Fix 26144; exception propagation for non-raising virtual methods (#26145)

ref #26144

The C backend must not use `sfNeverRaises` to remove exception checks
from
virtual method calls. The flag describes only the selected base method
body,
while a vtable override may raise a catchable exception.

This change makes `canRaiseDisp` conservative for `skMethod` symbols and
adds a
regression test covering an exception raised by a child method invoked
through a
base reference.

(cherry picked from commit 8cb406cd7a)

compiler/ccgcalls.nim

@@ -11,7 +11,11 @@
 
 proc canRaiseDisp(p: BProc; n: PNode): bool =
   # we assume things like sysFatal cannot raise themselves
-  if n.kind == nkSym and {sfNeverRaises, sfImportc, sfCompilerProc} * n.sym.flags != {}:
+  if n.kind == nkSym and n.sym.kind == skMethod:
+    # A base method may be overridden by a branch with a wider exception set.
+    # Its inferred effects describe only the base body, not every vtable target.
+    result = true
+  elif n.kind == nkSym and {sfNeverRaises, sfImportc, sfCompilerProc} * n.sym.flags != {}:
     result = false
   elif optPanics in p.config.globalOptions or
       (n.kind == nkSym and sfSystemModule in getModule(n.sym).flags and

tests/method/tmethod_virtual_raise.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''caught'''
+"""
+
+type
+  Base = ref object of RootObj
+  Child = ref object of Base
+
+method run(value: Base): string {.base.} =
+  result = "base"
+
+method run(value: Child): string =
+  raise newException(ValueError, "child")
+
+let value: Base = Child()
+try:
+  discard value.run()
+  quit "virtual method did not raise"
+except ValueError:
+  echo "caught"