azp: Fail on invalid elseif/else sequence

src/Sdk/DTObjectTemplating/ObjectTemplating/TemplateUnraveler.cs

@@ -894,14 +894,31 @@ private void StartIfInsertion()
             var parentSequenceState = expressionState.Parent as SequenceState;
             bool skip = false;
             bool metastate = false;
-            if(expressionState.Value.Type == TokenType.IfExpression || (parentMappingState != null && parentMappingState.IfExpressionResults.TryGetValue(parentMappingState.Index - 1, out skip) || parentSequenceState != null && parentSequenceState.IfExpressionResults.TryGetValue(parentSequenceState.Index - 1, out skip)) && skip) {
+            Func<bool> shouldRun = () => {
+                if(parentMappingState != null) {
+                    if(parentMappingState.IfExpressionResults.TryGetValue(parentMappingState.Index - 1, out skip)) {
+                        return skip;
+                    }
+                    m_context.Error(expressionState.Value, $"This {expressionState.Value.ToString()} must be chained after an if token in the parent mapping");
+                } else if(parentSequenceState != null) {
+                    if(parentSequenceState.IfExpressionResults.TryGetValue(parentSequenceState.Index - 1, out skip)) {
+                        return skip;
+                    }
+                    m_context.Error(expressionState.Value, $"This {expressionState.Value.ToString()} must be chained after an if token in the parent sequence");
+                }
+                return false;
+            };
+            if(expressionState.Value.Type == TokenType.IfExpression || shouldRun()) {
                 metastate = skip = expressionState.Value.Type == TokenType.ElseExpression ? false : !PipelineTemplateConverter.ConvertToIfResult(m_context, new BasicExpressionToken(expressionState.Value.FileId, expressionState.Value.Line, expressionState.Value.Column, expressionState.Value.Condition).EvaluateTemplateToken(m_context, out _));
             } else {
                 skip = true;
                 metastate = false;
             }
             if(parentSequenceState == null) {
-                parentMappingState.IfExpressionResults[parentMappingState.Index] = metastate;
+                if(expressionState.Value.Type != TokenType.ElseExpression) {
+                    // Don't set this for ${{ else }}: tokens, because an else token is the last in the sequence
+                    parentMappingState.IfExpressionResults[parentMappingState.Index] = metastate;
+                }
                 var nestedValue = parentMappingState.Value[parentMappingState.Index].Value;
                 var nestedMapping = nestedValue as MappingToken;
                 var removeBytes = 0;
@@ -948,7 +965,10 @@ private void StartIfInsertion()
                     expressionState.End();
                 }
             } else {
-                parentSequenceState.IfExpressionResults[parentSequenceState.Index] = metastate;
+                if(expressionState.Value.Type != TokenType.ElseExpression) {
+                    // Don't set this for ${{ else }}: tokens, because an else token is the last in the sequence
+                    parentSequenceState.IfExpressionResults[parentSequenceState.Index] = metastate;
+                }
                 var nestedValue = (parentSequenceState.Value[parentSequenceState.Index] as MappingToken)[0].Value;
                 var nestedSequence = nestedValue as SequenceToken;
                 var removeBytes = 0;

testworkflows/azpipelines/else-after-else-error/pipeline.yml

@@ -0,0 +1,9 @@
+# ExpectedException: TemplateValidationException
+# ExpectedErrorMessage: else
+steps:
+- ${{ if true }}:
+  - ${{ if false }}:
+  - ${{ else }}:
+  - ${{ else }}:
+- ${{ else }}:
+  - script: noop

testworkflows/azpipelines/else-with-colon-exec-error/pipeline.yml

@@ -0,0 +1,8 @@
+# ExpectedException: TemplateValidationException
+# ExpectedErrorMessage: else
+steps:
+- ${{ if true }}:
+  # This else token is an error if not skipped by if
+  - ${{ else }}:
+- ${{ else }}:
+  - script: noop

testworkflows/azpipelines/else-with-colon-no-error/pipeline.yml

@@ -0,0 +1,6 @@
+steps:
+- ${{ if false }}:
+  # This else token is not an error if skipped by if
+  - ${{ else }}:
+- ${{ else }}:
+  - script: noop

testworkflows/azpipelines/else-without-colon-error/pipeline.yml

@@ -0,0 +1,7 @@
+# ExpectedException: TemplateValidationException
+# ExpectedErrorMessage: else
+steps:
+- ${{ if false }}:
+  - ${{ else }}
+- ${{ else }}:
+  - script: noop