Fix invalid subtyping relationship in xml

[poorna2152]

Purpose

Fixes #34779

Approach

Update isAssignable to not to allow assigning of value with static typexml to value with static type xml:Comment|xml:Element|xml:ProcessingInstruction|xml:Text. But this assignment is allowed in the type binding pattern in theforeach loops. Thus in order to handle this case new function is introduced constituentTypesAssignable.

Samples

Provide high-level details about the samples related to this feature.

Remarks

List any other known issues, related PRs, TODO items, or any other notes related to the PR.

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.48%. Comparing base (e8b5bc5) to head (04cb468).
Report is 248 commits behind head on master.

Files with missing lines	Patch %	Lines
...llerinalang/compiler/semantics/analyzer/Types.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #42221      +/-   ##
============================================
- Coverage     77.55%   77.48%   -0.08%     
+ Complexity    58752    58448     -304     
============================================
  Files          3448     3432      -16     
  Lines        219728   218416    -1312     
  Branches      28922    28870      -52     
============================================
- Hits         170411   169234    -1177     
+ Misses        39921    39804     -117     
+ Partials       9396     9378      -18     

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[MaryamZi]

Why are we changing these? isAssignable should produce correct results.

[poorna2152]

This is a special case. With the modification to isAssignable function in this PR, variable with static type of xml is not assignable to xml<xml:Element>|xml<xml:Comment>|xml<xml:ProcessingInstruction>|xml:Text or xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text(Which is the correct behaviour and which is expected).

The function given here is used to validate the types in a foreach loop. In a foreach loop context, variable with static type xml is assignable to loop variable of type xml<xml:Element>|xml<xml:Comment>|xml<xml:ProcessingInstruction>|xml:Text. Thus here it is not possible to use isAssignable

Example code,

function foo() {
    xml x = xml `<a></a><!--comment-->text<?data?>`;
    foreach xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text item in x {
        
    }
}

[MaryamZi]

The function given here is used to validate the types in a foreach loop. In a foreach loop context, variable with static type xml is assignable to loop variable of type xml<xml:Element>|xml<xml:Comment>|xml<xml:ProcessingInstruction>|xml:Text. Thus here it is not possible to use isAssignable

This definition isn't entirely correct, is it? In the loop/foreach case we should be checking the sequence member type (xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text) against the foreach variable type, not xml.

[poorna2152]

Right, yes. But we cannot isAssignable for this check right, since for variable with static type xml it can't be assigned to xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text.

[MaryamZi]

What I meant was that we should be using isAssignable with

• source type - xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text against
• target type - xml<xml:Element>|xml<xml:Comment>|xml<xml:ProcessingInstruction>|xml:Text

which will (should) evaluate to true.

xml is xml<xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text>, therefore, iteration value type is xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text. https://ballerina.io/spec/lang/master/#section_5.1.6

[poorna2152]

In the case of type parameter the assignability is checked using the isAssignable(sourceParam, targetParam)
In the case of xml we cannot use this check.
Consider the following code

xml x = xml `<a>a</a>`;
x.forEach(function (xml entry) {
});

Here the targetType is xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text and the source type is xml. Since an xml value cannot be assigned to ``xml:Element|xml:Comment|xml:ProcessingInstruction|xml:Text` the check for xml is made special case for type params in this commit

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