fix: don't error on invalid FindInMap in false-condition resources

CloudFormation skips all validation for resources with false conditions,
including Fn::FindInMap lookups against non-existent mapping keys.
_partial_resolve_find_in_map was re-raising InvalidTemplateException
for string-literal keys that don't exist in Mappings, causing SAM CLI
to reject valid templates that CloudFormation would accept.

Now returns the partially-resolved FindInMap form instead of erroring,
matching CloudFormation's behavior.

Also updated LanguageExtensionResult docstring to accurately document
the no-LE path aliasing contract (expanded_template and
original_template share the same reference when had_language_extensions
is False).

Moved templateFnInMapInWhenFalseConditionInResourceWithInvalidStringPath
from ERROR_TEMPLATES_PASSING to SUCCESS_TEMPLATES_PASSING in the Kotlin
compatibility tests, with expected output.

Author: bnusunny

samcli/lib/cfn_language_extensions/api.py

@@ -315,14 +315,12 @@ def _partial_resolve_find_in_map(self, value: Dict[str, Any]) -> Any:
             try:
                 result = self._resolver.resolve_value({"Fn::FindInMap": resolved_args})
                 return result
-            except InvalidTemplateException:
-                # Re-raise - string literals that don't exist should error
-                raise
-            except Exception:
-                # Other errors - return with partially resolved args
+            except (InvalidTemplateException, Exception):
+                # For false-condition resources, CloudFormation skips validation
+                # entirely — including FindInMap lookups. Don't error on missing
+                # keys; return the partially-resolved form instead.
                 LOG.debug(
-                    "Partial-resolve Fn::FindInMap: unexpected error resolving %r; "
-                    "returning partially-resolved args",
+                    "Partial-resolve Fn::FindInMap: could not resolve %r; " "returning partially-resolved args",
                     resolved_args,
                     exc_info=True,
                 )

samcli/lib/cfn_language_extensions/sam_integration.py

@@ -47,6 +47,10 @@ class LanguageExtensionResult:
     All template fields are independent deep copies.  Callers may mutate
     them freely without affecting the original input or other callers.
 
+    Exception: when ``had_language_extensions`` is False, both fields
+    alias the caller's input dict (no copy overhead).  Callers that need
+    to mutate should ``copy.deepcopy()`` first in that case.
+
     Attributes
     ----------
     expanded_template : Dict[str, Any]

tests/unit/lib/cfn_language_extensions/compatibility/templates/templateFnInMapInWhenFalseConditionInResourceWithInvalidStringPath_expected.json

@@ -0,0 +1,34 @@
+{
+  "AWSTemplateFormatVersion": "2010-09-09",
+  "Transform": "AWS::LanguageExtensions",
+  "Mappings": {
+    "A": {
+      "B": {
+        "C": "D"
+      }
+    }
+  },
+  "Conditions": {
+    "TrueCondition": {
+      "Fn::Equals": [
+        "false",
+        "true"
+      ]
+    }
+  },
+  "Resources": {
+    "MyTopic": {
+      "Condition": "TrueCondition",
+      "Type": "AWS::SNS::Topic",
+      "Properties": {
+        "TopicName": {
+          "Fn::FindInMap": [
+            "A",
+            "Undefined",
+            "C"
+          ]
+        }
+      }
+    }
+  }
+}

tests/unit/lib/cfn_language_extensions/compatibility/test_kotlin_compatibility.py

@@ -192,7 +192,6 @@ def test_foreach_conditions(test_name: str, input_path: Path, expected_path: Pat
     "fnFindInMapWithUnsupportedFunctionFnGetAtt",
     "fnFindInMapWithUnsupportedFunctionFnRef",
     "fnFindInMapWithUnsupportedFunctionInMapName",
-    "templateFnInMapInWhenFalseConditionInResourceWithInvalidStringPath",
 ]
 
 # Templates that should error but have compatibility issues
@@ -311,6 +310,9 @@ def test_error_templates_with_placeholder(template_name: str):
     "toJsonStringWithSimpleTags",
     "toJsonStringWithSplitThatResolves",
     "toJsonStringWithStackNamePseudoParam",
+    # False-condition resources: CloudFormation skips validation, so invalid
+    # FindInMap keys are preserved rather than erroring.
+    "templateFnInMapInWhenFalseConditionInResourceWithInvalidStringPath",
 ]
 
 # Templates with known compatibility issues