Add type inference for functions without OutputType attribute and anonymous functions (PowerShell#21127)

Author: MartinGC94

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

@@ -150,6 +150,8 @@ public TypeInferenceContext(PowerShell powerShell)
 
         public TypeDefinitionAst CurrentTypeDefinitionAst { get; set; }
 
+        public HashSet<IParameterMetadataProvider> AnalyzedCommands { get; } = new HashSet<IParameterMetadataProvider>();
+
         public TypeInferenceRuntimePermissions RuntimePermissions { get; set; }
 
         internal PowerShellExecutionHelper Helper { get; }
@@ -1339,6 +1341,13 @@ private void InferTypesFrom(CommandAst commandAst, List<PSTypeName> inferredType
                 }
             }
 
+            if (commandAst.CommandElements[0] is ScriptBlockExpressionAst scriptBlock)
+            {
+                // An anonymous function like: & {"Do Something"}
+                inferredTypes.AddRange(InferTypes(scriptBlock.ScriptBlock));
+                return;
+            }
+
             PseudoBindingInfo pseudoBinding = new PseudoParameterBinder()
             .DoPseudoParameterBinding(commandAst, null, null, PseudoParameterBinder.BindingType.ParameterCompletion);
 
@@ -1421,6 +1430,21 @@ private void InferTypesFrom(CommandAst commandAst, List<PSTypeName> inferredType
                 }
             }
 
+            if ((commandInfo.OutputType.Count == 0
+                || (commandInfo.OutputType.Count == 1
+                && (commandInfo.OutputType[0].Name.EqualsOrdinalIgnoreCase(typeof(PSObject).FullName)
+                || commandInfo.OutputType[0].Name.EqualsOrdinalIgnoreCase(typeof(object).FullName))))
+                && commandInfo is IScriptCommandInfo scriptCommandInfo
+                && scriptCommandInfo.ScriptBlock.Ast is IParameterMetadataProvider scriptBlockWithParams
+                && _context.AnalyzedCommands.Add(scriptBlockWithParams))
+            {
+                // This is a function without an output type defined (or it's too generic to be useful)
+                // We can analyze the code inside the function to find out what it actually outputs
+                // The purpose of the hashset is to avoid infinite loops with functions that call themselves.
+                inferredTypes.AddRange(InferTypes(scriptBlockWithParams.Body));
+                return;
+            }
+
             // The OutputType property ignores the parameter set specified in the OutputTypeAttribute.
             // With pseudo-binding, we actually know the candidate parameter sets, so we could take
             // advantage of it here, but I opted for the simpler code because so few cmdlets use

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

@@ -650,8 +650,10 @@ Describe "Type inference Tests" -tags "CI" {
     }
 
     It "Infers type from variable with AllowSafeEval" {
-        function Hide-GetProcess { Get-Process }
-        $p = Hide-GetProcess
+        # Invoke-Expression is used to "hide" Get-Process from the type inference.
+        # If the typeinference code is updated to handle Invoke-Expression, this test will need to find some other way to set $p
+        # so that the type inference can't figure it out without evaluating the variable value
+        $p = Invoke-Expression -Command 'Get-Process'
         $res = [AstTypeInference]::InferTypeOf( { $p }.Ast, [TypeInferenceRuntimePermissions]::AllowSafeEval)
         $res.Name | Should -Be 'System.Diagnostics.Process'
     }
@@ -1492,6 +1494,17 @@ Describe "Type inference Tests" -tags "CI" {
         $null = [AstTypeInference]::InferTypeOf($FoundAst)
     }
 
+    It 'Should infer output from anonymous function' {
+        $res = [AstTypeInference]::InferTypeOf( { & {"Hello"} }.Ast)
+        $res.Name | Should -Be 'System.String'
+    }
+
+    It 'Should infer output from function without OutputType attribute' {
+        function MyHello{"Hello"}
+        $res = [AstTypeInference]::InferTypeOf( { MyHello }.Ast)
+        $res.Name | Should -Be 'System.String'
+    }
+
     It 'Infers type of command with all streams redirected to Success stream' {
         $res = [AstTypeInference]::InferTypeOf( { Get-PSDrive *>&1 }.Ast)
         $ExpectedTypeNames = @(