fixes #25803: add genCppConstructorExpr for full type-prefixed expression

[ringabout]

fixes #25803

This pull request introduces a new approach for generating C++ constructor expressions in the Nim compiler's C++ backend, ensuring that type-prefixed construction is used when needed (such as in assignments), rather than just braced initializer lists. It also adds a new test case (with corresponding C++ header) to verify correct behavior when assigning to types with overloaded assignment operators and constructors.

C++ code generation improvements:

• Added genCppConstructorExpr in ccgtypes.nim, which generates a full type-prefixed constructor expression (e.g., Foo(a, b)) for C++ code generation, as opposed to just a braced initializer list. This is important for contexts like assignments where the type must be explicit.
• Updated resetLoc in cgen.nim to use genCppConstructorExpr instead of genCppInitializer when initializing imported C++ types, ensuring correct code generation for assignments.

Testing:

• Added a new C++ header file, tcpp_default_ctor_assignment.h, defining a struct AmbiguousAssign with overloaded assignment operators and constructors to test ambiguous assignment scenarios.
• Added a corresponding Nim test, tcpp_default_ctor_assignment.nim, which exercises construction and assignment for the imported C++ type, ensuring the new code generation logic works as intended.

[Copilot]

Pull request overview

This PR aims to fix C++ code generation for default(T)/sequence initialization of importcpp types by avoiding untyped {} assignment expressions that can trigger ambiguous C++ assignment overload resolution.

Changes:

• Adds genCppConstructorExpr for type-prefixed C++ construction expressions.
• Updates resetLoc to use the new constructor expression for imported C++ types.
• Adds a C++ regression test with an imported type that has ambiguous assignment overloads.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
compiler/cgen.nim	Uses the new constructor-expression helper when resetting imported C++ types.
compiler/ccgtypes.nim	Adds genCppConstructorExpr alongside existing C++ initializer generation.
tests/cpp/tcpp_default_ctor_assignment.nim	Adds a regression test compiling newSeq for an imported C++ type.
tests/cpp/tcpp_default_ctor_assignment.h	Defines the C++ type with ambiguous assignment overloads used by the test.

Comments suppressed due to low confidence (2)

compiler/ccgtypes.nim:715

• Using a parenthesized constructor expression here changes C++ overload resolution from the braced initialization that genCppInitializer uses elsewhere. A typed braced construction expression would still disambiguate the RHS of assignments, while preserving initializer-list/default-list initialization semantics for imported C++ types that rely on std::initializer_list overloads or are only list-initializable.

        let k = lastSon(n[i])

compiler/ccgtypes.nim:714

• The assertion message contains a typo: “doesnt” should be “doesn't”.

      of nkOfBranch, nkElse:

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