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

compiler/ccgtypes.nim

@@ -696,6 +696,24 @@ proc genCppInitializer(m: BModule, prc: BProc; typ: PType; didGenTemp: var bool)
       if prc == nil:
         assert p.blocks.len == 0, "BProc belongs to a struct doesnt have blocks"
 
+# Unlike genCppInitializer which returns just the braced value list (e.g. "{a, b}"),
+# genCppConstructorExpr returns a full type-prefixed expression (e.g. "Foo(a, b)").
+# This is used when a standalone construction expression is needed — e.g. on the
+# right-hand side of an assignment — whereas genCppInitializer is used in variable
+# declarations where the type is already written separately before the initializer.
+proc genCppConstructorExpr(m: BModule, prc: BProc; typ: PType; didGenTemp: var bool): Snippet =
+  var params = ""
+  if typ.itemId in m.g.graph.initializersPerType:
+    let call = m.g.graph.initializersPerType[typ.itemId]
+    if call != nil:
+      var p = prc
+      if p == nil:
+        p = BProc(module: m)
+      params = genCppParamsForCtor(p, call, didGenTemp)
+      if prc == nil:
+        assert p.blocks.len == 0, "BProc belongs to a struct doesnt have blocks"
+  result = getTypeDesc(m, typ, dkVar) & "(" & params & ")"
+
 proc genRecordFieldsAux(m: BModule; n: PNode,
                         rectype: PType,
                         check: var IntSet; result: var Builder; unionPrefix = "") =

compiler/cgen.nim

@@ -526,7 +526,7 @@ proc resetLoc(p: BProc, loc: var TLoc) =
   if isImportedCppType(typ):
     var didGenTemp = false
     let rl = rdLoc(loc)
-    let init = genCppInitializer(p.module, p, typ, didGenTemp)
+    let init = genCppConstructorExpr(p.module, p, typ, didGenTemp)
     p.s(cpsStmts).addAssignment(rl, init)
     return
   if optSeqDestructors in p.config.globalOptions and typ.kind in {tyString, tySequence}:

tests/cpp/tcpp_default_ctor_assignment.h

@@ -0,0 +1,30 @@
+#ifndef TCPP_DEFAULT_CTOR_ASSIGNMENT_H
+#define TCPP_DEFAULT_CTOR_ASSIGNMENT_H
+
+struct AmbiguousAssign {
+  int x;
+  const char* y;
+
+  AmbiguousAssign(): x(0), y(nullptr) {}
+  AmbiguousAssign(int x, const char* y): x(x), y(y) {}
+
+  AmbiguousAssign& operator=(int v) {
+    x = v;
+    y = nullptr;
+    return *this;
+  }
+
+  AmbiguousAssign& operator=(const char* s) {
+    x = 0;
+    y = s;
+    return *this;
+  }
+
+  AmbiguousAssign& operator=(const AmbiguousAssign& other) {
+    x = other.x;
+    y = other.y;
+    return *this;
+  }
+};
+
+#endif

tests/cpp/tcpp_default_ctor_assignment.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim cpp $file"
+"""
+
+type
+  AmbiguousAssign {.importcpp, header: "tcpp_default_ctor_assignment.h".} = object
+    x: cint
+    y: cstring
+
+proc main =
+  var xs = newSeq[AmbiguousAssign](3)
+  doAssert xs.len == 3
+
+main()