Fix code fix crash when adding an argument to a conditional access invocation

[meziantou]

Problem

ArgumentListHelper.BindingContext.GetTargetMethod binds the new invocation with GetSpeculativeSymbolInfo at the position of the original invocation. When the original invocation is part of a conditional access (x?.M()), the expression of the new invocation starts with a MemberBindingExpressionSyntax (or ElementBindingExpressionSyntax) that has no parent ConditionalAccessExpressionSyntax, and Roslyn throws a NullReferenceException in SyntaxFactory.FindConditionalAccessNodeForBinding.

This crashes every code fixer that uses ArgumentListHelper.AddArgument or ArgumentListHelper.GetTargetMethod: UseIFormatProviderFixer (MA0011), UseStringComparisonFixer (MA0074), UseAnOverloadThatHasCancellationTokenFixer_Argument (MA0040), UseAnOverloadThatHasTimeProviderFixer and UseAnOverloadThatHasMidpointRoundingFixer.

System.DateTime? dt = null;
_ = dt?.ToString(); // MA0011 code fix throws NullReferenceException

Fix

• BindingContext detects when the invocation's expression starts with a member or element binding. It follows chains such as x?.A.B(), x?[0].M(), x?.a!.M() and nested conditional accesses.
• In that case, each new invocation is bound in a copy of the compilation where it replaces the original invocation. The conditional access is then available, whatever the enclosing context: method body, lambda, field initializer or expression-bodied member.
• When the fix imports a namespace, the using directive is added in the same copy.
• Other invocations still use the cheaper speculative binding at the position of the original invocation.

Notes for reviewers

• Binding a single statement speculatively (TryGetSpeculativeSemanticModel) would be cheaper, but it doesn't cover invocations in field or property initializers or expression bodies. The copy of the compilation is created only for invocations that are part of a conditional access, up to 3 times per diagnostic in AddArgument.
• ((string)null)?.ToLower() doesn't reach this code: MA0011 offers no fix for ToLower even without a conditional access, as the CultureInfo parameter doesn't pass the fixer's IFormatProvider check.

Tests

• MA0011: nullable value type (dt?.ToString()), element binding (values?[0].ToString()), conditional access in a field initializer
• MA0074: value?.IndexOf("v"), a call in a member access chain (sample?.Value.IndexOf("v").ToString()), an extension method whose namespace is imported by the fix
• MA0040: sample?.Run()

All 7 new tests fail without the fix. dotnet test --max-parallel-test-modules 2 passes for every Roslyn version (26,824 tests), and dotnet run --project src/DocumentationGenerator produces no changes.