Improve assignment type inference (#21143)

src/System.Management.Automation/engine/parser/TypeInferenceVisitor.cs

@@ -1241,7 +1241,18 @@ namespace System.Management.Automation
 
         object ICustomAstVisitor.VisitAssignmentStatement(AssignmentStatementAst assignmentStatementAst)
         {
-            return assignmentStatementAst.Left.Accept(this);
+            ExpressionAst child = assignmentStatementAst.Left;
+            while (child is AttributedExpressionAst attributeChild)
+            {
+                if (attributeChild is ConvertExpressionAst convert)
+                {
+                    return new List<PSTypeName>() { new(convert.Type.TypeName) };
+                }
+
+                child = attributeChild.Child;
+            }
+
+            return assignmentStatementAst.Right.Accept(this);
         }
 
         object ICustomAstVisitor.VisitPipeline(PipelineAst pipelineAst)

test/powershell/engine/Api/TypeInference.Tests.ps1

@@ -1469,6 +1469,26 @@ Describe "Type inference Tests" -tags "CI" {
         $res.Count | Should -Be 0
     }
 
+    It 'Infers right side of assignment expression' {
+        $res = [AstTypeInference]::InferTypeOf( { $Test1 = "Hello" }.Ast.Find({param($ast) $ast -is [Language.AssignmentStatementAst]}, $true))
+        $res.Count | Should -Be 1
+        $res.Name | Should -Be "System.String"
+    }
+
+    It 'Infers left side of assignment expression when it is a ConvertExpression' {
+        $res = [AstTypeInference]::InferTypeOf( { [string]$Test1 = 42 }.Ast.Find({param($ast) $ast -is [Language.AssignmentStatementAst]}, $true))
+        $res.Count | Should -Be 1
+        $res.Name | Should -Be "System.String"
+    }
+
+    It 'Infers left side of assignment expression when there is a ConvertExpression among other attributes' {
+        $res = [AstTypeInference]::InferTypeOf( {
+            [ValidateLength()] [string] [ValidatePattern()]$Test1 = 42
+        }.Ast.Find({param($ast) $ast -is [Language.AssignmentStatementAst]}, $true))
+        $res.Count | Should -Be 1
+        $res.Name | Should -Be "System.String"
+    }
+
     It 'Infers type of all scope variable after variable assignment' {
         $res = [AstTypeInference]::InferTypeOf( { $true = "Hello";$true }.Ast)
         $res.Count | Should -Be 1