWriter.GetDiagnostics();
         }
 
+        private IEnumerable<IDiagnostic> ValidateTypeAssignability(SyntaxBase typeSyntax, TypeSymbol assignedType)
+        {
+            if (typeSyntax is not SkippedTriviaSyntax &&
+                assignedType is not ErrorType &&
+                TryGetArmPrimitiveType(assignedType, typeSyntax) is null)
+            {
+                yield return DiagnosticBuilder.ForPosition(typeSyntax)
+                    .TypeExpressionResolvesToUnassignableType(assignedType);
+            }
+        }
+
+        private TypeSymbol? TryGetArmPrimitiveType(TypeSymbol type, SyntaxBase syntax) => type switch
+        {
+            BooleanLiteralType or BooleanType => LanguageConstants.Bool,
+            IntegerLiteralType or IntegerType => LanguageConstants.Int,
+            StringLiteralType or StringType => LanguageConstants.String,
+            ResourceType when features.ResourceTypedParamsAndOutputsEnabled => LanguageConstants.String,
+            ObjectType or DiscriminatedObjectType => LanguageConstants.Object,
+            TupleType or ArrayType => LanguageConstants.Array,
+            UnionType when TypeHelper.TryRemoveNullability(type) is { } nonNull => TryGetArmPrimitiveType(nonNull, syntax),
+            UnionType when IsExplicitUnion(syntax) => LanguageConstants.Any,
+            UnionType union when union.Members.Select(m => TryGetArmPrimitiveType(m.Type, syntax)).ToArray() is { } mTypes &&
+                !mTypes.Any(t => t is null) &&
+                mTypes.ToHashSet() is { } mUniqueTypes &&
+                mUniqueTypes.Count == 1 => mUniqueTypes.Single(),
+            _ => null,
+        };
+
+        private static bool IsExplicitUnion(SyntaxBase syntax) => syntax switch
+        {
+            UnionTypeSyntax => true,
+            ParenthesizedTypeSyntax parenthesized => IsExplicitUnion(parenthesized.Expression),
+            NonNullableTypeSyntax nonNullable => IsExplicitUnion(nonNullable.Base),
+            NullableTypeSyntax nullable => IsExplicitUnion(nullable.Base),
+            _ => false,
+        };
+
         private IEnumerable<IDiagnostic> ValidateIdentifierAccess(SyntaxBase syntax)
         {
             return SyntaxAggregator.Aggregate(syntax, new List<IDiagnostic>(), (accumulated, current) =>